A few corn fields are beginning to display stalk rot symptoms, but the majority of stalk rot problems will appear in September and October. In order to prevent lodging losses, it is wise to scout fields for stalk rot during the next few weeks. Fields that have problems can be identified and targeted for the earliest possible harvest.
Scouting should be done before black layer, about 40-50 days after pollination. While scouting for stalk rot, look for visible symptoms and test stalk firmness by pinching the lower internodes with thumb and forefinger. Healthy stalks are firm and can't be compressed. If a stalk can be compressed or feels soft, it is rotted and is a good candidate for lodging. Check at least 100 plants per field, in different locations. Different hybrids and fields with different tillage, rotation, or fertilization histories should be scouted separately. If a field has more than 10-15 percent of the stalks rotted, significant lodging is likely.
Prior to black layer, it is difficult to distinguish among the different stalk rot fungi. The most common in Iowa are Fusarium, Gibberella, and Colletotrichum (anthracnose). Any stalk rot can result in wilting and death of the plants. Leaves turn a gray-green color as they die, similar to frost damage. There may be dark external lesions at the lower nodes, and internally the stalk base will be dark brown and decayed. Later in the season, the stalk pith appears shredded and discoloration can become obvious in the pith and on the rind. Stalks infected with anthracnose will eventually become very discolored, and they may be showing initial stalk lesions now.
High levels of stalk rot susceptibility are a result of plant stress. Many types of stress can influence stalk rot, including inadequate soil moisture or excessive soil moisture. This year, areas of the state that experienced periods of low rainfall after pollination have the potential for stalk rot problems.
Future stalk rot problems can be avoided by crop rotation, insect and weed control, adequate potassium fertilization, appropriate plant population and adapted hybrids, avoidance of root and stalk injury, good drainage, proper irrigation (where applicable), and use of hybrids that have good stalk ratings and are resistant to foliar diseases. Hybrids with good stalk strength ratings suffer less lodging. For more descriptions of symptoms and details about prevention of stalk rot, see ISU Extension publication, Corn Stalk Rot in Iowa, IPM-50.
This article originally appeared on pages 164-165 of the IC-478(21) -- September 2, 1996 issue.