Golf courses and home lawns throughout the state have been experiencing sick turf recently. The symptoms include circular patches of bronzed or browned turf and patches or rings of straw- colored turf. The problems have been diagnosed as summer patch or take-all, two distinct fungal diseases that attack the roots and crowns of grass plants.
Summer patch, once known as Fusarium blight, is caused by the fungus Magnaporthe poae. Patches, rings, or arcs of tan- to bronze-colored grass can range from 6 inches to 3 feet across. Symptoms usually appear when typical warm summer weather begins. This year, prolonged heat starting about 2 weeks ago has promoted disease development. In the Plant Disease Clinic, we wash out roots of plants from the margins of patches and examine them under a dissecting or compound microscope for the presence of superficial, dark-colored fungal growth called ectotrophic hyphae.
In Iowa, we frequently see summer patch in home lawn situations where sod has been laid on heavy clay soil within the last few years. Many times, these sites occur in newer housing developments in which little topsoil has been allowed to remain. These sites seem to be prone to environmental stress during midsummer, and summer patch has often been linked to stressed turf.
To minimize the risk of summer patch, it's helpful to minimize stress on the lawn. Such practices as core aeration, timely fertilization (but not overfertilization with nitrogen), thatch control, mowing at a 2.5-inch or higher level, and timely watering are thought to aid root growth and keep stress levels down. A watering practice that can help to mitigate summer patch is to syringe (sprinkle for 10 to 15 minutes at a time) the lawn one or more times per day during midday during hot, humid days. Overseeding damaged turf with blends of Kentucky bluegrass cultivars with tolerance to summer patch, or with species mixes incorporating turf-type tall fescue or perennial rye as well as Kentucky bluegrass, can prevent serious outbreaks in the future.
Chemical treatment is an option for summer patch, but the trick is to start early. Systemic fungicides such as Rubigan, Banner, or Bayleton should be applied first when night temperatures don't fall below 70o F, with repeat applications at intervals of about 30 days until the warm summer weather subsides.
Take-all patch. This disease is most serious on the creeping bentgrasses that make up most golf greens in Iowa. Recently seeded greens are the most vulnerable, especially where drainage is poor. Scattered, more or less round patches of bronzed to light brown grass, from 4 inches to several feet across, appear in late spring. Some scattered green blades of grass may remain within the patches. Patches may continue to enlarge for several years, then sometimes disappear. Dark brown to black ectotrophic, "runner" hyphae are evident on the surfaces or roots and crowns, which may turn black and die themselves.
Drainage is a key to take-all patch control, especially when new seedings are planned. In areas where take-all has been a problem, it may be helpful to incorporate elemental sulfur (3 to 4 lb per 1,000 ft2) into the top several inches of soil prior to seeding, because an acid environment suppresses the pathogen. Acidifying fertilizers, such as ammonium sulfate, can also help to keep the soil pH down. Rubigan or Banner can sometimes help to suppress take-all, but results are erratic. Two applications in spring, the first shortly after mowing begins and the second a month later, can be helpful. However, the cultural controls are often the key to suppressing take-all.
This article originally appeared in the June 30, 1995 issue, p. 101.