CORN
Bacterial wilt has not been detected in Russia and is subject to external quarantine. First recorded in the USA and described by Stewart in 1897. Currently, the disease has also been registered in Mexico, South Africa, and Italy.
It first appears on the lower leaves of plants in the form of light green longitudinal dashed spots, which quickly turn yellow and spread along the vein along the blade. Later, streaked yellow spots or stripes appear on the stems and upper leaves. Small droplets of exudate often appear at the sites of lesions. Affected plants develop poorly and then wither and die. Sometimes there is premature ejection of male inflorescences and their whitening. On a cross section of the stem of a diseased plant, yellow mucus is released from the vessels.
Early ripening varieties of sweet corn are particularly affected. With early damage, plant seedlings wither and die. On other varieties, the disease usually appears after the appearance of male inflorescences.
The disease is caused by the bacteria Erwinia stewartii (Smith) Dye (syn. Aplanobacterium stewartii Me. Cull., Bacterium stewartii Sm.). They develop at temperatures of 8-39 and die at 53°.
Typical tracheobacteriosis: bacteria, accumulating in vascular bundles, clog and destroy them, as a result of which plant transpiration sharply decreases.
During the growing season, the pathogen is spread by insects. The source of infection can also be contaminated seeds and unrotted remains of affected plants. With severe development of the disease, the yield shortfall can reach 20-25%.
Bacterial stem rots are caused by three species of bacteria: Pseudomonas holci Kendrick, Pectobacterium carotovora Waldee and Erwinia dissolvens Burkh. When affected by the bacterium Ps. holci usually before throwing out the panicle, blurry cream-colored spots from 1 to 10 cm long with a wide dark burgundy or purple border appear in the upper part of the stem. The tops and middle leaves of the affected plants dry out at the edges, while the lower ones remain green. The internal tissues in the affected areas first take on the appearance of a thick light gray, and later a dark brown mass with an unpleasant, cloying odor. Such plants are sterile. The disease is more common on irrigated lands.
When P. carotova bacteria are infected, the central leaf of the plant wilts, curls and turns yellow, and then the entire plant wilts. Spots do not form on the stems, but a gray, rotting, fetid mass is found inside them. The disease appears more often on corn shoots under irrigated conditions.
E. dissolvens most often infects corn in the first half of the growing season. Sick plants are stunted and their leaves become discolored. Brown, then darkening spots appear on the stems, cob wrappers and leaves, especially at the base of the stem, turning into a rotting soft slimy mass with an unpleasant odor. After some time, the rotten tissue dries out and only the vascular bundles remain visible. At the site of the lesion, the stem breaks and falls to the ground. This disease develops especially intensively at excessive humidity and temperatures of 25-30°. Infection of plants increases when they are damaged by insects and tillage tools.
Sources of infection are predominantly unrotted plant debris, as well as wintering weeds affected by rot.
All rots cause thinning of crops and greatly reduce plant productivity. In addition, lodging of affected crops significantly complicates mechanized harvesting.
The hybrids Dneprovsky 460 MB and Collective 150 TV are relatively resistant to stem and root rot.
Distributed in all corn-growing regions of Russia. It appears during the period of milky ripeness in the upper part of the grains in the form of depressed spots of pale gray color, 2-3 mm in diameter. With a more severe development of the disease, the spots become wrinkled or ulcer-like, with a brownish-yellow color. On plants of white-grain varieties, the spots have a narrow dark gray border, while in yellow-grain varieties it is less pronounced.
The number of affected grains on the cobs usually does not exceed 30-40. They are often placed several in a row closer to the top of the cob. The cobs become infected in the field. The main carrier of the pathogen is the bread bug, in whose body live the bacteria Bacillus mesentericus var. vulgatus Flugge. By damaging the shell of the grain, the bug introduces bacteria into the wound.
In the second half of summer, bread bugs accumulate on millet and mogar crops, as well as on mice that infest corn crops. From these plants, insect vectors fly to corn during its milky ripeness. Cobs are especially often affected, the tops of which are open or poorly covered with wrappers, which is observed with excess moisture.
The harmfulness of bacteriosis lies in the reduction in the keeping quality of the cobs and their rapid molding, as well as in the deterioration of the seed qualities of the affected grains - underdevelopment, reduction in weight and germination.
Preventive measures against bacteriosis are the removal from the field and burning of all post-harvest residues, followed by deep autumn plowing, as well as the destruction of insect carriers.
White rot was found in Ukraine and the Krasnodar region. It appears on the lower part of the stem in the form of large dark weeping spots with a white fluffy cotton wool-like coating. Dark brown sclerotia about 1 cm in diameter are formed on it, as well as in the middle of the affected stems.
The causative agent of the disease is the marsupial fungus Whetzelinia sclerotiorum Korf. et Dumont. It overwinters in the form of sclerotia, which, germinating in the spring, form a mycelium that penetrates the plant, or apothecia - saucer-shaped bodies on cylindrical legs. In the upper part of the apothecium, bags with sacspores are formed. The bags are cylindrical, colorless, measuring 130-135 x 8-10 microns. Each of them contains 8 elliptical colorless sacspores measuring 7-12 x 4-6 µm. The latter also germinate and infect plants. The disease is especially severe in humid, warm years.
The most typical manifestation of leucorrhoea is the formation of one smooth or angular-winding wide crack, which greatly disfigures the shape of the grain. Ruptures of the seed coat are often observed with a generally normal appearance of the caryopsis and its epidermis. Sometimes the endosperm does not have cracks, but, growing in one place, it protrudes through the cracked seed coat in the form of a wart.
Cracks appear at the beginning of the waxy ripeness of the grains, and towards the end of this ripeness their formation stops.
The cause of the disease is a discrepancy between the intensity of growth in the grain of the epidermis and the seed coat, which occurs as a result of a sharp change from drought to excess moisture.
White cobs practically do not reduce the corn yield, but significantly deteriorate its quality. On affected cobs in the field, and even more often during storage, fusarium and mold fungi intensively develop.
Brown spot (helminthosporiosis)
It is most often found in Transcarpathia, Ukraine, Georgia and the Primorsky Territory, where it sometimes causes significant crop shortages. In some years it appears in the North Caucasus, southern Ukraine, Armenia and the Baltic republics.
Leaves of young and older plants, underground and above-ground parts of stems, less commonly roots, and sometimes cobs are affected. Initially, small whitish and then brown spots with a dark or reddish-brownish border appear on the leaves. In the center the spot is covered with a brownish-olive coating. Over time, the spots increase in size, often merge and cover almost the entire leaf blade, as a result of which they dry out and die.
The disease usually begins on the lower leaves.
Greenish or dark spots of various shapes with a border are formed on underground and aboveground internodes. In this case, the parenchyma of the core is almost not destroyed.
On cobs, helminthosporiosis can appear in the form of a thick dark brown coating at their base and in the recesses between the rows of grains.
The disease develops intensively in conditions of sufficient moisture and elevated temperatures. Late corn crops are more severely affected.
The causative agent of the disease is the imperfect fungus Helminthosporium turcicum Pass. Its mycelium first spreads intercellularly in the parenchyma, and then in the vascular system of the leaves, as a result of which the lesion has the character of leaf tracheomycosis. On the surface of the leaves in places of spots, conidial sporulation of the fungus in the form of tufts is formed. Conidiophores are olive-brown, straight or slightly curved, with three septa up to 150 µm long. Conidia are elongated, fusiform, pointed at the ends, olive, with 5-8 septa and a thickened shell. The size of conidia is 85-110 x 20-24 microns. They germinate in dripping moisture at temperatures from 10 to 38° (optimum 23-30°), infecting plants through the stomata, and occasionally through the epidermis.
The duration of the incubation period depends on the age of the plant and the condition of the leaf surface. On young plants it lasts from 3 to 7, and on older plants it lasts 7-11 days. During the growing season of plants, the fungus can produce 2-3 generations of conidia.
On the soil surface and at a depth of 10 cm, the pathogen persists in the form of a mycelium, on which new conidial sporulation is formed in the spring, spreading to corn crops. It usually dies in soil at a depth of 20 cm. In the form of conidia it can also spread with seeds.
The disease causes a significant shortage of grain and green mass. There are no hybrids or varieties of corn with high resistance to the disease.
Diplodiasis
Diplodia is common in Georgia. It affects ears, stems, leaf sheaths and leaves. The stems turn brown mainly at the lower internodes, then soften and break. White mycelium often appears under the leaf sheath. In places of browning, point-shaped, initially white, and later black pycnidia are formed, protruding outward from under the epidermis. Brown spots, sometimes with pycnidia, also appear on the affected leaves.
The cobs are covered with a continuous cotton-like coating, spreading to the adjacent wrappers. On the grains, especially near the embryo, inside the stem and on the involucres adjacent to the cob, pycnidia are formed in the form of black, slightly convex dots.
The causative agent of the disease is the imperfect fungus Diplodia zeae Lev. Its pycnidia are round or slightly flattened, blackish-gray, 350-500 µm in diameter. A large number of dark brown oblong straight or slightly bent two- and three-celled pycnospores measuring 13-33 x 3-7 µm are formed in them. The optimal temperature for spore germination is 20°, and for the development of the disease - from 28 to 30°.
Sources of infection are infected seeds and remains of infected plants, on which the pathogen persists for 3-4 years.
Red rot is more often found in fairly humid areas of the North Caucasus and the Far East. It usually appears during the period of milky-waxy ripeness of the grain in the form of a bright pink coating on the top of the cob. The wrappers fit tightly to the cob and acquire a brick-red color. The grains are also painted the same color. They become fragile and are filled with mycelium inside.
The causative agent of the disease is the imperfect fungus Fusarium graminearum Schwabe, which has conidial and marsupial stages. Conidia are fusiform-crescent-shaped, with 3-6 septa, measuring 25-75 x 3-6 µm in mass, whitish-pink, golden-yellow or carmine-purple. Microconidia are usually not formed.
Sporangiophores are 330-1200 µm long, sporangia are round, 110-165 µm in diameter, and sporangiospores are yellow, elliptical, 5.5-9.3 x 4.5-7 µm in size. The fungus develops intensively at excessive or moderate humidity and a temperature of 30-35°, so during storage the cobs are almost not affected. The pathogen persists in the form of mycelium and sporangiespores on plant debris in the field. The infection is practically not transmitted by seeds.
With early damage, the cobs become underdeveloped, the seeds lose their viability and become very moldy during storage.
In the south of Ukraine, grain harvest losses from this rot are small (0.05-0.3%), but in some areas they reach 3-4%.
The causative agent is Zea virus I Storey. The virus is transmitted by leafhoppers.
With severe development of the disease, mosaic streaks and spots appear on leaf sheaths, cob wrappers and stems. The productivity of affected plants is reduced. At the same time, the shortfall in grain yield can reach 5-10%.
Diseases caused by a lack of nutrients in the soil slow down the growth and development of both individual organs and the entire corn plant.
With a lack of nitrogen in the soil, corn leaves turn yellow and die, starting from 
lower.
A lack of phosphorus leads to slower root growth, and the leaves become dull in color with a reddish or purple tint.
With a lack of potassium in the soil, yellowing of the tops and edges of first the lower and then the upper leaves is observed. Often the leaves die off completely.
A deficiency of manganese, magnesium, boron and other microelements is characterized by the formation of yellow-green spotting and chloroticness on the leaves.
Distributed in all corn growing areas. It especially often manifests itself under irrigation conditions during the period from the beginning of milk to full ripeness of grain. Reproductive buds, cobs, leaf sheaths, and stems are affected.
The first signs of the disease are the loosening of the stalk and lower part of the cob stem, as well as the appearance of small black piles of spores at the base of the stem and on the lower grains, clearly visible through a magnifying glass. Later, a black dusty coating forms on the cobs, reproductive buds and leaf sheaths.
Affected stems acquire a dirty gray or bluish tint and become macerated with partial destruction of the parenchyma. Under the affected epidermis, which is easily separated from the phloem, there is a noticeable accumulation of spores in the form of a sooty coating.
With severe damage, the cobs are underdeveloped, their stem is loose, grayish with a blue tint, and often breaks up into separate bundles of fibers. The grains in the affected cobs are usually underdeveloped, dull, slightly grayish, and not densely located.
The causative agent of the disease is the imperfect fungus Nigrospora oryzae Fetch. Its mycelium is loose, grayish, located in the grooves between the rows of caryopses, the conidiophores are pale olive, simple or irregularly branched, with septa, slightly swollen at the top. At the top of the conidiophore, one spherical or elliptical conidia is formed, first translucent and then black, with a thick shell. The diameter of the conidia is 12-15 microns. The fungus is spread by conidia overwintering on plant debris and grain.
More intensive development of nigrospora occurs with sufficient humidity and a temperature of 20-25°, especially in late sowings. The harmfulness of the disease is great. Affected cobs lose their marketable quality and are quickly destroyed by mold fungi during storage. Seeds have reduced germination.
Lodging of seedlings
Lodging of seedlings is of two types and is more often found in areas with sufficient moisture, especially on soils prone to waterlogging and poorly drained. The first type of lesion is characterized first by the blackening of thin fibrous and then large roots; the stem at the root collar turns black. Thinning of the stem and drooping of the plants are also observed. Often, a white coating of mycelium is noticeable in the affected areas.
The causative agents of this type of disease are fungi from the genus Pythium Pring., most often P. debaryanum Hesse. It forms a branched mycelium that penetrates plant tissue. Sporulation is asexual - zoosporangia with zoospores and sexual - oospores. Zoosporangia are ovoid or spherical, 15-26 µm in diameter, oospores are smooth, 12-20 µm.
In addition to corn, the pathogen affects sorghum, wheat, oats and other plants. It is not transmitted with seeds, but is stored on the remains of affected plants in the form of oospores.
The second type of infection is characterized by the appearance of weak shoots with a pale green color. The roots rot, causing the plants to fall out. Often at the base of plant stems there is a faint pink coating with sickle-shaped colorless conidia 8.4-6.0 x 12.4-4.5 µm, with several septa.
The causative agent of this type of disease is the fungus Fusarium moniliforme Scheldon. It persists on seeds and remains of affected plants in the soil.
Lodging of seedlings can cause large thinning of corn crops.
Blackening of vascular bundles was found in the Krasnodar region and in the south of Ukraine. Appears at the beginning of milky ripeness of the grain. The above-ground parts of plants (stems, leaves and leaf sheaths) acquire a red-purple or brown color. The color change begins with the upper leaves and gradually spreads to the lower leaves. Typically on leaves, the red-purple coloration first appears along the central vein and then covers the rest of the blade.
In humid weather, a whitish-pink, delicate, velvety coating appears on the affected vagina. On a cross section of the stem, blackening of the vascular bundles is clearly visible, which is the most characteristic sign of the disease. Often the disease causes excessive thickening of the stem.
The causative agent is the imperfect fungus Serhalosporium acremonium Corda. On its mycelium, vertical, simple, slightly thinned at the top conidiophores 30-60 µm long are formed. At their ends, ovoid unicellular conidia measuring 3-4 x 1-1.5 microns, collected in spherical heads, are successively formed. Plants become infected during the growing season with conidia. The fungus spreads through the vascular system of the stem and often reaches the cobs, where it infects the seeds. When seeds germinate, the mycelium penetrates the sprout and spreads diffusely throughout the plant.
The harmfulness of the disease lies in the fact that plants grown from the affected seeds do not bear fruit. Plants infected before the panicles are thrown out produce puny seeds. When the grain is infected at the beginning of milky ripeness, a significant reduction in yield is not observed.
Rust The causative agent of the disease is the dioecious fungus Puccinia sorghi Schw., which forms uredinia with urediniospores and telia with teliospores (black pustules) on corn. Urediniospores can be spherical or elliptical, pale brown, finely spinous, 21-35 µm in diameter. Teliospores are oblong, club-shaped, dark brown, two-celled, 31-50 x 18-22 µm in size, with a thick and elongated brown stalk. The fungus also has spermogonial and aecial stages, which occur occasionally on sorrel weeds (Oxalis L. spp.), a very common corn weed in the southern regions. However, in most cases, corn is infected by urediniospores, which in the south overwinter well on the remains of infected corn plants in the field. Urediniospores can germinate at temperatures from 4 to 34° (optimum 17-18°) and high air humidity. The incubation period of the disease usually lasts 5-8 days. During the summer period, the fungus can produce 2-3 generations of urediniospores. During the harvesting period, urediniospores fall on the cobs, and together with the seeds, on new corn crops. The role of teliospores in maintaining infection is small. They germinate in the spring, forming basidia with basidiospores. The latter, scattering, can infect kisliki during germination, but in Russia this is observed very rarely. Aeciospores from sour sorrel can only be the primary source of infection of corn. The harmfulness of rust is manifested in the premature drying of leaves, as a result of which the ears are underdeveloped and puny grains are formed in them. Tooth corn varieties have increased resistance to rust, and sweet corn varieties have reduced resistance. Pink mold
Gray rot is common in the southern regions of Ukraine, Rostov and other regions of Russia. It appears at the beginning of the milky-waxy ripeness of the grain in the form of a thick gray coating between the rows of grains. It only affects grain on the cob. First, rot appears in the upper part of the cob, and then quickly spreads to the lower part. Affected grains turn brown, die and crumble easily.
Rust is more common in Transcaucasia, on the Black Sea coast of the Caucasus, in Transcarpathia and less often in other regions of Ukraine. It appears on the leaves and sometimes in the stalks of corn in the second half of the growing season. Initially, inconspicuous light yellow spots appear on the leaves, on which brownish oblong, randomly scattered pustules up to 1 mm in size, covered with epidermis, are formed. Over time, the epidermis ruptures, the pustules are exposed, and urediniospores scatter from them. Towards the end of the growing season, pustules appear on the corn for the second time, but, unlike the previous ones, they are black, larger and located on the leaves in the form of elongated spots.
Pink mold is a consequence of infection by fungi from the genera Trichothecium Link., Sporotrichum Link, etc. The disease, as a rule, first develops on grains affected by linen, and then spreads to the entire cob. Its intensive development is observed at a temperature of 8-10° and grain humidity on the cob is about 19%. Severe molding of cobs and grain also occurs when storage facilities are poorly ventilated and especially when grains are damaged. Very affected cobs in the piles completely rot. As a result of molding, the energy of germination and germination of seeds sharply decreases, and they become unsuitable for sowing. Grain infected with certain species of Aspergillus cannot be fed to animals or used for human nutrition, as it can cause diseases of the hearing and respiratory tract (inhalational mycoses of the lungs, etc.).
The causative agent of the disease is the lower fungus Rhizopus maydis Bruderl., which forms an abundant nonseptate mycelium with stolons and rhizoids that penetrate plant tissue. On the stolons, sporangiophores with sporangia and sporangiespores are formed one by one or in groups.
Sporangiophores are 120-330 µm long, sporangia are spherical, 110-165 µm in diameter, sporangiospores are yellow, elliptical, 5.5-9.3 x 4.5-7 µm in size. The fungus develops intensively with sufficient humidity and a temperature of 30-35°, so gray rot does not spread during the storage period of the cobs. The pathogen persists in the form of mycelium and sporangiespores on corn residues in the field and other places.
Seeds do not play a significant role in the spread of infection.
P When corn is damaged early, the cobs become underdeveloped, the seeds lose their viability and become very moldy during storage.
Gray-green mold is quite common. Its causative agents are fungi from the genera Penicillium Link., Aspergillus Micheli et Fr., Botrytis Micheli, Mticor Micheli, etc. Most of them begin to develop at a temperature of 8°, and Penicillium species - even at 2-5°, while suppressing other fungi located on the cobs.
Sclerosporosis was discovered in the North Caucasus. It appears on leaves and stems in the form of whitish-yellow or dirty green stripes, on which a barely noticeable whitish coating forms (on leaves mainly on the underside). The affected leaves, and often the entire top of the stem, turn brown and become deformed. Often the stem becomes loosened and the fibers of the conductive bundles become twisted. The panicles thicken and turn into a large number of underdeveloped, densely growing leaves.
The causative agent of the disease is the lower fungus Sclerospora maydis Butler. Its zoosporangiophores are thick and short, with branched apices, at the ends of which colorless spherical or elliptical zoosporangia measuring 28-45 x 16-28 µm are formed. During germination, up to four zoospores can be formed from one zoosporangium.
Plants become infected in the presence of drip moisture during periods of heavy rain and dew. The pathogen persists in the affected plant remains on the soil surface.
Dark mold is caused by fungi of the genera Cladosporium Link, Alternaria Fr., Macrosporium Fr., Coniosporium Link, etc. It is characterized by the formation of black or dark olive-colored mold, mainly in the upper part of the cobs. Primary infection is usually observed at the site of damage to the grain. The development of the disease is promoted by temperatures above 12°.
Charcoal rot occurs in almost all corn growing areas. Causes browning or discoloration of the lower part of the stem and root. Under the epidermis, small black sclerotia in the form of dots are found scattered along the stem. The parenchyma of the core is almost completely destroyed, the stem dries out or becomes soft and easily breaks.
The causative agent of the disease is the imperfect fungus Sclerotium bataticola Taub. In its development cycle, it forms sclerotia measuring 50-152 x 22-32 microns and mycelium, which infects plants. The development of the disease increases with low rainfall and elevated temperatures.
Fusarium is widespread. It appears in the field on the cobs at the end of the milky or beginning of the waxy ripeness of the grain and develops before harvesting, and sometimes during storage of the crop at elevated temperatures and humidity.
A cobwebby or denser pale pink coating appears in patches on the cobs. The grains in such places become dirty brown and are easily destroyed, especially when threshing the cobs.
The causative agents of the disease are imperfect fungi from the genus Fusarium Link, mainly Fusarium moniliforme Sheldon. In addition to abundant mycelium, it forms microconidia and, in small quantities, macroconidia.
Microconidia are colorless, fusiform-ovate, unicellular or with one septum, 4-30 X 1.5-4 µm in size, formed on the tops of conidiophores in the form of disintegrating chains or false heads. Macroconidia are colorless, spiny or slightly sickle-shaped, gradually tapering towards both ends, usually with 3-5, rarely 6-7 transverse septa, measuring 9.4-6.0 x 4-4.5 µm.
Occasionally, dark blue spherical sclerotia with a diameter of 80-100 microns are formed on the mycelium.
Severely affected corn seeds lose their viability, and seeds with a healthy embryo produce weak sprouts that usually die before reaching the soil surface.
The main source of infection is post-harvest corn residues and especially cob wrappers, the tissues of which contain mycelium, which gives rise to a new generation of conidial sporulation in the spring. Infestation usually begins on ears damaged by insects or affected by corn linen.
The fungus F. moniliforme has no toxic properties, so the cobs affected by it can be fed to animals.
Fusarium rot is known in all corn growing areas. It usually appears on two or three lower nodes and internodes in the form of brown or yellow spots of various shapes, which in wet weather are covered with a red-white or white-pink coating. The stem of affected plants is hollow inside and often becomes macerated. The underground internode and roots become red, and the parenchyma of the core is destroyed.
The causative agents of Fusarium rot are various imperfect fungi from the genus Fusarium Link. They spread by conidia, which are of two types: macroconidia - fusiform or sickle-shaped, with 3-5 septa, and microconidia - one- or two-celled. The latter are often formed during the budding of macroconidia. The disease develops more strongly at elevated temperatures and light precipitation.
The article presents corn diseases that manifest themselves at the initial stages of seedlings, germination, growth and development of the crop.
Corn seed mold disease during germination
Corn disease manifests itself on seeds contained in the soil. Its symptoms are characterized by the appearance of plaque of various colors on the surface of the seeds, characteristic of certain types of fungi. The disease is caused by saprotrophic micromycetes. For example, the development of fungi from the genera Penicillium Link, Aspergillus Micheli, Mucor Micheli, Botrytis Micheli and others leads to the formation of gray-green mold. Dark mold is caused by fungi of the genera Cladosporium Link, Alternaria Nees, etc., and pink mold is caused by fungi of the genera Trichothecium Link, Sporotrichum Link and others.
Grains in which the seed coat is damaged or affected by pathogens of other diseases become moldy especially quickly. Conditions characterized by a decrease in soil temperature (cooling) after sowing seeds contribute to their damage.
Diseased corn seeds do not form seedlings, and in humid conditions they are completely covered with abundant mycelium of the pathogen. Seeds in which the embryo is not infected germinate late, forming weak seedlings that are often unable to reach the soil surface and die before reaching its surface. Young plants grown from slightly damaged seeds are weakened, stunted, and somewhat chlorotic. 
The causative agents of this corn disease are Fusarium spp. The development of the disease is possible in the presence of a seed infection (latent, since heavily affected seeds are discarded at the stage of preparing the seed material), pathogens in the soil and under the influence of favorable abiotic environmental factors for the development of the disease. Among the latter are low temperatures during seed germination, violations of seed placement depth, and increased crop density.
Brown spot, or helminthosporium blight of corn
Symptoms of this disease in corn can be found on the leaves, underground and aboveground internodes, and on the ears of the plant. At the same time, according to our observations, leaves are most intensively affected. The development of the disease begins on the lower leaves and gradually covers the upper ones. During its diagnosis, it is important to take into account the following typical symptoms: on the affected leaves, small whitish spots (which then turn brown) first appear along the leaf blade with a narrow dark brown or reddish-brown border, in the center of which a brownish-olive coating forms. Subsequently, the affected areas increase in size, often merge and cover almost the entire leaf blade, causing it to dry out and die. Often the size of the spots reaches a length of about 25 cm or more. On cobs, the disease can appear at their base and in the depressions between the rows of grains in the form of a thick dark brown felt coating.
The harm of brown spot is the premature death of leaves, which leads to a decrease not only in grain yield, but also in green mass. Severely affected plants are susceptible to infection by stem rot pathogens.
The causative agent of the disease is the fungus Exserohilum turcicum (Pass.) Leonardet Suggs (synonyms - Bipolaris turcica (Pass.) Shoemaker, Helminthosporium turcicum (Pass.), Drechslera turcica (Pass.). It is characterized by high variability of pathogenic properties and the presence of races.
The corn disease, as a rule, develops under conditions of sufficient and excessive moisture, heavy dew, the presence of water on the leaf surface for 6-18 hours and elevated temperatures (23 ... 30 ° C) and an optimal relative air humidity of more than 90%. Late corn crops are more severely affected. The incubation period of the disease depends on the age of the plant and the condition of the leaf surface. On young plants it lasts up to seven days, and on adults - up to 11 days. Sporulation is intensively carried out at 100% relative air humidity. Poor, poorly fertilized soils increase the susceptibility of corn to brown spot.
Typically, on the soil surface and at a depth of 10 cm, the fungus is preserved in plant debris in the form of a mycelium, which produces new conidial sporulation in the spring. It dies in the soil at a depth of 20 cm. The pathogen may be retained by conidia on the surface of the seeds.
Corn disease: rust
The disease usually appears on corn leaves in the second half of the plant's growing season. At the beginning of the disease, inconspicuous light yellow spots appear on the leaves, on which brownish oblong (up to 1 mm) urediniopustules (they contain urediniospores) are then formed, which are covered for quite a long time by the epidermis. They are placed randomly on the leaves of corn. Over time, the epidermis ruptures, the pustules are exposed, and a spore mass is released and spreads. By the end of the corn growing season, instead of urediniopustules, black teliopustules appear, which contain teliospores. With severe development of the disease, the leaves dry out prematurely, and underdeveloped heads of cabbage with flat grains are formed.
The causative agent of the disease is the dioecious fungus Puccinia sorghi Schw. On corn plants it forms uredinio- and teliopustules. Intermediate life-giving agents for the fungus are wood sorrel (Oxalis spp.), weeds that are common in corn crops in the southern regions of Ukraine.
Favorable conditions for the development of the disease are moderate temperature, high relative humidity, frequent precipitation and heavy dew. The incubation period lasts five to eight days. The source of corn rust infection is mainly urediniospores, which in the southern regions overwinter well on leaf debris in the field. The role of teliospores in the preservation of the pathogen in winter and the restoration of infection is small.
Measures to combat corn diseases
Measures to protect corn from diseases should be based on the cultivation of resistant hybrids, timely implementation of agrotechnical practices that provide the best conditions for growth and development (observance of crop rotation, favorable sowing dates and optimal plant density, as well as provision of balanced fertilizer). For sowing, you should use high-quality seeds treated with approved disinfectants. If necessary, fungicides are used during the growing season of plants. You should also comply with the requirements regarding the optimal timing of harvesting and high-quality seed processing. 
Disinfection of seeds allows you to control pathogens located on its surface (for example, the causative agent of flying smut) and inside (causative agents of Fusarium, red rot, etc.), as well as protect against mold in the soil during germination.
To treat seeds, disinfectants approved in Ukraine are used. The “List of Pesticides and Agrochemicals Permitted for Use in Ukraine” contains a wide range of preparations based on various active ingredients:
Alios, TN (triticonazole, 300 g/l), 1.0-2.0 l/t (against mold, bubbly and dusty smut, root rot);
Vaxa, KS (carboxin, 375 g/l + thiramide, 375 g/l), 2.0 l/t (against root and stem rot, loose smut);
Viking, v.s.k. (Carboxin, 200 g/l + thiram, 200 g/l), 2.5-3.0 l/t (against flying and blister smut, root and stem rot, molding of seeds);
Vispar, KS (carboxin, 375 g/l + thiram, 375 g/l), 2.0 l/t (against root and stem rot, loose smut);
Vitavax 200 FF, v.s.k. (Carboxin, 200 g/l + thiram, 200 g/l), 2.5-3.0 l/t (against flying and blister smut, root and stem rot, molding of seeds);
Granivit, TN (carboxin, 200 g/l + thiram, 200 g/l), 2.5-3.0 l/t (against flying and bubbly smut, root and stem rot, molding of seeds);
Inshur Perform, t.k.s. (Triticonazole, 80 g/l + pyraclostrobin, 40 g/l), 0.5 l/t (against mold, blister and loose smut, root rot);
Maxim Quatro 382.5 FS, TN (thiabendazole, 300 g/l + azoxystrobin, 15 g/l + fludioxonil, 37.5 g/l), 1.0-1.5 l/t or 8.5 ml per 50 000 seeds (root rot, flying and blister smut, molding of seeds);
Maxim 025 FS, TN (fludioxonil, 25 g/l), 1.0 l/t (red rot, fusarium, pythium rot, seed mold);
Maxim XL 035 FS, tk.s. (Fludioxonil, 25 g/l + metalaxyl-M, 10 g/l), 1.0 l/t (against root and stem rot, molding of seeds);
Royalflo, v.s.k. (Tirami, 480 g/l), 2.5-3.0 l/t (against root and stem rot, molding of seeds);
Stamina, TN (pyraclostrobin, 200 g/l), 0.25 l/t (against rhizoctonia, pythium, fusarium, seed mold);
TMTD, KS (thiram, 400 g/l), 3.0-4.0 l/t (against root and stem rot, seed mold, smut);
Fever 300 FS, TN (prothioconazole, 300 g/l), 0.6-0.9 l/t (against loose smut and root rot);
Fluosan, t.k.s. (thiram, 533 g/l), 3.0 l/t (against seed mold, root and stem rot, blister smut) and other approved drugs.
Systematic monitoring during the growing season of plants makes it possible to timely identify the onset of the development of dangerous diseases and make the right decision regarding the use of fungicides.
Abacus, mk.e. (pyraclostrobin, 62.5 g/l + epoxiconazole, 62.5 g/l), 1.5-1.75 l/ha (against fusarium, rust and helminthosporium);
Acanto plus 28, KS (picoxystrobin, 200 g/l + cyproconazole, 80 g/l), 0.75-1.0 l/ha (against helminthosporium and rust);
Amistar Extra 280 SC, KS (azoxystrobin, 200 g/l + cyproconazole, 80 g/l), 0.5-0.75 l/ha (against fusarium, helminthosporiosis, grain mold, rust, loose smut);
Coronet 300 SC, KS (tebuconazole, 200 g/l + trifloxystrobin, 100 g/l), 0.6-0.8 l/ha (against rust and helminthosporium);
Custodia, KS (tebuconazole, 200 g/l + azoxystrobin, 120 g/l), 1.0-1.2 l/ha (against helminthosporiosis, rust, fusarium);
Retengo, EC (pyraclostrobin, 200 g/l), 0.5 l/ha (against fusarium, rust and helminthosporium), etc.
When using chemical protective agents, you should adhere to the spraying periods recommended by the manufacturers of the drugs and the regulations for their use.
M. Pikovsky, Ph.D. biol. sciences,
M. Kirik, dr. biol. sciences,
A. Stolyar, Ph.D. agricultural sciences,
NUBiP of Ukraine
Citation information
“The very first” diseases of corn / M. Pikovsky, M. Kirik, A. Stolyar // Proposition / - 2017. - No. 4. - P. 128-130
During development, corn stalks and fruits can be affected by a wide variety of diseases. In our article we will describe such popular corn diseases as smut, helminthosporium blight, rust, and white rot. Let's take a closer look at the symptoms of these diseases and methods to combat them.
Description and symptoms
The first signs of smut, pink or greenish swellings, can be seen on the cobs or stems of the crop. After some time, the affected areas begin to deform, the swellings increase in size, affecting up to 20 centimeters of the stem. Then the bubbles darken and burst, releasing a huge number of spherical spores.
Damage to corn by smut can occur up to 4 times during the summer. On infected ears, the upper grains are damaged; on panicles, individual inflorescences suffer from this disease. This disease is especially dangerous for young plants; rough swellings soon appear on their leaves, which subsequently lead to the death of the crop.
The disease in question develops intensively during drought, as well as during late sowing of corn, with an excess of nitrogen fertilizers in the soil, as a result of improper watering. If weather conditions are favorable, smut can eat up to 40% of the crop. Under the influence of this disease, high-quality grain turns into powdery spores. The fungus overwinters in organic debris, as well as on the surface of seed material.
Struggle
One of the main measures to combat bubbly smut is considered to be treatment of seed material and compliance with crop rotation of grain crops (fields with corn must be located at least 1 kilometer from last year’s areas with this crop). The use of corn varieties and hybrids with high resistance to the disease in question.
Prevention
Preventive actions are aimed at destroying affected organic residues. In small areas, such plant parts are collected and burned or buried deep into the soil. In addition, it is necessary to follow the rules of crop rotation; the crop can be replanted on the site no less than after three years. Seed material is collected only from healthy, fully ripened heads of cabbage.
Dusty smut
Description and symptoms
Loose smut is common in Kuban and Transcaucasia. The disease develops intensively during seed germination at high temperatures (about 30 degrees). Other sources of the disease are contaminated soil or untreated seed material.
Struggle
One of the main measures to combat bubbly smut is considered to be treatment of seed material, compliance with crop rotation of grain crops (fields with corn must be located at least 1 kilometer from last year’s plots. The use of varieties and hybrids of corn with high resistance to the disease in question.
Prevention
The main preventive action is the destruction of the affected parts of the plant. In small areas, affected crops are collected and burned or buried in the soil to a depth of 50 centimeters. In addition, it is necessary to follow the rules of crop rotation - corn is grown in the same area every three years. Seed material is collected from high-quality cobs that are not affected by diseases.
Helminthosporiosis
This disease occurs not only on corn, but also on sorghum or Sudan grass. In most cases, this disease affects the leaves of the crop, less often the roots or cobs.
Description and symptoms
The first symptoms of the disease in question are the appearance of brown oblong spots on the leaves of corn. After some time, the center of the affected areas dries out, and a brown border forms on their edges. At the first stage of the lesion, the spots are small, then they increase in size and can completely cover the leaf blade.
Spindle-shaped olive spores of the fungus first infect the lower leaves of the crop, then move to the upper tiers. Helminthosporiasis usually appears in mid-summer, and especially at low temperatures combined with high air humidity. The fungus mainly affects the leaves, causing them to dry out. The main source of infection is considered to be untreated seeds and organic residues left after harvesting.
Struggle
The main methods of control are careful calibration, cleaning and sorting, as well as dressing of seed material. Plant immunity to this disease can be increased by applying complex mineral fertilizers and foliar fertilizers. It is advisable to feed corn with microelements with the addition of urea during the period of tillering or booting.
Prevention
The main preventive action is to observe crop rotation and remove organic residues. Deep plowing of the soil, as well as the use of resistant hybrids and varieties, will help prevent the disease.
Dry rot
Signs of the disease can be identified by the affected ears and stems.
Description and symptoms
The first symptoms of the disease in question are the appearance of a cotton-like coating near the base of the cobs. The fungus moves between the grains, causing their surface to turn brown and the grain itself to crumble. If you look closely, you can see black dots on the cobs; these are fungal pycnidia with a diameter of up to 500 microns. Subsequently, spores form in these places.
Dry rot mycelium is usually located in the lower internodes. On the affected areas of the plant, brown spots with microscopic black inclusions are clearly visible. Damaged stems lose stability and can be damaged by sudden gusts of wind. Diplodia also appears on the leaves of the crop in the form of brown spots. The described disease can affect corn at all stages of the growing season, and especially at the stage of milky or waxy maturity. The fungus attacks grain during storage in a warehouse.
The infection enters the plant from infected organic matter, although in some cases the infection occurs through untreated seed material. Intensive development of the disease is possible in humid and hot weather, which is often observed in the southern regions of our country from August to September.
Struggle
Selection of high-quality grain and processing of seed material before sowing will help prevent the appearance of dry rot on corn bushes. It is also necessary to apply complex mineral fertilizers. To prevent the appearance of fungus in grain, it is necessary to harvest the crop in a timely manner and dry the seeds to the required moisture content.
Prevention
Preventive actions are aimed at maintaining crop rotation. When this disease appears, corn is returned to its original place only after four years. Autumn plowing of fields with careful sealing of crop stumps is also necessary. Due to the fact that the pathogen remains in organic residues, the affected parts of the crop must be burned or embedded in the soil.
Rust
The intermediate host of the disease in question is wood sorrel, so such a plant should not be located near corn plantings. Rust mainly spreads from uredospores that are found on overwintered plant remains.
Description and symptoms
At the initial stage of the infection, small red spots (up to 2 millimeters) form on the leaves of the crop. For some time, these areas are covered by a layer of epidermis, then the upper cells of the corn leaf rupture, making room for the spores to move. The fungus can infect crops at all stages of the growing season.
Primary damage occurs through teleitospores, which form on the lower and upper parts of the leaves. With intensive development of the disease, numerous dark stripes appear on the leaves of corn. Corn rust is common in Ukraine, the southern part of the Russian Federation, and also in Transcaucasia.
Struggle
To prevent the occurrence of the disease in question, it is necessary to carefully destroy plant remains of the crop and follow the rules of crop rotation. Treating the seed material, as well as using rust-resistant hybrids and varieties, has a good effect.
Sclerotinia or white rot
The causative agent of this disease causes harm not only to corn, but also to other agricultural crops.
Description and symptoms
Sclerotinia affects the cobs, as well as the lower tiers of the stem. The first symptoms of white rot are identified by brown spots in the lower tier of the plant. Usually the affected areas are covered with a white coating, similar to cotton wool. With intensive development, sclerotinia covers a significant part of the corn stalk, which subsequently leads to cell death. During the initial lesion, dark sclerotia of different diameters appear on the roots of the plant.
The disease most often appears in late summer or during grain storage. Intensive development of the disease is observed at high air humidity, and especially in fields where sunflower was grown before corn. The affected remnants of predecessors are considered to be carriers of infection.
Struggle
Thorough cleaning and drying of seed material to the required conditions. Application of complex mineral fertilizer before sowing, compliance with crop rotation. Constant fight against weeds. Destruction of organic residues, deep plowing of the field.
Red bacteriosis
This disease affects not only corn, but also sudangrass and sorghum.
Description and symptoms
The causative agent of the infection is a fungus that affects the ear membranes, plant stems and leaves. The first symptoms of this disease can be identified by the formation of red spots and stripes on the surface of the leaves. After some time, the specks merge into a single whole, which is why the surface of the sheet becomes like marble. Pathogenic bacteria accumulate in the lower part of the leaf blades, which later lead to leaf drying out.
Foci of the spread of red bacteriosis are observed in the southern part of our country. The disease develops rapidly in persistent, humid and warm weather. The infection can persist on seed coats and plant remains, as well as on weeds.
Struggle
Use of resistant varieties and high-quality treated seeds. Plowing up post-harvest residues, maintaining crop rotation, and constantly fighting weeds.
Corn leaves are affected by many diseases, but only a few cause serious harm.
Most of these diseases cause characteristic stripes, spots or swellings on the leaves. On dead areas of such lesions, spores of the causative fungus develop. The source of primary inoculum is usually crop residues. Once leaves are infected, secondary infection through conidia may occur. Leaf tissue becomes damaged or dies and the amount of green chlorophyll decreases. Severe infestation reduces plant vigor and yield, weakens stems and causes the formation of lightweight, filmy grains. Some leaf diseases can attack seedlings and kill plants at a very early age. In this case, pathogens penetrate into all parts
plants and infects it entirely (systemic infection). Later infections usually affect only the leaves. Hereditary factors, unfavorable environmental conditions and unbalanced application of mineral fertilizers can cause similar symptoms in plants.
Northern helminthosporiosis of corn leaves. (Helminthosporiurn turcicum) (Trichometa spaeria turcica).
The fungus Helminthosporiurn turcicum is widespread throughout the world and affects all forms of corn. It causes the most severe damage in humid areas where heavy dew, heavy rainfall and warm summer weather are common. Susceptible plants become infected as they approach maturity. Losses from this disease can range from barely noticeable to 50% of the grain yield, depending on the duration and extent of the damage. Diseased leaves lose some of their nutritional value and many die prematurely. Early maturing corn lines are usually more susceptible than late maturing ones.
The causative agent of northern helminthosporium leaf blight (H. turcicum) causes long (2.5 to 15 cm) elliptical, grayish-green or reddish-brown lesions on leaves. Lesions appear first on the lower leaves and may increase in number and size until almost all living tissue is affected. Heavily infected plants resemble frost-damaged plants in appearance. The cobs are not affected by the disease, and therefore the possibility of disease transmission by seeds is unlikely.
To combat the disease, resistant hybrids should be sown. Seed treatment and crop rotation help little in the fight against it. Pollination or spraying with fungicides is too expensive.
Polyfactorial resistance can impart very low to high resistance to plants. The stability of hybrids is usually proportional to the number and degree of stability of the inbred lines included in their pedigree. Breeders have developed resistant inbred lines that can be used to produce partially resistant hybrids.
Allstrup, Robert, and others, using backcrosses, selfing, and selection, transferred the polyfactorial resistance of inbred lines from the southern United States to susceptible inbred lines in the Corn Belt.
Resistance to infection does not become clear until plants are 6-8 weeks old. Therefore, the resistance of inbred lines cannot be assessed during the germination phase, just as it is impossible to confidently select resistant plants from splitting generations in a greenhouse. This work should be carried out in the field under conditions of strong disease development. Since in F 2 highly resistant plants usually make up less than 1% and resistance is determined by many genes, relatively large segregating populations are required to implement a breeding program. Jenkins, Robert, and Findlay showed that periodic selection was an effective method for concentrating resistance genes to leaf blight caused by H. turcicum.
Jenkins and Robert found that in the reaction with R4, the degree of dominance of polyfactorial resistance of inbred lines NC34 and Mo21A was the highest, L97 - average, and in lines T528 and C. I.23 - the least. Inbred lines NC34 and Mo21A contained the main gene determining resistance. The responses of the three susceptible inbred lines R4, 38-11 and K64 to the resistant inbred line NC34 differed significantly. It was established that the inbred line contains the main recessive gene that determines extreme susceptibility.
Hughes and Hooker described the nature of the action of the gene that determines the quantitative type of resistance of corn to northern helminthosporium blight caused by H. turcicum. By analyzing generational averages, additive, dominant and epistatic effects of the gene were discovered. In all crosses, the additive effect of the gene was most important. Non-additive effects were generally weak and varied depending on the study population and study year. The resistance of these lines to helminthosporium was determined by a relatively small number of genes that have a predominantly additive effect. Breeding for resistance to this disease must be effectively accomplished by periodic phenotypic selection or mass selection.
The discovery of single-gene resistance to northern helminthosporiosis facilitates selection for resistance. This type of resistance is more complete than polyfactorial, and resistant plants are easy to identify. To transfer this type of resistance to inbred lines in the Corn Belt, simpler breeding methods are required, since it is determined by the action of a single dominant gene. The work can be carried out on small populations.
The effect of gene dosage on single-gene controlled resistance to northern helminthosporiosis was studied by Dunn and Namm. They assessed the effect of different ploidy levels on resistance characterized by chlorotic lesions. Diploid, triploid and tetraploid seedlings containing two, three and four doses of the ht allele, respectively, did not differ in the degree of susceptibility. Monoploid (ht) seedlings were significantly more susceptible than seedlings at the other three ploidy levels. The increased susceptibility of monoploid ht seedlings, which are generally less vigorous than seedlings of other genotypes, may result from interactions between other deleterious recessive genes and the ht locus.
Most researchers divide the relative resistance of plants into six classes: class 0 means immunity; class 0.5 - very mild damage and class 5.0 - extreme susceptibility. Three other classes fall somewhere between these extremes.
Southern helminthosporiosis of corn leaves. (Helminthosporium maydis) (Cochliobolus heterostrophus).
The causative agent of southern helminthosporium leaf blight is the fungus Helminthosporium maydis. The disease occurs wherever corn is grown in warm, humid conditions. Apparently, the disease develops especially strongly at slightly higher temperatures than those that favor the development of northern helminthosporiasis.
Typical lesions with southern helminthosporiasis reach a size of 0.6X1.2-1.9 cm. They have an oblong shape with parallel sides and are colored reddish or straw-yellow. The lesions are smaller and lighter in color than with northern helminthosporium. The disease causes the death of green leaf tissue. A decrease in effectively functioning leaf surface area can lead to reduced grain yield, weakened stems and poor forage quality. Severely affected fields have a scorched appearance.
This epiphytoty was caused by a new race T of the fungus. All inbred lines and hybrids possessing Texas-type CMS are susceptible to this race to varying degrees. Inbred lines and hybrids with normal cytoplasm and non-Texas type CMS are resistant. Many states now require that when corn seed is sold, the label must state whether it was obtained from plants with normal cytoplasm (N), from plants with Texas-type CMS (T), or from a mixture (B) of seeds obtained from plants with normal cytoplasm and from plants with Texas type CMS; the mixture must indicate the percentage of each type of seed it contains.
Corn is affected by two races of N. maydis: T and O. In addition, other races may exist or appear in the future. Race T produces a specific pathotoxin that damages the cytoplasm of susceptible plants. The race is highly pathogenic, specific for plants with Texas-type CMS. It affects the leaf, leaf sheath, stipe, involucres and cobs. The old common O race does not produce a specific pathotoxin and attacks mainly leaves. Both races differ in one gene that controls toxin formation and pathogenicity.
Resistance to race O is based on nuclear genes and in most sources is manifested quantitatively and is inherited polygenically. One source, characterized by the inheritance of a single recessive gene, manifests itself in the form of chlorotic lesions and weakened sporulation of the fungus. Resistance to race T is determined by the cytoplasm and nucleus. Normal (non-male sterile) and many male sterile cytoplasms, such as the CMS-C and CMS-S types, give corn high resistance in the field. Resistance to race T in forms with CMS-T is partial and is probably determined by the same core genes that confer resistance to race O. Resistance to H. maydis is manifested in the number and size of lesions, their type, spores of the fungus in lesions, resistance to the action of pathotoxin, the period of death of the cob, the period of death of the plant and yield losses.
Diversity of cytoplasms in hybrids and selection for high levels of resistance determined by nuclear genes against the background of resistance to stem rot are indicated as goals of breeding and seed production. This can be achieved through selection by phenotype, periodic selection or backcrosses.
The best way to combat southern leaf helminthosporiosis is the breeding and use of resistant hybrids. Crop rotations or seed treatments provide little benefit.
Southern leaf spot. (Helminthosporiurn carbonum) (Cochliobolis carbonum).
Southern leaf spot is caused by the fungus H. carbonum. Three known physiological races of the fungus produce different symptoms, but none of them is of major economic importance. The spores of all three races are similar in size, shape and color. The disease, caused by race I, eliminates susceptible inbred lines. However, most inbred lines are resistant to this disease. The development of the fungus is favored by humid environmental conditions.
Allstrup pointed out that with a disease caused by race I, reddish-brown lesions measuring 1.2 X 2.5 cm appear on the leaves. The lesions are oval or round in shape; Concentric zones are observed within the affected area. Leaves of susceptible inbred lines are completely covered with lesions. The disease also affects the cobs, giving the grains a characteristic charred appearance. Race II of the fungus causes the appearance of oblong chocolate-colored spots up to 0.5X2.5 cm in size.
Susceptibility to infection by race I H. carbonum is controlled by a single recessive gene, hm, which is localized on chromosome I. The recessive nature of the gene determines the secondary economic importance of this disease. When a susceptible inbred line is crossed with a resistant one, resistant hybrids are obtained. The genes are arranged in the following order: P-hm-br-fi. The hm gene lies 33 units to the right of the P locus and 14 units to the left of the Lr locus. It appears that one or more modifier genes in some inbred lines suppress full expression of susceptibility. The inheritance of susceptibility to infection by race II H. carbonum is probably determined by many genes.
Southern leaf spot can be controlled using resistant inbred lines. Since only one gene determines susceptibility, and since backcrossing is expected to result in a 1:1 ratio of resistant to susceptible lines, only 10 or 12 plants are required to effectively select for resistant types in a backcross generation. Inbred lines Pr, Mo21A, K61, K44 and NC37 are susceptible to race I.
Allstrup gave two examples of selection for resistance to this disease. Inbred lines Mo21A and Pr are susceptible. Outcrossing of these lines with stable inbred lines and backcrosses in the 5th-7th generation make it possible to select stable restored forms that are in all respects similar to the original inbred lines, but are resistant.
Corn leaf spot, caused by Helminthosporium rostratum. Lesions appear on adult plants in the form of small, light yellow, elongated spots measuring 1-2X2-5 mm. Mature lesions are straw-yellow in color with light brown edges and reach a size of 2-3X2-40 mm. The development cycle of the disease is similar to the development cycle of northern and southern helminthosporiasis. Important factors determining the development of the disease are temperature and relative humidity. The main control measure is the use of resistant varieties.
A new form of helminthosporium leaf blight. Hooker et al report a new corn leaf disease caused by Helminthosporium spp. that is widespread in the northern US Corn Belt.
The disease mainly affects the leaf blade. The size and shape of the lesions are variable, but they are usually oval in shape and reach 4-6 mm in width and 12-15 mm in length. Inbred lines used for grain production vary in susceptibility to this disease.
Yellow leaf spot, or phyllosticosis(Phyllosticta maydis).
Yellow leaf spot, or phyllostictosis, appears to be a recent development. If plant infestation occurs early and environmental conditions favorable to its continued development are maintained, the ears become lightweight, filmy and resemble the ears from plants affected by northern helminthosporium leaf blight. At the same time, stem rot may also increase.
Yellow spot lesions are oval or elliptical in shape and tan, cream, or yellow in color. Sometimes each spot is surrounded by a clearly defined yellow border. The size and shape of the spots are very variable, their size on average is 0.3X1.3 cm. Older leaves are infected first, which later turn brown and die. The fungal spores formed on these leaves are spread by splashing rain and by physical contact with the upper leaves. Leaf sheaths and outer involucres are also susceptible to the disease. Sometimes on patches of dead tissue you can find pycnidia of the fungus, which look like very small black specks. These specks make it possible to distinguish phyllosticosis from southern helminthosporium leaf blight.
The causative agent of the disease is the fungus Phyllosticta maydis. It overwinters on corn or on the remains of weeds and next year can infect corn at any stage of its development. Young corn plants that grow through a thick layer of plant debris in rainy weather are especially susceptible to phyllosticosis.
The best way to control is to develop resistant varieties. Unfortunately. Texas-type CMS confers susceptibility to inbred lines and hybrids, although these same lines with normal cytoplasm are more or less resistant. The response of hybrids to the disease varies, but they are usually less resistant than their parental lines. In areas where the disease is currently endemic, seeds of hardier varieties obtained without the participation of Texas-type CMS should be preferred for sowing (if available).
Early spring infestations are reduced by careful tillage or crop rotation. In contaminated areas, these activities are especially recommended in seed plots. Correct application of fertilizers and drainage of the soil is very important, since stress factors dramatically increase the disease. Fungicides can also be used to control this disease, although they are usually used only in seed production.
Cabatiella blight. (Kabatiella zeae).
This disease was first discovered in 1968, when its epiphytoty broke out in several areas of the northern corn belt. During this exceptionally wet summer, the plants were severely damaged in August and died by early September. This led to significant yield losses and a decrease in the quality of corn. Since then, this disease has been observed sporadically or at the very end of the growing season, but has caused little or no harm.
Numerous small, oval or round spots ranging from 1 to 4 mm in diameter appear on infected leaves. The center of each type of spot is tan in color and may be surrounded by a purple, brown, or watery border. Each spot is surrounded by a yellow rim, which shines through when the leaf is viewed against the light; this feature serves as a very useful diagnostic symptom. Plants can be infected from the seedling stage to maturity, but older plants appear to be most susceptible. In cold and damp weather, the infection can cover the entire field fairly evenly, affecting both the lower and upper leaves equally. Foci of infection sometimes appear on the field even in dry weather.
The causative agent of the disease is the fungus Kabatiella zeae. It overwinters in corn residue. Under appropriate conditions of humidity and temperature, the spores germinate and penetrate the plant through the surface of the leaves, on which lesions appear after 7-10 days. After this, secondary spread of the disease is possible. The spores are easily carried by the wind, and therefore cabatiella blight is not limited to areas of permanent corn crop.
Resistant lines of corn are available. Apparently, agricultural practices have little effect on the disease, since the spores of the plant are easily transported by air currents.
Brown spot, or fisodermoz(Physoderma maydis).
The fungus Physoderma maydis causes a disease of corn and teosinte known as brown spot or physdermosis. It is most important in warm and damp areas where corn is grown. Very small, oval or round, yellow pittings appear on the tissues of the leaves and stems. Watery lesions coalesce, forming reddish or chocolate brown spots. Affected stems often break off.
For the disease to develop, high temperature and abundant moisture are required. The fight against brown spot involves carrying out phytosanitary measures, observing crop rotations and using resistant hybrids. Resistance to this disease is likely polygenic.
Kress and Thompson describe a method for early assessment of brown spot resistance in corn. Broyles found that severe disease damage affects yield and stem lodging.
Moll et al., Thompson et al., and Thompson reported yield losses and described the quantitative pattern of inheritance of resistance to Physoderma maydis. These researchers showed that heritable variation in response to brown spot is additive in nature. Thompson et al provide evidence for dominance but not epistasis. Moll et al. did not establish dominance but reported epistasis in some populations. Thompson tested the additive, dominant, and epistatic effects of genes by analyzing generational averages in 15 populations. The most important role was played by the cumulative additive effect. The combined effect of dominance was statistically significant in five populations, and epistasis in four populations.
Anthracnose(Colletotrichum graminicolum). Anthracnose is of economic importance in France, Germany, India and the Philippines, but may also pose a threat to US corn production in the future. Symptoms of the disease on the leaves are watery spots of round or irregular shape. Sometimes severe rot of roots and stems develops. The fight lies in the use of resistant varieties, the use of crop rotation, phytosanitary measures and balanced soil fertility.
Phaeospherellosis leaf spot(Phaeosphaeria maydis). Spot, caused by the fungus Phaeosphaeria, initially causes small, light green or chlorotic lesions to appear on the leaves. Heavy rains favor the development of the disease. The disease can be found in India, areas adjacent to the Eastern and Central Himalayas, and Brazil.
Curvular leaf spot(Curvularia spp.). Leaf spots caused by the fungus Curvularia were described by Schutleff et al.
Fusarium leaf spot (Fusarium moniliforme). Fusarium leaf spot is especially harmful in dry, warm areas near the Caribbean Sea. Watery spots appear first in the axils of the leaves.
Cercospora leaf blight(Cercospora zeae-maydis). The Cercospora fungus causes pale brown stripes to appear on mature leaves. The disease is found in the United States, Europe, Africa, Southeast Asia, India, China, South America and the Philippines.
Bacterial wilt, or Stewart's disease. (Erwinia stewartii).
Erwinia stewartii (Bacterium stewartii) is the causative agent of the most dangerous bacterial leaf disease of corn. It goes by several names: Stewart's disease, Stewart's wilt, Stewart's leaf disease, and bacterial wilt. The disease is particularly hard on sweet corn, as well as susceptible varieties of dent, flint, starchy and popping corn. The combination of bacterial wilt and dental helminthosporiosis in sweet corn is very important for corn growers.
Before the development of resistant hybrids, wilt destroyed large areas of sweet corn in the United States and was the main reason for the movement of canning varieties to the northern regions of the country. Bacterial wilt has been reported from many countries.
The causative agent of bacterial wilt is the bacterium Erwinia stewartii. Infected sweet corn plants wilt quickly. Narrow and long light green or yellow stripes appear on the leaves, which soon dry out and turn brown. The yield is reduced and the susceptibility of plants to stem rot increases.
The bacteria overwinter in the body of corn flea beetles. In the spring after wintering, when the first small, black, oval-shaped beetles appear in the fields, they feed on young corn plants, and through the wounds they cause, bacteria penetrate the plants, infecting primarily the leaves. During the growing season, beetles continue to spread the disease from infected plants to healthy ones, often migrating from south to north. The prevalence of bacterial wilt varies from year to year depending on the number of overwintered flea beetles. During mild winters, many beetles usually survive and contribute to the infection of corn, which continue to spread the disease throughout the growing season. In cold winters, the number of beetles decreases, so in early spring the infestation is insignificant and the disease does not spread over large areas.
To combat wilting, it is best to grow resistant hybrids. The first sustainable sweet corn hybrid was Golden Cross Bantam. There are now many other varieties of sweet corn that combine resistance to bacterial wilt, high yield and good quality. But at the one to three leaf stage, even resistant forms of sweet corn are susceptible to this disease. Early-maturing, short-growing forms of sweet and dent corn appear to be more susceptible than late-maturing, tall-growing inbred lines.
In dent corn, there may be a positive correlation between resistance to late leaf stage disease and resistance to northern corn leaf blight.
Spraying or dusting seedlings with an insecticide such as carbaryl (Sevin) to kill corn flea beetles will partially control the spread of the disease. Adequate potassium levels in the soil can reduce disease development, while high nitrogen levels promote bacterial wilt infection in corn. Treating seeds and spraying plants with fungicides do not give positive results in the fight against this disease.
Wellhausen discovered three dominant genes for resistance to bacterial wilt - Sw, Sw 2 and Sw 3. These genes are inherited independently of each other and behave as additional genes (modifiers). One of these genes was linked to the P gene, which determines the color of the cob. Artificial infection helps select stable inbred lines. According to the degree of resistance, plants can be divided into six classes. Class 0 indicates no infection, Class 1 indicates traces of infection and the presence of a few small lesions, and Class 5 indicates premature death of plants due to disease. Other classes define intermediate stages of the disease.
Bacterial stem rot and leaf spot(Pseudomonas alboprecipitans). The disease is found in parts of the Midwestern United States. Apparently, its development is favored by hot (29-35°C) weather with heavy rains. The economic damage from this disease is small. The disease can be localized in certain areas of the field. The nature of the lesions ranges from small elliptical spots to narrow stripes almost the entire length of the leaf. They often merge, occupying most of the leaf in width. At first they are olive-green in color and have an oily or watery appearance, and later they dry out and take on a reddish-brown color, sometimes with reddish-brown edges. Severely affected leaves are easily split, especially under the influence of wind and heavy slanting rains.
Stem rot usually develops above the attachment point of the cob, causing browning of the affected tissue and splitting of the core. As the rot progresses, the tops of the plants die. When infected early, the plants become dwarf and often produce many sterile ears, which subsequently rot. The causative agent of the disease is the bacterium Pseudomonas alboprecipitans. Control measures have not been developed. Sweet corn appears to be slightly more susceptible than corn. Popping corn is most susceptible.
Bacterial spot(Pseudomanas syringae). It usually affects the lower leaves of corn. Watery lesions are initially dark green in color, but may later dry out and become creamy white or tan in color with reddish or brown spots. They have an elliptical shape, and their size ranges from very small specks to spots reaching 1.3 cm in diameter. The disease is caused by the bacterium Pseudomonas syringae. Wet weather favors the development of the disease.
Bacterial wilt and leaf spot of corn in Nebraska(Corynebacterium nebraskense). The lesions appear as watery streaks parallel to the veins on the leaves of seedlings or older plants. The pathogen overwinters on infected leaves and stems remaining on the soil surface.
Control measures include deep plowing of infected crop residues, using crop rotation, sowing with healthy, uninfected seeds, and using resistant varieties.
Bacterial stripe spot(Pseudomonas andropogoni). P. andropogoni causes oily, clear, amber or olive green lesions with parallel sides to appear on the lower leaves. Long periods of warm and damp weather favor the development of the disease. The use of resistant varieties is recommended.
Bacterial stem rot(Erwinia dissolvens). A reliable symptom of bacterial rot is flattened and twisted stems. They often emit an unpleasant odor. Bacterial stem rot is rare and its distribution is limited. Sometimes rot is caused by other types of bacteria.
Brown (common) rust(Puccinia sorghi). Brown rust of corn is caused by the fungus Puccinia sorghi. In the United States this disease is usually of little importance, but in some other countries it can cause significant leaf mortality. An infestation that has reached a significant extent by the time threads appear is likely to cause grain yield losses.
Brown rust produces small, oval or elongated pustules that are initially golden brown or light brown but turn brownish-black as the crop matures. Pustules can appear on any above-ground part of the plant, but are especially numerous on the leaves, where they are scattered on both surfaces.
Moderate temperatures and high relative humidity with lush development of succulent plants of a susceptible host favor the development of the fungus. Inbred lines vary in their degree of resistance to leaf rust.
Southern rust(Puccinia polysora). Southern corn rust is caused by the fungus Puccinia polysora. It is difficult to distinguish P. sorghi from P. polysora based on symptoms. Heavy dew and high temperatures increase infection and spread of the disease.
Southern corn rust is common in southern and central North America, Mexico, Africa, Asia, Australia, Central and South America. In these areas, rust diseases develop very strongly and can cause premature death of plants.
Tropical rust(Physopella zeae). Tropical rust is limited to warm, humid areas of Mexico. Central and South America and the Caribbean islands. Pale yellow pustules develop on the upper surface of the leaves. The best method of control is the cultivation of resistant varieties.
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Corn smut.
Pathogen - Ustilago zeae (Beckm.) Unger. It affects the aboveground organs of the plant and young roots, but more often - the ears and stems, on which bubble-like swellings and nodules form. The affected parts of the plant are deformed. At first, the swellings are small and have a greenish or pinkish-white color. Gradually they reach enormous sizes, increasing in diameter up to 20 cm, and darken. Ripe swellings burst, releasing a mass of chlamydospores. The spores of bladder smut are spherical, yellow-brown, with a reticulate pattern, 8-13 microns in diameter. Infection occurs throughout the growing season (over the summer the fungus can produce 3-4 generations of spores). In the cob, individual grains located in its upper part are usually affected; in the panicle, individual flowers are affected. Small swellings in the form of rough wrinkles form on the leaves and shoots. As a result, the young plant dies.
The development of the disease is facilitated by low air humidity, uneven supply of water to plants, dense crops and late sowing of corn, as well as the application of excess nitrogen fertilizers. Under favorable conditions for the development of the disease, up to 40% of the cobs are completely transformed into a mass of powdery spores. Spores overwinter on post-harvest residues, in soil, on the surface of cobs and grains.
The disease is common in all corn growing areas.
Control measures.
Pre-treatment of seeds. Spatial isolation (at a distance of no closer than 1 km) of corn crops from the fields where it was grown in the previous year. Compliance with crop rotation with growing corn in the same place no earlier than two years later. Collecting and destroying swellings before they ripen and burying them in the soil to a depth of 40-50 cm. Selection of seeds only from healthy ears. Destruction of post-harvest residues and deep plowing of fields. Sowing corn with first generation hybrids that have increased resistance to smut.
corn
The causative agent is Sorosporium Reilianum Mc Alp. The fungus splits into two biological forms, confined to corn and sorghum. Only the cobs or panicles are affected. The cobs turn into a black lump, consisting of spores and remnants of vascular bundles, covered with short involucres.
Corn panicles are partially affected; in sorghum they turn into rounded swellings covered with a white, slightly pinkish shell. Sick plants are severely stunted in growth, bushy, sometimes do not produce ears at all, and are deformed. The spores are dark, spiny, 9-14 microns in diameter. Infection occurs during the period from the beginning of seed germination to the emergence of seedlings. Subsequently, the mycelium of the fungus spreads throughout the plant as it grows, so the first signs of the disease appear first on the cob, and then on the panicle.
The development of the disease is facilitated by the monoculture of corn and high (28-30°) air and soil temperatures during the period of seed germination. Sources of infection are seeds and soil.
The disease is widespread in the North Caucasus, Transcaucasia, Kuban, Moldova and the republics of Central Asia.
Control measures are the same as for corn smut.
corn
The causative agent is Helminthosporium turcicum Pass. In addition to corn, it affects Sudan grass and sorghum. Mostly the leaves become diseased, less often the cob wrappers, as well as the cobs themselves and the root collar. Elongated brown spots with a dark reddish-brown border appear on the leaves. Later, in the center of the stain, the tissue dries out, acquiring a light color; a wide brown border remains at the edges. The spots are small at first, then they grow up to 15 cm in length and 3-4 cm in width, merge and cover almost the entire surface of the leaf. In humid weather, they sometimes develop a grayish or dark olive coating. Spores (50-110x11-24 microns) fusiform, olive, with 3-12 septa and a thickened shell. Initially, the leaves of the lower tiers are affected, but gradually all leaves can become diseased. Severely affected leaves dry out. The diseased root collar of the plant withers. The disease appears in the second half of summer. Its development is favored by heavy rainfall and low air temperatures. The main source of the disease is post-harvest residues, possibly seeds.
The zone of constant spread of the disease is the Far East, Transcarpathia, Ukraine, Transcaucasia, and in some years Abkhazia, the environs of Sochi and Latvia.
Control measures.
Thorough cleaning and sorting of seeds. Etching. Early optimal sowing dates. The use of organic and complete mineral fertilizers, as well as foliar fertilizing with saltpeter and microelements (boron, manganese) during the tillering phase and plant emergence into the tube. Thorough cleaning of fields from post-harvest residues and early autumn plowing. Maintaining crop rotation. Introduction of disease-resistant varieties.
The causative agent is Diplodia zeae (Sw.) Lev. It affects all plant organs, most often the cobs and stems. A white cotton-like coating forms at the base of the cobs, which spreads between the grains, covering them and the inner wrappers. Affected grains become dull brown and crumble easily. In the initial stage of the disease, when there is still no mycelium on the surface of the cob, the presence of infection is indicated by its wrinkled tip. On the grains and cobs the fruiting bodies of the pathogen appear in the form of black dots of pycnidia measuring 300-500 microns. In pycnidia, stylospores of olive or dark brown color are formed in large numbers, usually with one septum. Stems become infected mainly in the lower internodes. Brown spots with numerous black dots are visible on the affected tissue near the stem nodes. A white mycelium forms inside the vagina near the nodes. Severely affected stems break. Elongated, irregularly shaped purple or brown spots appear on infected leaves and leaf sheaths. The disease manifests itself throughout the growing season, but corn is more susceptible in the stages of milky and milky-waxy ripeness. Cereals can also become infected during storage. The strong development of the disease is facilitated by high (25-30°) temperatures and frequent heavy rainfall in August-September. The source of the disease is contaminated plant remains and seeds. The disease occurs in the southern regions of the country.
Control measures.
Selection of healthy seed cobs. Pre-sowing seed treatment. Application of complete mineral fertilizer or phosphorus, phosphorus-potassium fertilizers before sowing. Timely harvesting and immediate drying of the cobs to the required moisture content. Thorough harvesting of plant residues, as well as stems, threshed cobs, autumn plowing with good sealing of stumps. Compliance with correct crop rotation with the return of corn to the original site no earlier than after 4 years, avoidance of monoculture.
Corn rust.
The causative agent is Puccinia maydis Veg. At the beginning of the disease, small oblong reddish-brown pustules (uredopusules) from one to several millimeters long are formed on the lower and upper sides of the leaves, covered for a long time by the epidermis. By
When pustules mature, the epidermis of the leaf ruptures, releasing a mass of single-celled reddish spores (21-35x22-29 microns), capable of causing repeated infection of plants during the growing season. Winter spores (teleitospores) are formed on both sides of the leaves. When the infection is severe, the teletopustules coalesce, forming dark stripes along the leaves. Teleitospores are two-celled, ellipsoidal, slightly compressed at the septa, on a long, non-falling stalk. The spore shell is dark brown, thickened at the apex.
The intermediate species is wood sorrel, but in most cases the renewal of rust is possible with the help of uredospores that have overwintered on plant debris.
The disease is widespread in Transcaucasia, on the Black Sea coast of the Caucasus, in Ukraine and in Transcarpathia.
Control measures.
Compliance with crop rotation, thorough cleaning of plant residues. In areas of severe rust severity, sowing of odontoid varieties of corn is the least affected. Seed dressing.
White rot (sclerotinia) corn
Infection of the cobs occurs in the field. The disease develops throughout the growing season, but more often in the second half of summer and during storage. The development of the disease is facilitated by increased air humidity and sowing corn in fields that have grown out of sunflowers. Sources of infection are post-harvest residues of affected plants.
The causative agent is Bacterium andropogonis E. F. Smith. In addition to corn, it affects sorghum and Sudan grass. The disease affects leaves, stems, and spike membranes. It appears in the form of well-defined spots or stripes, without signs of sporulation and fruiting of mushrooms. On the leaves, the spots are round, oval or elongated in the form of red stripes, or yellow-gray with a red-brown rim. Merging, the spots capture a significant part of the leaf blade, making the leaf appear marbled. On the underside of the leaf, scales of drying bacterial mucus are observed. Affected leaves dry out. The disease occurs in early summer. Its development is favored by warm, humid weather.
The infection persists on post-harvest residues, seeds, wild plants and weeds of the genus Sorghum.
Red bacteriosis occurs in the southern regions of the country.
Control measures.
Sowing with healthy seeds. Seed dressing. Control of weeds and wild Sorghum species. Destruction of post-harvest residues. High agrotechnical background.