Physoderma brown spot is making an appearance in some Illinois cornfields, says Carl Bradley, University of Illinois Extension  plant pathologist. Severe symptoms of this disease, caused by the pathogen Physoderma maydis, have been observed in parts of southern Illinois, he says. Symptoms appear as small, round to oblong spots on the leaves, which generally occur in bands.
"This disease can be confused with purple leaf sheath, because symptoms also can occur on the leaf sheath as dark purple circular lesions," Bradley says. "However, if no lesions on the leaves are present, then the leaf sheath symptoms likely are caused by purple leaf sheath."
Purple leaf sheath occurs when pollen grains are trapped between the leaf sheath and stalk and are colonized by saprophytic fungi, which is cosmetic only and does not cause damage, he said. On the other hand, infections by P. maydis generally take place in the leaf whorl or in places where free moisture is present.
"A quick scan of fungicide  labels revealed that a few fungicides list Physoderma brown spot as a target disease," Bradley says. "Although the disease symptoms may be somewhat severe in some fields, an application of a foliar fungicide at this stage may not be beneficial."
Bradley says most affected cornfields are now at a stage where the leaf whorl is no longer present, reducing the likelihood of new infections. In fields with severe symptoms, the best-management practices  are to rotate to a non-host crop the following year and plant the most resistant hybrid available the next time corn  is grown in those fields.
To confirm if you have Physoderma brown spot in your field, send samples to the U of I Plant Clinic .