Annual bluegrass, Kentucky bluegrass, bentgrasses, and fescues are most commonly affected.
From a distance, infected turf appears clumpy and patchy, due to the killing of individual plants and the more upright growth of infected plants. Smutted plants are most evident in the spring and fall when they appear pale-green to slightly yellow or brown, stunted, and more erect than healthy plants. Single plants may be affected, and irregular patches up to 1 foot or more in diameter may occur. Narrow, yellow-green streaks develop between the veins. These streaks turn silvery to dull gray and extend the entire length of the leaf blade and sheath. The grass epidermis covering the streaks soon ruptures, releasing blackish brown dusty masses of smut spores. The leaves soon twist, split, shred into ribbons, turn brown, curl from the tip downward, and die. Affected plants do not tiller as profusely or produce as many rhizomes or stolons as healthy plants, nor do they develop as extensive a root system. Smut-infected plants usually die during hot, dry weather; weed invasion soon follows.
Stripe smut (caused by Ustilago striiformis) and flag smut (caused by Urocystis agropyri) are favored by low soil moisture and temperatures between 50 and 68 F. Turf that is 2 to 3 or more years old and locations where pH is below 6.0 are more likely to have smut. The pathogens grow systemically throughout a grass plant; once infected, a plant remains so for life.
Leaf smuts are less common in the U.S due to the widespread use of resistant varieties in blends and mixes. Thus, the best control strategy is to renovate and overseed infected turf with a blend or mixture of several highly resistant varieties. Maintain a balanced fertility level based on soil tests; spring and summer applications of nitrogen can enhance smut severity. Yearly treatment with a systemic fungicide is expensive, but when done properly, will eradicate the pathogens from within the grass hosts. Drench the fungicide into the soil in late fall, just before winter dormancy, or at green-up in the early spring.