Corn fields in southern Indiana are exhibiting symptoms of holcus leaf spot, a disease caused by the bacterium Pseudomonas syringae pv. syringae. Infected plants will have round, discrete lesions that are initially pale yellow to white and then enlarge and turn gray or brown.
Lesions have a water-soaked halo and on certain hybrids, the margin of the lesion may appear brown or purple (Figure 1). Lesions are typically distributed from the center to the tips of leaves (Figure 2). Holcus leaf spot can easily be confused with symptoms caused by herbicide drift or injury. To determine if the symptoms are caused by disease or an abiotic disorder, it may be necessary to submit a sample to a diagnostic lab where it will be checked for the presence of the bacterium.
Holcus leaf spot development is favored by warm temperatures (mid 70s-to mid-80s) and rainy, windy weather. Although rain has been scarce in Indiana, most symptoms appeared after rains that occurred at the end of May. The bacterium enters the plant through wounds or through stomates, and is not known to spread from infected leaves to healthy leaves. The bacterium will overwinter in infected residue, and also on other grass species, including weeds.
Holcus leaf spot is not a common disease in Indiana, and is not known to limit yield. Although the disease may cause concern based on symptom appearance, no in-season treatment is available or necessary. Fungicide applications will not have efficacy against this bacterial disease.