Description and Symptoms
Holcus leaf spot (Pseudomonas syringae) usually does not cause major losses. Leaf spots are initially oval to irregular, dark green, water-soaked at the margins, and may have a light brown border. Later, lesions dry out and turn light tan, usually without a border. The dry lesions have a papery texture. Holcus leaf spot often follows warm (77°F –86°F), rainy weather early in the season. Most hybrids have adequate resistance. Other management practices are rarely warranted and fungicides are not effective against this disease.
The best time to scout is prior to R1. Confused with eyespot and drift or application injury caused by certain herbicides, fertilizers, or spray additives. Often appears suddenly after a heavy rain, but then does not spread to new leaves.
The most favorable condition for the disease is warm (75-85 degrees Fahrenheit), wet, windy conditions early in the season favor infection and the development of Holcus leaf spot. Symptoms often appear suddenly after a heavy rain, but then do not spread to new leaves. Corn is most susceptible prior to tasseling. Holcus leaf spot is mostly cosmetic and does not result in yield loss. Fungicides are not effective against this bacterial disease.
Photo by Daren Mueller