We have begun to receive corn samples with stalk rot symptoms at the ISU Plant Disease Clinic. A few fields are beginning to display symptoms, but the majority of stalk rot problems will appear in September and October. 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 the black layer stage, 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 your 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, 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 other symptoms may be seen.
Decay of stalk base caused by Fusarium.
High levels of stalk rot susceptibility are a result of plant stress. Many types of stress can influence stalk rot. This year we may see stalk rot occurring in fields that are killed prematurely by frost. With the lateness of the crop, it is almost certain that some fields will be caught by frost. When leaves are killed, grain fill continues for a short time as carbohydrates move from the stalk tissue. The result is a weakened stalk that cannot resist stalk rot.
Corn plant killed by stalk rot.
Future stalk rot problems can be avoided by crop rotation, insect and weed control, adequate potassium fertilization, appropriate plant population and use of hybrids adapted to your area, 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 description of the symptoms and details on stalk rot prevention, see ISU Extension publication, Corn Stalk Rot in Iowa, IPM-50.
This article originally appeared on pages 160-161 of the IC-476(23) -- September 3, 1996 issue.