Problem: Fusarium Blight
These lawn diseases are caused by two different fungi that cause symptoms so similar they are hard to tell apart. Ophiosphaerella korrae causes a summer patch disease called Necrotic Ring Spot. Magnaporthe poae causes Summer Patch. Formerly lumped together and called Fusarium Blight, these diseases were believed to be caused by a fungus called Fusarium. Summer patch diseases primarily attack Kentucky bluegrass and annual bluegrass, beginning as a small spot then growing. As the grass dies, the spot fills in with resistant plants, usually weeds, causing the frog''s-eye look. These diseases are favored by hot, dry, windy conditions.
Irrigate regularly to keep grass evenly moist. Avoid heavy nitrogen fertilization. Control thatch, which favors the disease, and mow the lawn at 2 1/2 to 3 inches tall in hot weather. When replanting, select resistant varieties of bluegrass or choose ryegrass or fine fescue. Or, plant a mix of 20% perennial ryegrass and 80% Kentucky bluegrass, which is more resistant than pure bluegrass.