Brown Leaf Spot (fungus – Physoderma maydis): The fungus causing this disease occurs in most fields but seldom does economic damage. Infection requires high temperatures and presence of surface moisture. The first symptom of the disease is small circular spots. As they mature, they turn dark brown. Rotation and deep burial of stalks will help reduce losses to this fungus.
Charcoal Rot (fungus – Macrophomina phaseolina): Many plants are susceptible to this soil borne fungus and symptoms vary according to type. Infected stem tissue shows evidence of shredding with tiny black dots (sclerotia) between the remaining tissues. This gives those plant parts an ashy-gray appearance. Stalks such as corn or sorghum show a shredded appearance when split longitudinally. Charcoal rot occurs most consistently when plants are experiencing moisture stress due to drought. The fungus is widely distributed and builds up in soil when susceptible host plants are present and conditions favor its development. Rotation with unrelated crops help reduce the population of the fungus in the soil. Avoid moisture stress by increasing the moisture holding capacity of the soil and, if available, using irrigation when needed. Rotate with crops that are not seriously affected by this organism. Practices which hasten decomposition of crop residue may help decrease the population of the fungus in the soil.
Common Rust (fungus – Puccinia sorghi): Common rust occurs in most home gardens and commercial fields, but seldom causes economic losses. Infected leaves have raised spots or pustules formed primarily on the upper surface. The pustules are rectangular to oval, brick red, and may occur in bands on the leaf (See Photo). Spores are produced in the pustules, which are blown to neighboring leaves where infection can be repeated. Infection is encouraged by high humidity and cool temperatures (60oF to 70oF).
Common Smut (fungus – Ustilago maydis): Common smut is easily found in fields of sweet corn. The fungus overwinters as spores in the soil or in manure. They can survive 2 or 3 years. Spores may be windborne for long distances. Younger plants are more susceptible. Any above-ground part is susceptible. Ears are most commonly infected. Hail provides open wounds and greatly increases infection. Galls are formed as the common smut fungus causes cells of the corn plant to increase in size and number. These galls at first are covered with a thin white membrane. As the gall ages, the membranes break open to reveal a black powdery spore mass underneath (See Photo). The spores are blown to adjoining corn plants where infection in repeated. The smut fungus is favored by high temperatures and high moisture. Optimum spore germination occurs from 79 to 100oF. Little infection occurs below 61oF. Plants grown in soils high in nitrogen or plants damaged through cultivation are most susceptible to infection. Seed treatment is not effective and breeding for resistance has not been successful.
Corn Stunt (spiroplasma): Corn stunt occurs in a small percentage in most sweet corn fields. It seldom reaches levels high enough to cause economic loss. The corn stunt spiroplasma is transmitted by leafhoppers. Infected plants are stunted, young leaves are yellow in color, and with age they take on a reddish-purple color. Internodes are reduced in length and infected stalks are sterile. Control is not required due to the very low percentage of plants that normally show this symptom in the field.
Crazy Top Downy Mildew (fungus – Sclerophthora macrospora): This disease is a problem when fields become flooded early in the life of the plant. The fungus produces swimming spores which require water for mobility. Infected plants are sterile and have numerous shoots at the base of the stalk. Leaves are thickened, distorted, and a lighter green color than normal leaves. Tassels and ears develop green, leafy shoots. The fungus is commonly found in grasses along the edge of the field. Spores are washed into the field in flood water. Infection of the young corn plants takes place at this time. Varieties vary in their reaction to this fungus. Due to the low percentage of occurrence, little has been done to rate varieties for their reaction. Avoid fields that flood regularly and plant on a raised bed which will help reduce the exposure of young seedlings to standing or flowing water.
Downy Mildew (fungus – Peronosclerospora sorghi): Infected plants are chlorotic, stunted and have striped leaves. Infected leaves have a downy growth on the underside, toward the basal part. Potential infection is increased when the crop is grown in soil previously grown to infected sorghum, field corn, or sweet corn. Although high populations of spores are produced on the leaf surface, they are short-lived and require extended periods of high humidity for infection. Overwintering spores produced between leaf veins exist in the soil for long periods. Practices which hasten the breakdown of crop residue will help reduce the amount of inoculum carried over in the soil. Varieties vary in their reaction to this disease. Growers should consult their county Extension agent for current hybrids and their reaction to this disease.
High Plains Disease (Unknown): This disease is found on the Texas High Plains on corn and wheat. It is suspected to be a virus vectored by the wheat curl mite. Symptoms consist of yellow areas that eventually form yellow bands (See Photo). The earlier the infection the more destructive the disease. Early infected plants are stunted (See Photo). Ears will be reduced in size and partially filled (See Photo). The disease continues throughout the season. Hybrids will differ in susceptibility [see table below]. Suggested controls for sweet corn include planting early, using resistant hybrids, plowing volunteer wheat, and in general, staying away from grain such as wheat, barley, and rye. Weeds and native grasses that serve as hosts should also be avoided [Yellow Foxtail, Green Foxtail, Stink Grass, Crab Crass, Switch Grass].
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Maize Dwarf Mosaic (virus): Maize Dwarf Mosaic virus is the most common virus disease of sweet corn in Texas. Infected plants have mottled upper leaves that are lighter in color than healthy leaves. The mottled or mosaic pattern consists of alternate yellow and green islands in the leaf tissue. Aphids transmit virus particles from surrounding Johnsongrass. Johnsongrass rhizomes serve as the overwintering host for this virus. Early infected plants may be sterile. Late infection will reduce yields and quality of corn produced. Insect control is not successful due to the feeding pattern of the aphid. Elimination of Johnsongrass and isolation of sweet corn fields from Johnsongrass stands will help reduce the occurrence of this disease. There are a number of hybrids of sweet corn that are resistant [see table below].
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Plant Parasitic Nematodes: (See Root Knot and Other Nematodes sections)
Northern Corn Leaf Blight (fungus – Exserohilum turcicum): This disease is found in most sweet corn fields in wetter areas of Texas, but it is seldom severe enough to cause economic loss. Spots produced are larger than those caused by the southern corn leafspot fungus. Spots are from one to six inches long and one-half to one inch wide. With maturity, the center of the spot has a dark brown color, usually due to spore production. Infection occurs first on the lower or older foliage (See Photo). High humidity and temperatures between 60oF and 80oF favor disease development. Varieties vary in their reaction to the fungus.
Seed Rots and Seedling Disease (fungi – Pythium spp. Macrophomina phaseolina, Gibberella zeae, Penicillium oxalicum and others): Both seed rots and seedling disease can cause poor stands. In cold soils, seeds decay and seedlings may die before they break the soil surface. In warmer soils, they more commonly emerge, but will have rotted roots and stems at the ground line. Cool wet soils slow seed germination and development of young seedlings so that there is exposure to fungi for a longer period of time. Low quality seed also produce seedlings that are weak and survive poorly in cold wet soils. Control is obtained by using high quality seed which have been treated with protective fungicides. Sweet corn should be planted on a raised bed after the soil temperature is above 55oF.
Southern Corn Leaf Blight (fungus – Bipolaris maydis): The disease is easy to recognize under field conditions. Spots on the leaves are tan to brown in color. On the ear the fungus causes oblong, bleached spots which penetrate through the shuck layers and finally into the ear. The fungus overwinters in crop residue and produces spores which can be carried for long distances by wind.
Stalk Rot and Kernel Rot (fungus – Fusarium spp.): Fusarium fungi survive on corn residue in soil and on seed. Stalk rot is associated with moisture stress and over-fertilization. Cutting the stalk will reveal a pink discoloration of the pith. Infected plants are stunted and delayed in maturity (See Photo). During periods of high wind plants often lodge. The Gibberella stage of the fungus will infect kernels causing them to be pink in color. Infected ears have a strong odor and should not be used as food or feed. Control stalk rots by rotating with non-related crops, planting in well drained soils, and by using treated seed. Avoid excess nitrogen and avoid root pruning when cultivating and injecting fertilizer.