Plant diseases caused by fungi are numerous, including various types of root rot and leaf spot. Fungal diseases are usually caused by humid weather, poor drainage and roots sitting in moist damp soil; the cause of fungal infection in corn depends on the specific kind of fungus. Treating fungal diseases can be difficult, but there are measures that can prevent infection.
Main Fungal Disease
Corn grown in soil high in nitrogen is most susceptible to corn smut, Ustilago maydis, the most common fungal disease of sweet corn. Corn smut develops during hot, humid weather. It causes large swellings or galls on stems, leaves, ears and tassels. The galls are white or grayish white when they are young and turn brownish as they mature.
Root and Stalk Rot
The fungi diplodia, fusarium, pythium and macrophomina can cause root and stalk rots. These fungal diseases thrive in soil that is poorly drained, and when overcrowded, plants allow little air movement for drying. Some of these fungi enter the roots and move up into the stalk; others enter the stalk directly. Wounds caused by insects can allow fungi to enter. They can cause stalks to fall over and result in undeveloped ears.
Rust and Crazy Top
Rust on sweet corn, caused by the fungus Puccinia sorghi, is spread by red, wind-borne spores. It appears when temperatures are 60 to 70 degrees and a relative humidity is 98 to 100 percent. It causes raised, brick-red spots on the tops of leaves. Crazy top, caused by the fungus Scierophthora macrospora, causes deformed leaves and ultimately warped ears. It appears when seedlings four to six inches tall are flooded and allowed to sit for one or two days in waterlogged soil.
Infections of several species of fungi, including diplodia, fusarium, pythium and penicillium can cause seeds to rot and seedlings to die. Cool, wet soils delay germination, increasing the time that seeds are exposed to fungi. Soils colder than 55 degrees can cause the seeds to rot and die before they can germinate. In soils warmer than that, seedlings can emerge but have rotted roots and stems.
Prevention and Treatment
We know generally what causes fungal infections in sweet corn. Since there is little we can do about the onset of humid weather, preventing and treating fungi is more complicated.
No chemical controls are available for corn smut. Plant cultivars that are resistant to smut. Pick and destroy developing galls when they're young and have not yet released spores. Do this carefully; spores can easily blow to plants nearby. In the winter, corn smut stays on plant debris. Don't till infected debris into the soil. Do not plant your corn near wheat fields.
To avoid root and stalk rot, plant your sweet corn in soil that drains well. Don't crowd plants. Test your soil to keep it balanced. Follow recommended treatment for insect pests.
Fungicides containing azoxystrobin, chlorothalonil, mancozeb, maneb and propiconazole are registered for treating rust, according to Dr. M. K. Hausbeck of Michigan State University. Studies in Florida found that these fungicides should be rotated to be most effective.
To avoid seed rot, plant high-quality seeds that have been treated with protective fungicides. Treated seeds will be pink. Plant sweet corn in a raised bed after soil temperatures are above 55 degrees.