NEWS for North Dakotans
Agriculture Communication, North Dakota State University
7 Morrill Hall, Fargo, ND 58105-5665
July 29, 1999
The sultry, steamy dog days of summer can produce more than lethargy in humans and other animals. The heat, thunder and intense downpours this weather often generates can produce symptoms of turfgrass diseases caused by several fungi, says a horticulturist with the North Dakota State University Extension Service. Regardless of fungal source, the plant diseases are known by a collective term, "Summer Patch Syndrome."
"The symptoms show up with heat stress, when soil temperature exceeds 70 F in the top 2 inches for at least 48 hours," says Ron Smith, extension turfgrass specialist at NDSU. "The symptoms show up then, but the disease activity actually begins earlier, when the topsoil temperature hits 65 F."
Two of the disease-causing fungi (Leptosphaeria korrae and Magnaporthe poae) produce the same symptom: rings of dead grass. These organisms mostly affect lawns with newer grass varieties and those with peat-backed sod. Lawns with heavy thatch are also more susceptible, Smith says.
The third organism, Helminthosporium sativum, typically produces disease in older lawns, those established in the 1950s and 1960s with common grass varieties. Symptoms begin as spots on individual leaves but can develop into dead patches resembling turfgrass under drought stress. The advanced stage of this disease is called "Melting Out."
Smith says, "With Melting Out disease, you'll see big, irregular patches in your lawn. The dead rings the other pathogens cause are more uniform, perfectly circular in some cases."
The causes of all three diseases are similar. Excessive moisture, either from rainfall or improper irrigation, pushes oxygen out of the soil. This oxygen deprivation weakens grass plants and allows the fungi to invade the vascular tissue and reduce its ability to take up water and nutrients and move carbohydrates from the foliage to the roots, Smith explains. Heavy thatch also produces oxygen deprivation and causes turfgrass to re-root in the thatch and not in the soil.
As with the causes, the treatments for all three diseases are similar and can involve a combination of cultural practices and chemical applications, Smith says. First, home owners should reduce their normal lawn-watering cycles. Mowing lawns to a height no shorter than 3 inches and using a sharp blade will also help, as will adequate fertility.
Whether a lawn is shaded or exposed to full sun will determine its fertility needs. For newer lawns in full sun, home owners should apply 0.5 pound of nitrogen per 1,000 square feet of lawn every three to four weeks during the growing season. Older lawns in shaded areas probably require less fertilization, perhaps only one per year, Smith says. Slow-release forms of nitrogen fertilizer are more effective at battling these turfgrass diseases than quick-release forms such as urea.
The general-purpose fungicides maneb and chlorothalinol can halt the spread of Melting Out once it has begun. For the ring-spot diseases, home owners have the option of preventative and curative fungicides. Preventive fungicides, such as triadimefon or fenarimol, must go on before symptoms develop. Curative fungicides, such as maneb and clorothalinol, are designed to treat existing problems; the curative fungicides available for turfgrass ring-spot diseases include benomyl or thiophanate methyl. Smith advises home owners to look for products containing these active ingredients and follow label directions when applying any of the fungicides.
"Remember, the objective in controlling these diseases is to get oxygen into the root zone," Smith concludes. "Annual core aeration in the spring followed by light, adequate nitrogen fertilization will help get the turfgrass back on track. But cures do not come overnight, nor likely in a single season. With persistence, the disease should gradually subside."
###
Source: Ron Smith (701) 231-8161
Editor: Dean Hulse (701) 231-6136