Grass blades exhibiting white spots indicate a fungal disease called powdery mildew. Powdery mildew is the most widespread disease that infects plants, according to Colorado State University. However, gardeners can have peace of mind knowing that the fungal spores that cause lawn powdery mildew will not infect your other plants.
Powdery mildew typically infects lawns during July through September, according to the University of Rhode Island. Fungal spores quickly germinate in a few hours when temperatures are cool and lawn is moist. Furthermore, powdery mildew fungal spores are almost always present in the lawns, but may not be infecting grass. Spores overwinter in dead grass debris and spread when the weather favors the disease. Because the disease can infect grass in two hours, gardeners can see severe powdery mildew symptoms in a week, reports the University of Rhode Island.
Upon close examination, gardeners can see tiny white spots on their grass. Spots may appear to look like powder. This powder is a combination of mycelium and fungal spores. Mycelium is the branching parts of the fungal spores. The disease sucks up the nutrients from the surface of the grass blades. Grass blades become malnourished and turn yellow from the result of the lack of nutrients. Also, severe powdery mildew infections cause small to large areas of the lawn to die out.
Treating Powdery Mildew in the Shade
Grass growing in the shade is the most susceptible to contracting powdery mildew. Shady environments favor the germination and spread of powdery mildew fungal spores. Gardeners can control powdery mildew by cutting back overhanging branches in these areas. Trees that cast too much shade can be removed. Also, planting grass that tolerates shade will reduce the chances of powdery mildew damaging the lawn. Healthy grass tolerates moderate infections of powdery mildew. Shade tolerant grass types include St. Augustine, zoysia, fescues and supina bluegrass.
Cultural Management Strategies
Avoid overfertilizing your lawn to prevent stressing out the grass. Stressed out grass dies out when infected with powdery mildew. Avoid using more than 1 lb. of slow release fertilizer per 1,000 square feet at one time. Using too much nitrogen can burn grass and weaken grass blades. Furthermore, reduce your irrigation if you give your lawn more than 1 inch of water a week. Typically, lawns only need more than one inch of water each week to keep them from drying out.