Within the past week, Iowa State University has had several reports of corn seedlings with symptoms of post emergent damping off 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 (PIDC) 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).
Survival of young corn seedlings depends on a healthy kernel and mesocotyl which should remain firm and white through at least growth stage 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 the “pipeline” for translocation of nutrients from the kernel and seminal roots to the seedling stalk and leaf tissues.
Rotted 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 F) soil conditions predispose seedlings to infection by a number of fungi.
We isolated Pythium and Fusarium from the diseased seedlings we received. 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. 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 is the time to evaluate corn stands
Dig up smaller seedlings and check for symptoms of seedling disease. This will also give you an opportunity to evaluate the effectiveness of the seed treatment that was applied to the seed 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”.