Physoderma brown spot (PBS) and physoderma stalk rot (PSR) are fungal diseases caused by the Physoderma maydis pathogen. PBS is not usually considered an economically important disease in the Midwest but PBS occurrence has increased over the last decade.1
Foliar symptoms of PBS include numerous small (about 1/4 inch in diameter) yellowish to brown, round to oval lesions that usually occur in broad bands on leaves (Figure 1 and 2).1 Dark purplish to black lesions can also appear on the leaf midrib, stalk, leaf sheath and husks (Figure 2). Lesions on the leaf midrib often coalesce to form larger dark blotches. Symptoms typically appear prior to tasseling. Foliar leaf symptoms appear similar to eyespot and southern rust (Figure 3), however those diseases do not have the dark, larger lesions on the leaf midrib.1
PBS leaf symptoms are often not visible on plants expressing physoderma stalk rot (PSR).1 Plants affected with PSR often appear healthy, however nodes are weakened and often the stalks snaps in two at the lower nodes. Stalk nodes where the breakage occurs are black and rotted (Figure 4).
The pathogen survives in overwintering structures called sporangia, which can be viable for 2 to 7 years in crop residue from infected plants.2 The sporangia are wind dispersed or splashed onto corn plants. Infection commonly occurs in the plant whorl where water tends to accumulate from rain and irrigation. Since the pathogen requires a combination of sunlight, water, and warm temperatures (75 to 85 °F)3 to germinate, infection in the whorl tend to develop lesions in bands across the leaf (Figure 2).
Management of PBS and PSR includes reduction of available inoculum with crop rotation or tillage although sporangia can be transported long distances and live for multiple years in the soil.1Planting tolerant products also reduces infestations. Products are being evaluated for their reactions to the stalk breakage symptoms.
Many fungicides are labeled for PBS, although there is limited efficacy or application timing data. Fungicide application is not recommended for PBS at this time. Further research is underway at public universities to evaluate effectiveness of fungicide application in preventing stalk breakage due to PSR.
Sources: 1 Robertson, A.E., Jesse, L., Munkvold, G., Salaau-Rojas, E., and Mueller, D.S. 2015. Physoderma brown spot and stalk rot of corn caused by Physoderma maydis in Iowa. Plant Health Brief BR-15-0003. Plant Health Progress. Vol. 16, No. 2. The American Phytopathological Society. https://www.plantmanagementnetwork.org/ 2 Robertson, A. 2015. Physoderma brown spot and stalk rot. Integrated Crop Management. Iowa State University. https://crops.extension.iastate.edu/cropnews/2015/07/physoderma-brown-spot-and-stalk-rot 3 Wise, K., Kennedy, B., Mehl, K., and Bradley, C.A. 2018. Physoderma brown spot. University of Kentucky. https://plantpathology.ca.uky.edu/ 4 Jackson, T. Physoderma brown spot. Crop Watch. University of Nebraska-Lincoln. http://cropwatch.unl.edu/plantdisease/corn/physoderma Web sources verified 05/10/18. 130819060802