Summer patch is a moderately common turfgrass disease of Kentucky bluegrass, annual bluegrass and fine fescues. Reports and samples of summer patch disease from Kentucky bluegrass lawns in Ames and Des Moines have been received over the past few weeks.
Summer patch is caused by the fungus Magnaporthe poae. Characteristic symptoms of the disease are rings of affected turf that start with a reddish brown discoloration and then turn tan or light straw color. The center of the tan discolored patch usually remains green giving rise to the alternate name, “frogeye patch.” See the symptom in the picture below.
The fungal pathogen survives in the soil and can cause damage in the same location every year. Among the most important management practices are: avoid excessive nitrogen fertilization, dethatch and core-aerate the lawn at least once a year. There are preventive chemical controls for summer patch that include fungicides in the azoxystrobin, fenarimol, propiconazole active ingredient group and others but these must be used in combination with the cultural practices mentioned above. More information about summer patch can be found in ISU Extension publication SUL 17.