Physoderma Brown Spot (Corn)

Physoderma maydis

Corn leaf infected by Physoderma Brown Spot
Corn leaf infected by Physoderma Brown Spot. Photo courtesy of Daren Mueller. Licensed under CC BY 3.0.
Physoderma brown spot is not a serious disease in the United States. It is recognized by its small brown lesions located mainly on the leaves of corn.
The PBS fungus survives can survive 2 to 7 years in either infected corn tissue or the soil. It is more common in continuous corn and conservation tillage systems. PBS prefers a moist environment between 73 and 90 degrees F. Its spores often can be found in water held behind the leaf sheath or whorl.
Infection most often happens between stages V5 and V9. PBS appears on leaves as numerous small, round or oblong lesions. They can be purple or yellowish to brown. These spots can also occur in the stalk, leaf sheath, and husks. If infection sets in the stalk, it often results in stalk breakage at one of the lower nodes. It can be easily confused with southern corn rust or eyespot infections, but can be identified as different upon closer inspection. PBS is the only one that grows in a banded pattern across the leaf.
Stalk Breakage Due to PBS
Stalk breakage due to PBS. Photo courtesy of Clemson University. Licensed under CC BY 3.0.
Damage for Physoderma brown spot are very minimal. It is much more dangerous when it takes the form of stalk rot. This weakens the lower nodes, often resulting in breakage.
It is best to rotate crops with non-host species such as soybeans. Tillage is also very helpful in breaking down corn residue which otherwise acts as a host for the fungus spores. Hybrids have generally not been developed due to the minimal economic impact and sporadic nature of the fungus.
Some hybrids do have some resistance to this disease. Research is being done to develop fungicide that will effectively control this fungus. Please contact your local plant pathologist to learn what is most effective in your area and how to use it best. Always read and follow all label instructions.