Some varieties of sweet corn are more resistant to disease than others. Stewart's bacterial wilt, smut, common rust and sorghum downy mildew are just a few of the fungus and bacteria that can affect quality and yield of a corn crop. Wet weather, planting schedules and monthly temperatures can all play a crucial role in sweet corn disease. Understanding and recognizing the diseases that affect sweet corn is the first step in ensuring a healthy crop.
Stewart's Bacterial Wilt
Stewart's disease of corn is referred to as Stewart's bacterial wilt in sweet corn, according to Cornell University Cooperative Extension. Stewart's bacterial wilt is a fungus caused by the bacterium Xanthomonas stewartii, which is carried by corn flea beetles that overwinter in the soil. The disease is transmitted when the beetles feed on the corn seedlings in the spring. Symptoms of the disease include foliage wilting, stunted plant growth and death. Yellow or brown streaks on the leaves may also be present. This disease can be mistaken for fungus leaf blights or injury from frost. Control measures include growing varieties that are tolerant to the disease and using insecticidal sprays to control the corn flea beetle population.
Smut is caused by the fungus Ustilago madis which overwinters in the soil. The disease develops when an opportune injury damages the integrity of the plant allowing smut spores to invade the plant. The smut spores are carried in the wind, so hot dry conditions promote the disease. Galls (abnormal growths) on the leaves, ears, stems or tassels characterize smut. They can be white or brown and full of spores. No real control measures exist for smut and no varieties that are completely disease tolerant when weather conditions are conducive for smut.
Common rust, caused by the Puccinia sorghi fungus, is exacerbated by extended planting schedules, which results in high concentrations of spores in the air from previous corn crops. The appearance of rust begins with cinnamon or brown colored pustules on leaves and sometimes ears or tassels. The pustules break open and spread red fungus spores, which once mature become black spores that overwinter in the fields. Temperatures of 61 to 70 degrees F and high humidity advance the disease, reports Penn State College of Agricultural Sciences. Control of the disease includes planting resistant varieties and use of fungicides containing mancozeb.
Sorghum Downy Mildew
Sorghum downy mildew, caused by the fungus Peronosclerospora sorghi, affects corn and sorghum. The fungus overwinters in plant debris and emerges in the spring. The fungus can become systemic in young plants and causes yellow streaks with white mold usually at the base of the leaves. Sorghum downy mildew thrives in warm humid conditions. Control of the disease involves using seeds that have been treated with a fungicide containing metalaxyl and rotating crops so the fungus is tilled under.