Stripe Smut & Flag Smut of Kentucky Bluegrass

Ned Tisserat 
  Extension Specialist, Plant Pathology

Symptoms

Stripe and flag smuts were at one time considered to major diseases of Kentucky bluegrass. With the introduction of resistance cultivars, smut is now a relatively rare problem in bluegrass lawns.  Nevertheless, smut can still result in significant damage to certain susceptible Kentucky bluegrass cultivars.

Symptoms of smut on individual plants are most noticeable in spring and fall because cool temperatures (50 to 60 degrees F) favor disease development. Infected plants, which are pale green to slightly yellow, may occur in patches, or may be scattered throughout the entire lawn. Individual leaf blades affected with the disease are slightly curled and have tattered or shredded tips, similar in appearance to damage caused by a dull mower blade. Leaves also develop black sooty or powdery stripes that run parallel with the leaf veins. The sooty substance is composed of masses of fungal spores. The presence of the black, sooty stripes on the leaves is diagnostic of smut. Infected plants often die during hot, dry weather, resulting in thinning and an uneven appearance to the turf. In some cases, the symptoms disappear during the hot summer months, only to show up again in the cool fall weather.

Figure 1. Stripe smut on leaves of Kentucky bluegrass.

Figure 2. Stripe smut on Kentucky bluegrass.

Conditions

Stripe (Ustilago striiformis) and flag (Urocystis agropyri) smut fungi survive adverse weather inside infected plants or as resting spores in the soil. Spores (called teliospores) germinate in the soil and thatch during cool weather. Most disease development occurs in the early spring and fall. Infection of new plants can only occur at new growing points, such as developing tillers. The pathogen then grows systemically. Thus, all new buds and rhizomes developing from an infected plant also will be diseased. Diseased plants are very sensitive to high temperatures or dry conditions and often die during the hot summer months.

Disease Management

Stripe and flag smuts are very difficult to control once they are established in the lawn. The best means of avoiding smut is to plant resistant or tolerant cultivars. Avoid planting the Kentucky bluegrass cultivars Windsor, Merion, Fylking, Pennstar, Galaxy, Sydsport, Baron, or Rugby in areas where smut has been a problem. Most of the newer Kentucky bluegrass cultivars have some tolerance to the disease. Because these smut fungi are quite variable, it is advisable to plant a blend of two or more resistant cultivars.

Controlling smut in lawns already infected is very difficult because the pathogen is systemic. Smut-infected lawns are intolerant of drought and high temperatures. Therefore, maintain an irrigation program to prevent drought stress. Maintain a balanced fertilization program and increase the mowing height during the summer.

Properly timed fungicide applications also will help suppress smut development in infected turfgrass. Systemic fungicides are effective in limiting new infections and systemic spread of the pathogen. However, these chemicals may not eradicate the fungus in systemically infected plants. Apply fungicides once in early spring, just as the turf breaks dormancy, and again in late summer to early fall when vigorous growth resumes Consult fungicide labels for specific information on chemical rates and methods of application.

Spring, fall

Heritage

Spring, fall

Rubigan

Spring, fall

Eagle

Spring, fall

Banner

Spring, fall

Cleary's 3336, Cavalier, Fungo, SysTec 1998

Spring, fall

Accost, Bayleton, Fungicide VII, Fungi-Fighter, others

Spring, fall

Compass

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Web updated 9/01/06
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