Agriculture’s Weblog

 Sweet corn is seldom seriously damaged by diseases in the home garden. Reduce many of the potential disease problems by following proper cultural practices including:

• Using commercially grown certified and treated seed to improve stands.
• Planting varieties recommended for South Carolina (see HGIC 1308, Corn).
• Planting corn when the soil temperature is above 55 °F to reduce most seedling rots.
• Keeping the garden free of nearby weeds, which can harbor viruses.
• Removing smutted corn ears promptly from the stalks and garden area.
• Removing corn plant debris after harvest to reduce diseases caused by rusts and corn smut.

Remember, preventing corn diseases in the garden is very important, since the rapid growth of sweet corn makes it very difficult to spray adequately with chemicals. More information on how to grow corn is available in the fact sheet HGIC 1308, Corn.

This disease is easy to find in the garden. Large, fleshy, brownish galls (swellings) containing a black sooty mass of spores develop on leaves, stems, ears or tassels. Young immature galls are white or grayish white. A fungus, Ustilago maydis, causes this disease which occurs throughout the Southeast, especially when temperatures are high (79 to 100 °F) and moisture is abundant. Prevention and Treatment: Pick off and destroy infected ears and galls while they are immature and have not yet released spores. Remove galls carefully, since spores can readily blow to nearby plants, causing more disease. Corn smut overwinters on plant debris in the soil, so do not put infected crop residues back into the soil. The most susceptible plants are those grown in soils high in nitrogen. No chemical controls are available.

Rust occurs in most home gardens on corn but is seldom a serious problem. It is caused by various species of the fungus Puccinia and causes raised, brick red spots primarily on upper leaf surfaces. Plant corn early, and avoid letting plants get under stress from drought. Rust develops best when temperatures are cool (60 to 70 °F) and humid.

Poor stands of corn can result from various species of fungi, including Pythium, Fusarium, Diplodia and Penicillium, which affect seed germination and seedling development. In cold soils that have a temperature less than 55 °F seeds can decay and seedlings may die before they reach the soil surface. In cool, wet soils seed germination and seedling development slows down, increasing the length of time exposed to fungi in the soil. Low-quality seed also produces seedlings that are weak and survive poorly, especially in cold soils. In warmer soils, seedlings may emerge, but have rotted roots and stems at the ground line.Prevention and Treatment: Control of these problems is obtained by using high-quality seed that has been treated with protective fungicides. Seeds will be pink to indicate they have been treated. Sweet corn should be planted on a raised bed, after the soil temperature is above 55 °F.

In South Carolina there are two main viruses that are a problem on corn, Maize dwarf mosaic virus (MDMV) and Maize chlorotic dwarf virus (MCDV). Symptoms of these virus diseases can easily be mistaken for other growth problems such as nutrient deficiencies. The upper leaves of MDMV-infected plants show pale green blotches or mottling. MCDV-infected leaves often have a pale green streaking of the upper leaves, and often these leaves turn a reddish color. Both virus diseases generally cause plant stunting. The viruses survive the winter on many perennial grasses, especially Johnsongrass. In the spring, insects transmit the virus to young corn plants. Prevention and Treatment: Control grassy weeds near corn plantings (Johnsongrass). Corn planted at the appropriate time, and not too late in the season, is often damaged less by viruses. There is no chemical control available.

Root and stalk rots are among the most destructive corn diseases. They are caused by various species of fungi, including Fusarium, Diplodia, Pythium and Macrophomina. Infected corn stalks fall over (lodging) and losses result from unharvested ears and poor ear development on infected plants. Some of these disease-causing organisms enter through the roots and move up into the stalk, while others enter the stalk directly at the nodes. Insect damage can increase infection by wounding the plant and allowing fungi to enter. Prevention and Treatment: Plant in well-drained areas, because stalk rots are most severe in poorly drained soil and where poor air movement slows drying. Do not exceed recommended plant densities. Keep soil fertility balanced based on soil tests. All insect controls should be carefully followed as described in HGIC 2205, Insect Pests of Sweet Corn.

This disease is caused by the fungus Helminthosporium maydis. Symptoms occur as leaf spots or cob rots and are fairly easy to recognize on plants. Spots on the leaves are tan to light brown and occur first on the lower leaves of the plant. Spots often enlarge and increase in number and can cause severe leaf blighting. On the ear the fungus causes oblong, bleached spots which penetrate through the shuck layers and finally into the ear .Prevention and Treatment: Rotate corn with nongrass-type plants in the garden. Turn under corn debris soon after harvest to promote rapid decay and destroy overwintering fungi.

This disease causes death of seedlings and wilting of mature plants. Long rectangular streaks, up to an inch wide, appear on the leaves, which later turn brown and die. A brown cavity is often formed inside of the stalk at ground level. It is caused by a bacterium, Erwinia stewartii, which survives the winter in flea beetles. Healthy plants become infected when the beetles begin feeding on them. Prevention and Treatment: There is no effective chemical control available for this disease. Remove and destroy all infected plants immediately. Controlling the insects that spread the disease can reduce disease levels. More information is available in HGIC 2205, Insect Pests of Sweet Corn.

Nematodes are microscopic roundworms that live in the soil and can feed on corn roots. They can damage the root system to the point where it cannot properly absorb water and nutrients. Above-ground symptoms include stunting and nutrient deficiencies. Below the soil surface, affected roots appear stubby, swollen, and often have an absence of small feeder roots. Nematode damage can only be confirmed by having your soil analyzed. Samples can be submitted to your local county Extension office for determination of the type and degree of infestation.Prevention and Treatment: An effective nematode control program should include crop rotation, sanitation and solarization. More information about controlling nematodes in the home garden is available in HGIC 2216, Root-Knot Nematodes in the Vegetable Garden.Diseases and Insects of Corn in South Carolina, EC601.

 Soil solarization is a non-chemical method for controlling weed seeds, bacteria, fungi, and nematodes. As a non-chemical control method it fulfills many gardeners’ wishes for organic or low toxicity control. It is a potential alternative to chemical fumigation.

Soil solarization is usually only considered as a remedy for challenging problems in the home garden. This is because the treated area will be out of production for nearly 2 months during the summer growing season. The basic procedure is to cover the soil with a clear plastic and trap the sun’s heat to raise soil temperatures high enough to kill weed seeds and harmful plant disease organisms.

Solarization can be applied in many areas of the home landscape, but no plant material can be present in the area to be solarized. The extremely high soil temperatures will kill any plants left in the solarized area, and plant remains will interfere with even heating.

Vegetable garden plots and annual flowerbeds are the best areas for control. The larger the plot area the better control achieved. Treated areas greater than 30 inches wide seem to have the best results. Areas smaller than 30 inches have a cooling affect on the outer edges.

Soil solarization works well in South Carolina because of the high temperatures during the summer months. The warmer the climate during the growing season and the longer you have to leave the plastic in place, the better results you will obtain. Long periods of heating are necessary to achieve good control of hard-to-kill weeds like crabgrass and nutsedge.

Soil characteristics such as color, structure, and depth have an effect on the effectiveness of soil solarization. Dark colored soil tends to adsorb more solar radiation in comparison to light colored soils. Loose, friable soils enable heat to be transmitted throughout the soil and allow heat to go deeper. Control is much better in the top few inches of the soil and decreases with soil depth.