Physoderma Brown Spot and Physoderma Stalk Rot in Corn

Physoderma brown spot (PBS) and Physoderma stalk rot (PSR) are fungal diseases caused by the Physoderma maydis pathogen. Physoderma brown spot was not previously considered an economically important disease of corn in the Midwest. However, between 2020 and 2025, estimated yield losses across the US due to PBS have ranged from as low as 200,000 bushels in 2023 to as high as 3.5 million bushels in 2022, with an average of about 1.3 million per year during that time period.¹

Foliar Symptoms of Physoderma Brown Spot (PBS) in Corn

The 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 (Figures 1 and 2).² 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 symptoms of PBS appear similar to eyespot and southern rust (Figure 3); however, those diseases do not produce darker, larger lesions on the midrib of the leaf.²

Corn Stalk Breakage Due to Physoderma Stalk Rot (PSR)

Physoderma brown spot leaf symptoms often are not visible on plants expressing Physoderma stalk rot (PSR).^2,3 Plants infected with PSR may appear healthy even though their nodes are weakened, and often the stalks snap at one of the lower nodes. Where the breakage occurs, stalk nodes are black and rotted (Figure 4).⁴

Infection and Development of Physoderma Brown Spot and Physoderma Stalk Rot in Corn

Both PBS and PSR are caused by Physoderma maydis. The pathogen survives in rust-colored, overwintering structures called sporangia, which can be viable for two to seven years in infested crop residue from infected plants (Figure 5).⁴ The sporangia are wind dispersed or splashed onto corn plants. For these reasons, PBS may be more common in continuous corn and conservation tillage systems.

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 (73 to 86 °F) to germinate, infection in the whorl may cause lesions in bands across the leaf (Figure 2).^3,5

Plants are most susceptible during early vegetative growth (V5 to V9 stages) and may become more resistant with age.^2,4 Stalk rot develops when nodes six and seven have a Physoderma infection.⁴

How to Manage Physoderma Brown Spot and Physoderma Stalk Rot in Corn

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.^3,4 Tolerant products should also be planted to help reduce infestations.⁵ Corn seed products are being evaluated for their reactions to the stalk breakage symptoms. Delaro® 325 SC fungicide and Delaro® Complete fungicide are labeled for PBS and are effective methods of management when applied at early vegetative corn stages (V4 to V7). Three modes of action from Delaro® Complete fungicide will provide consistent control of major corn diseases under many different environmental conditions. For more information on these fungicide products, please visit https://www.cropscience.bayer.us/crop-protection/fungicide/delaro-325-sc-fungicide or https://www.cropscience.bayer.us/crop-protection/fungicide/delaro-complete-fungicide.

Sources

¹Mueller, D., Wise, K., Sisson, A., et al. 2025. Corn disease loss estimates from the United States and Ontario, Canada – 2025. Crop Protection Network. CPN-2007-25. doi.org/10.31274/cpn-20260309-1. https://cropprotectionnetwork.org/publications/corn-disease-loss-estimates-from-the-united-states-and-ontario-canada-2025
²Robertson, 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 Progress. 16(2): 90–92. https://apsjournals.apsnet.org/doi/10.1094/PHP-BR-15-0003
³Jackson-Ziems, T. 2020. Physoderma brown spot. University of Nebraska-Lincoln, CropWatch. http://cropwatch.unl.edu/plantdisease/corn/physoderma
⁴Robertson, A. 2015. Physoderma brown spot and stalk rot. Iowa State University, Integrated Crop Management. https://crops.extension.iastate.edu/cropnews/2015/07/physoderma-brown-spot-and-stalk-rot
⁵Wise, K., Kennedy, B., Mehl, K., and Bradley, C.A. 2018. Physoderma brown spot. University of Kentucky. PPFS-AG-C-07. https://publications.mgcafe.uky.edu/sites/publications.ca.uky.edu/files/ppfs-ag-c-07.pdf
Web sources verified 07/24/26. 1211_145548