Within the past week, Iowa State University Extension plant disease specialists have received several reports of corn seedlings with symptoms of post emergence "damping off" which is a disease problem that sometimes can afflict young corn plants. The reports this spring came from a few fields in southern Iowa that were planted in early May.
Diseased corn seedlings received at the ISU Plant and Insect Diagnostic Clinic were stunted, compared to health seedlings from the same field (Figure 1), and the mesocotyl was rotted (Figure 2). Affected seedlings occurred in areas of the field where the soil was heavier and consequently wetter (Figure 3).
ISU Extension plant pathologist Alison Robertson, and her colleague at the ISU Plant Diagnostic Clinic at Ames, offer the following information and explanation.
Survival of young corn seedlings depends on a healthy kernel and mesocotyl which should remain firm and white through at least the corn growth stage of V6. Damage to the kernel or mesocotyl prior to establishment of the nodal root system can result in stunted, weak or dead seedlings. A developing corn seedling relies on the kernel endosperm for nourishment until the nodal root system has fully developed, usually around the 6-leaf stage. Thus, the mesocotyl acts as a "pipeline" for translocation of nutrients from the kernel and seminal roots to the seedling stalk and leaf tissues.
Can be several causes for rotted corn seedlings, not just disease
Rotted corn seedlings can be the result of pathogen infections, anhydrous ammonia injury, wireworms and cold injury. Seedling susceptibility to fungal infection increases the longer the seed sits in the ground, and the more stress germinating corn undergoes. Wet and cool (less than 55 degrees F) soil conditions predispose seedlings to infection by a number of fungi.
"We isolated Pythium and Fusarium from the diseased seedlings we received," says Robertson. "It is difficult to determine who got their first. We suspect Pythium, since the heavier areas of the field were more affected, and we have had periods of cool, very wet conditions this spring in Iowa. Cool, saturated soil conditions favor infection by this pathogen. There are several species of both Pythium and Fusarium that infect corn and soybeans. Recent work from Ohio has shown that sensitivity to fungicides used as seed treatments may vary within, and across species of Pythium and F. graminearum."
Now's the time to evaluate corn stands for signs of seedling disease
Dig up smaller corn seedlings and check for symptoms of seedling disease, advises Robertson. This will also give you an opportunity to evaluate the effectiveness of the seed treatment that was applied to the seed you planted. If you have significant seedling rot, you may have to replant. For replant decisions, please see Roger Elmore and Lori Abendroth's article on Assessing corn stands for replanting.