Summer patch first appears in the warm weather of summer as yellow to bronze-colored, irregular-shaped patches ranging from 6 in. to 3 ft (15.3 cm to 1 m) in diameter. Patches may coalesce, resulting in areas of affected turf that are several feet in diameter. Annual bluegrass is the predominant host, and in mixed stands such as golf course greens, fairways and tees, a "frog-eye" symptom can be seen with creeping bentgrass remaining healthy. Dark "runner hyphae" is evident on crowns and roots of affected plants when observed microscopically.
The disease first appears in the warm weather of the summer, typically after rainy periods or heavy downpours. Saturated soils have been shown to exacerbate disease development. Soil temperatures above 70° F (21.1° C) at a 2 in. (5cm) depth for at least 48 hours are key for disease onset as well. While symptoms are present in the warm weather, infection takes place in the spring when soil temperatures first reach 65° F (18.3° C) at a 2 in. (5 cm) depth. Weakened roots from saturated, anoxygenic soil conditions allow for further infection and symptom development.