Tall fescue (Festuca arundinacea) is cultivated as a turf grass in U.S. Department of Agriculture plant hardiness zones 2 to 7, where it features a deep root system that gives this species good drought and heat tolerance. A handful of diseases, including powdery mildew, can affect the appearance and health of a tall fescue lawn. Powdery mildew, caused by the fungus Blumeria graminis, appears on grass blades as a powdery, white growth; heavily infected leaves turn yellow to brown and die. Powdery mildew is a problem where conditions are shady and humid and when temperatures are between 60 and 70 degrees Fahrenheit. This disease on turf grass does not pose a threat to children or pets that use the area and will disappear when conditions no longer favor its growth. Chemical control is generally not warranted in home landscapes if the conditions that encourage powdery mildew are corrected.
Prune off overhanging tree branches and thin out adjacent shrubs to increase the amount of sunlight reaching the tall fescue and improve air circulation around the grass. Make any pruning cut just above a bud or branch junction. If you are cutting a branch off at a large branch or the main stem, position the cut just outside the branch collar, the area of raised tissue where the branch meets a larger branch or the trunk.
Mow the tall fescue at a height of 3 inches, the maximum recommended mowing height for this species. A lower mowing height, especially in shady sites, can weaken the grass, increasing the impact of powdery mildew and leading to thin growth.
Fertilize the tall fescue turf with a complete, slow-release fertilizer three or four times each year. Time applications for when the tall fescue is actively growing and apply fertilizer at a rate of 1 pound of nitrogen per 1,000 square feet of grass with each application. Excessive nitrogen encourages tender new growth that is especially susceptible to powdery mildew.
Water the tall fescue grass deeply and frequently, irrigating only when soil 1 to 2 inches below the soil surface feels dry to the touch and wetting the top 6 to 8 inches of soil each time you irrigate. Time your watering for early in the day.