Heavy rain and ponding in Indiana cornfields this spring have increased the prevalence of seedling blights. Two minor diseases, crazy top and Physoderma brown spot, may also be problematic in areas where corn was underwater for 24-48 hours.
Seedling blights are prevalent when cool, wet soil conditions persist after planting. These conditions favor infection by many of the organisms that cause soil-borne diseases. Cool, wet soils also slow plant growth and development and give pathogens more time to infect and damage the seedling.
Seedling blights are caused by a variety of soil or seed-inhabiting fungi. Infected seeds may rot after germination, preventing emergence, and plants that emerge have reduced root development and are often stunted. Roots of infected plants may be brown and discolored and can be soft or mushy. Infected plants may also have brown discoloration on the mesocotyl.
Two of the most common seedling blights of corn are caused by Pythium and Fusarium species. Remember that to accurately determine the specific organism responsible for a suspected seedling blight issue, it is necessary to submit samples to a diagnostic lab such as the Purdue Plant and Pest Diagnostic Lab.
This video demonstrates how to sample fields to diagnose seedling blight and stand establishment issues.
The risk of corn seedling blight decreases when crops are planted into warm, dry soils. These conditions allow seedlings to germinate and emerge rapidly. However, it is often necessary to plant into less than ideal soil conditions, and fungicide seed treatments provide some protection against seedling blights.
Crazy top is caused by a fungal-like organism called Sclerophthora macrospora. This pathogen survives in soil and infects young corn plants when there is excess rain or ponding in the spring. Crazy top symptoms are most commonly observed at tasseling when distorted and malformed tassels appear in areas that were ponded or saturated (Fig. 1).
However, in some fields symptoms may be less diagnostic, and include stunting, tillering, thin, yellow leaves, and barren plants.
Physoderma brown spot is caused by the fungus Physoderma maydis, and also survives in soil and residue and infects corn plants when plants are ponded or excess water remains in the whorl. The symptoms typically appear in the late vegetative stages through pollination and are characterized by very small chocolate brown or yellow lesions on the leaves and midrib (Fig. 2).
The lesions may appear in a banded pattern. The lesions can also be found on the stalk, leaf sheath, or ear husks. When summer weather is conducive for disease development, premature lodging due to stalk breakage may occur.
Physoderma brown spot management, but symptoms are usually not severe enough to warrant preventative fungicide applications.