Physoderma Brown Spot and Stalk Rot
Physoderma Brown Spot (PBS) and Physoderma Stalk Rot (PSR) are diseases in corn that have generally been considered minor until the last few years. Both versions of this disease are caused by Physoderma maydis, a soil borne fungus. PBS is generally not an economically important disease but PNR has recently caused trouble in many areas of Iowa, Illinois, Indiana, and Minnesota.
Symptoms
For PBS, small round, sometimes oblong, lesions appear on the midvein, leaf blade and/or leaf sheath. They can vary in color from yellow/red to a very dark brown. The small lesions frequently coalesce into large blotches on the leaf. Foliar symptoms often have little impact on yield.
PSR can have serious impacts to a field due to increased lodging. Infected plants regularly reach grain fill before any stalk symptoms are seen. Lesions occur at the mostly at the first or second node above the soil level. The node appears dark brown to black and can bleed the color into tissue above or below the node itself. Nodes become very brittle.
Disease Cycle
Physoderma maydis can over winter in the soil and crop residue for as long as seven years. Infection occurs when spores are splashed into the whorl of young corn plants and abundant rainfall keeps water sitting in the world for multiple days. Warm temperatures are also favorable for infection. When the water filled whorl is saturating nodes 6 and 7 during infection, stalk rot is more likely to occur.
Management
Most commercial hybrids have acceptable tolerance to the foliar PBS. Tolerance to the stalk rot is still be evaluated and enhanced but newer hybrids are likely to give ratings. If you have a field with a history of Physoderma infection, choose more tolerant hybrids. Crop rotation and residue management may also help reduce inoculum for future infections. At this time, foliar fungicide does not appear to reduce PSR.
Sources
Crop Protection Network. Pyhsoderma Stalk Rot or Corn. https://cropprotectionnetwork.org/resources/articles/diseases/physoderma-stalk-rot-of-corn
Corn States. Physoderma Brown Spot and Stalk Rot in Corn. https://www.corn-states.com/app/uploads/2018/07/physoderma-brown-spot-and-stalk-rot-in-corn.pdf
Kleczewski, N. Physoderma Brown Spot and Node Rot in Corn. University of Illinois. http://cropdisease.cropsciences.illinois.edu/?p=792