Heavy rain and ponding in Indiana corn fields this spring have increased the prevalence of two minor diseases of corn, crazy top and Physoderma brown spot. These diseases are rarely yield limiting, but it is important to correctly identify these diseases to prevent unwarranted management.
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 (Fig.3). The lesions can also be found on the stalk, leaf sheath, or ear husks.
These diseases rarely need management and are usually only problematic in wet years. Improving soil drainage and removing infected plants will reduce the disease risk for subsequent crops. Fungicides are labeled for Physoderma brown spot management, but symptoms are usually not severe enough to warrant preventative fungicide applications.