Having had mild summer weather in 2013 and 2014, diseases like summer patch have not been a big problem in the last two years. Summer patch is mostly a disease of Kentucky and annual bluegrass and creeping red fescue. It was the heavy rain storms and high temperatures in late June and July that brought summer patch back to the forefront in 2015.
Summer patch was once known as Fusarium blight and dubbed the SOB disease (Summer Obliteration of Bluegrass) by Professor Patricia Sanders at Penn State in the late 1970’s. In the early 1980’s, Dr. Richard Smiley at Cornell discovered that the causal agent was a darkly pigmented fungus that attacked roots and stems and not a species of Fusarium. It was not until 1989, when Dr.’s Peter Landschoot and Noel Jackson at the University of Rhode Island, found the real culprit, which turned out to be a previously undescribed fungal pathogen, which they named Magnaporthe poae.
The most important environmental factors required for summer patch development are for soils to be very warm (≥ 75oF) and wet. Generally, summer patch does not become evident in lawns before mid-July in the mid-Atlantic and usually appears during hot sunny weather following heavy rain events. Summer patch is most severe on sunny, exposed slopes or other heat-stressed areas such as those adjacent to sidewalks, driveways and south facing sides of homes and other buildings. Thunderstorm deluges that result in standing water often are associated with extreme damage to turf. Saturated soils absorb and hold more heat and are more deficient in oxygen compared to well drained soils, which promotes colonization and infection of roots and stem bases by the pathogen.
Symptoms of summer patch in Kentucky bluegrass initially appear as wilted spots or bronze-colored spots, not unlike dog urine damage. There are no diagnostic leaf lesions, but on close inspection, you are likely to find that leaves die-back from the tip and are bleached white. Affected areas soon turn white, tan or straw-brown. In severe cases, patches increase in size and may become crescent-shaped or remain circular. Fully developed patches generally range from 6 to 18 inches in diameter. In closely cut Kentucky bluegrass, a yellow or bronze-colored halo may be seen at the outer periphery of patches. Healthy turf (usually tall fescue in mixed stand) may persist in the center of patches, producing rings or frog-eye symptoms. Depressions in turf called “crater pits” are common. Patches may coalesce, and large non-uniformly shaped areas of turf can be destroyed within a two week period. Many lawns consist of mixes of Kentucky bluegrass and tall fescue. Since tall fescue is resistant, if not immune to summer patch, symptoms in mixed stands usually are diffuse and circular patches are not seen.