Leaf spot and melting out are diseases of stressed turf, especially Kentucky bluegrass, caused by several species of fungi including Drechslera (Helminthosporium). Leaf spot disease appears during cool, moist weather, spring or fall. The first symptoms of leaf spot are small, purple to black specks on the leaf blades. Some leaf spot can be found on most home lawns in the spring, but it normally does not cause significant damage to the turf. However, newly seeded lawns and certain cultivars of Kentucky bluegrass are particularly susceptible, and a disease outbreak can result in serious injury to the turf. Extended periods of cool, wet spring weather also may trigger leaf spot epidemics on Kentucky bluegrass.
Melting out disease is active during warmer weather. Melting out starts as a black to purple leaf spot, then works its way to the plant base and attacks the roots and crown. Both diseases grow in dry periods alternating with cloudy, wet weather and cool to moderate temperatures. Once the fungus reaches the crown and roots, the entire plant may be killed, causing large areas of grass to thin. Infected areas are irregularly shaped, and range in size from several inches to several feet in diameter. Severe infections may cover the entire lawn
The diseases are enhanced by the use of susceptible cultivars, excessive nitrogen fertilizer, excess water, and a short mowing height.
The most effective means of preventing leaf spot and melting out is to plant resistant cultivars.
Cultural Control: The severity of the disease can be controlled by proper cultural practices that maintain the grass at optimum vigour.
Use resistant varieties when establishing or re-establishing a lawn.
Core aerate the lawn once a year (spring or fall) to help reduce thatch buildup and improve soil condition. Core-Aeration is the best method to reduce thatch. Aeration increases the rate of organic matter decomposition. If the thatch accumulates in excess of 1 centimeter, it should be removed. Thatch provides an ideal medium for the fungi to multiply but it also interferes with proper movement of water and nutrients. If clippings are heavy, remove them to reduce thatch accumulation.
Mow grass as necessary to maintain a height of 3 inches. Make sure mower blades are sharp. Never remove more than one-third of the grass blade at a time. Mow frequently. Do not mow when grass is wet. During a disease outbreak, the diseased areas should be mowed last and clippings should be removed, to reduce the spread of disease.
Water to a depth of 6 to 8 inches as infrequently as possible without creating water stress. Water in the morning or midday so the leaf blades dry as quickly as possible. Avoid frequent light sprinklings.
Avoid excessive applications of fast release nitrogen fertilizer, which induces tender, succulent growth and more susceptible tissue. Apply nitrogen according to soil test results or at the rate of 1 pound per 1,000 square feet 3-4 times a year. Never apply more that 4 pounds of nitrogen per 1,000 square feet in an entire year.
Fungicides are rarely needed to control leaf spot disease. However, if melting out disease has occurred repeatedly in the same areas over a number of years, a fungicide may be warranted. Chemicals are most effective when combined with cultural controls.