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
Figure 1. Physoderma brown spot lesions. Note the darker, larger lesions on the leaf midrib.
Figure 2. Left: PBS leaf lesions appearing in a broad band. Right: Larger, darker, purplish PBS lesions on the leaf sheath. Photo's courtesy of Dr. Carl Bradley, University of Kentucky.
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 snap 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).
Plants are most susceptible during early vegetative growth and become more resistant with age. PBS may be more common in continuous corn and conservation tillage systems.
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.1 Planting tolerant products also reduces infestations. 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 control. Three modes of action from this fungicide provide consistent control under a variety of environmental conditions.
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.