Description and Symptoms
Symptoms of Physoderma brown spot (Physoderma maydis) usually appear on mid-canopy leaves. Leaf lesions are numerous, very small (approximately 1/4 inch in diameter), round to oval, yellowish to brown in color and usually occur in broad bands across the leaf. Alternating bands of infected and noninfected tissues are common. Dark purplish to black oval spots also occur on the midrib of the leaf. Symptoms may also occur on the stalk, leaf sheath, and husks. Physoderma brown spot is more prevalent in wet growing seasons. Hybrids vary in their susceptibility to the disease. Crop rotation and tillage reduce survival of inoculum. Foliar fungicides labeled for Physoderma brown spot are available.
The best time to scout for physoderma brown spot is V12 through R1. Chocolate brown lesions in the midrib are diagnostic for this disease. Misdiagnosed as eyespot or southern rust. May be more prevalent in fields with infected corn residue.
Some hybrids appear to be more susceptible to infection by P. maydis than others. Moreover, the very wet conditions that occurred during June across most of the state likely increased risk of disease. Many fungicides are labeled for Physoderma brown spot management, but there is no publicly available data on application timing. Research is underway to determine how fungicides could be used to manage the disease.
Photo by Daren Mueller
Physoderma Brown Spot and Stalk Rot