Andrew Penney, Iowa State University PhD student in Plant Pathology, discusses the disease Physoderma brown spot.
Physoderma brown spot is caused by the fungus Physoderma maydis. Symptoms of Physoderma brown spot usually appear on mid-canopy leaves. Leaf lesions are numerous, very small (approximately ¼ 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 also may occur on the stalk, leaf sheath, and husks. Physoderma brown spot is more prevalent in wet growing seasons. Infection requires a combination of light, free water, and warm temperatures (75-85°F). Leaf infection occurs in the whorl when water is present for an extended time. Hybrids vary in their susceptibility to the disease, and crop rotation and tillage reduce survival of inoculum. Foliar fungicides labeled for Physoderma brown spot are available. https://cropprotectionnetwork.org/res...