Leaf Smuts Leaf Smuts (Stripe smut (Ustilago striiformis) and Flag smut (Urocystis agropyri)): Symptoms: Leaf smuts are named for the powdery black spore masses produced by the smut fungi that infect Kentucky bluegrass and, occasionally,  perennial ryegrass. In the spring and fall, at temperatures of 50-65 degrees F,  plants may look stunted and yellow, turning grayish just before the streaks of black spores break through the leaf surface. Later, leaves appear twisted and frayed. During hot summer weather, smut symptoms often disappear, but the plants  are still infected. They are very susceptible to various stresses, such as heat and drought, and many will die. Smut symptoms may return in the fall, and the fungus will continue to live inside the plant until the plant dies. Smut is  usually not reported until lawns are three to four years old, when yellowing and thinning is first noticed and black streaks of spores are discovered. Disease Cycle: The black dusty smut spores survive in soil, thatch, and on seed. They are easily transported via air, soil, water, feet, seed and  equipment. Turfgrass plants are infected by smut only at growing points, and  they will remain infected until they die. Cultural Management: Only smut-free seed and sod should be used to establish new lawns. A few infected plants are easy to overlook, and smut is  usually not discovered until infections are more extensive in older lawns. In  established lawns, smut will be worse with excess thatch, soil pH below 6.0,  frequent watering and excess nitrogen fertilizers. These same factors encourage other diseases as well and should be avoided. After infected plants die out  during summer stress, reseed in the fall with a blend of resistant cultivars. Chemical Management: Only  penetrant /systemic fungicides are useful against smut fungi. They are expensive  and usually must be applied more than once. Fungicide applications are not  economical or practical for most lawn situations compared to reseeding damaged  areas.