Physoderma brown spot, caused by the pathogen Physoderma maydis, is making an appearance in Illinois cornfields. In some cases, severe symptoms have been observed. Symptoms appear as small, round to oblong spots on the leaves, generally occurring in bands. Symptoms also can occur on the leaf sheath as dark purple circular lesions, which may be confused with "purple leaf sheath." If there are no lesions on the leaves, 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; the results are cosmetic only, causing no damage.
Infections by P. maydis generally take place in the leaf whorl or where free moisture is present. A quick scan of fungicide labels reveals that Headline and Headline AMP list Physoderma brown spot as a target disease. Although the disease symptoms may be somewhat severe in some fields, applying a foliar fungicide now may not be beneficial. Most affected cornfields would be at the stage where the leaf whorl is no longer present, so the likelihood of new infections is considerably decreased (in other words, noninfected tissue would not necessarily need protection from a fungicide because new infections are unlikely). In fields with severe symptoms this season, the best management practices are to rotate to a nonhost crop next year and plant a resistant hybrid (if available) the next time corn is grown in those fields. To confirm whether you have Physoderma brown spot in your field, send samples to the University of Illinois Plant Clinic.