Leaf Spot and Melting Out Diseases

Quick Facts...

• Leaf spot and melting out are diseases of stressed turf caused by several species of fungi.
• The first symptoms of leaf spot are small, purple to black specks on the leaf blades.
• 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.
• The diseases can be controlled by proper cultural practices that maintain healthy grass.

Leaf spot and melting out diseases are caused by several species of fungi that used to be classified in the single genus Helminthosporium. Most plant pathologists use this name because it is commonly accepted by turf professionals. There are two diseases in this leaf, crown and root disease complex. The first disease is leaf spot caused by the fungus Bipolaris sorokiniana. The second disease, melting out, is the more serious of the two and is caused by the fungus Drechslera poae.

Both fungi attack cool season turfgrasses (Kentucky bluegrass, ryegrass and tall fescue). Leaf spot disease appears during cool, moist weather, spring or fall, while melting out disease is active during warmer weather.

Figure 1: Leaf spot and melting out disease on grass.

Symptoms

Leaf spot. Symptoms first appear as small, purple to black specks on the leaf blades (Figure 1). These spots become elliptical and may be surrounded by a dark purple border. Tissue in the center of the spot may die and turn a beige or straw color. If the spot extends across the leaf, the blades wither and die.

Melting out. This disease also starts out as a black to purple leaf spot. As the disease progresses, the fungus works its way to the plant's base and attacks the roots and crown. Basal tissues near the ground become dark brown and rot. This stage is called melting out because the grass gradually thins and "melts out" the diseased area. The turf appears yellowish, thin and shabby with irregular patches of dead grass. When these areas are raked, the dead grass plants are easily removed.

About the Fungi

The pathogens responsible for leaf spot and melting out survive from year to year as spores or mycelium (fungal threads) in dead plant debris, in the thatch layer, and in infected plant parts.

Both diseases are favored by dry periods alternating with cloudy, wet weather and cool to moderate temperatures. The diseases are enhanced by the use of susceptible cultivars (Table 1), excessive nitrogen fertilizer, excess water, and a short mowing height.

Control

Leaf spot and melting out are diseases of stressed turf. The severity of the disease can be controlled by proper cultural practices that maintain the grass at optimum vigor.

Cultural Control

Use resistant varieties when establishing or re-establishing a lawn (Table 2).

Core aerate the lawn once a year (spring or fall) to help reduce thatch buildup and improve soil condition.

Mow grass as necessary to maintain a height of 2 1/2 to 3 inches. Make sure mower blades are sharp. Never remove more than one-third of the grass blade at a time.

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 excessive applications of nitrogen fertilizer, which induce 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 four times a year: mid-May, June, September and two to three weeks before hard frost. Never apply more that 4 pounds of nitrogen per 1,000 square feet in an entire year.

Chemical Control

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. Broad spectrum fungicides such as chlorothalonil (Daconil Ultrex, Daconil Zn, Daconil Weather Stik – use of these products on home lawns is prohibited), iprodione (Chipco 26019 – prohibited at residential sites), or mancozeb (Fore) should give adequate control. Chemicals are most effective when combined with cultural controls.

Remove loose thatch before treatment. If a preventive spray program is used, apply the first fungicide at the first sign of leaf spot once the grass begins to grow in the spring. Make additional applications according to label directions as need develops. Turf should not be drought-stressed prior to spraying or irrigated immediately after application. Be sure to follow the instructions on the fungicide label for specific rates to use and timing of application.

¹L.P. Pottorff, Colorado State University Extension plant pathologist and horticulturalist, Integrated Pest Management Program, Jefferson County; W.M. Brown Jr., Extension plant pathologist and professor, bioagricultural sciences and pest management; and A.J. Koski, Extension turf specialist, horticulture and landscape architecture. 6/96. Reviewed 1/03.

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