Physoderma brown spot is caused by Physoderma maydis, the only class of fungi that produce zoospores, spores that have a tail (flagellum), and swim free in water. P. maydis can survive in soil and crop debris for 2 to 7 years. The pathogen can be dispersed by the wind or splashed into the whorls of the developing corn.
Corn is most susceptible to infection between growth stages V5 to V9. Therefore, even though we are seeing symptoms now as corn is into R-stages, infection occurred in standing water in the whorl. Dark purplish to black oval spots along the midrib of the leaf and on the stalk, leaf sheath and husks are distinguishing characteristic symptoms of Physoderma brown spot.
In addition, infected leaves have numerous very small round or oval spots that are yellowish to brown and occur in bands across the leaf. Management options for Physoderma are limited, there are a few fungicides that are labeled for Physoderma control, but there is limited information if a fungicide application would be economical in Indiana.
Rotation and tillage both can help manage residue where the pathogen will survive year to year – most susceptible sites are those in no-till and continuous corn.