When to Spray Grass for Leaf Spot

Leaf Spot

• Leaf spot fungal spores, almost always present in the lawn, overwinter in grass or weeds, germinating during rainy weather. This fungal disease is at its worst when temperatures reach 85 degrees Fahrenheit, according to the University of Rhode Island. Your lawn can become infected with leaf spot any time during the growing season, but rain and warm weather spur the disease to germinate and spread rapidly.

Symptoms

• Gardeners may notice red to purplish ovals with tan centers appearing on their grass blades, according to the University of Rhode Island. Several oval patches can occur on one blade until they coalesce and cause blighting. Eventually, the fungus can spread to the stems and root system of the grass. Once the fungus has invaded the roots, entire root systems can rot. Over time, small areas turn into larger areas, and the entire lawn is at risk of dying or melting out.

When to Spray

• Spray a fungicide to treat leaf spot before the lawn has melted out. Fungicides will not help a severely infected lawn, however, according to the University of Rhode Island. Use the fungicide every five to 10 days. Frequency is determined by the severity of leaf spot and how often it rains. While gardeners can use a fungicide that contains mancozeb, captan or fludioxonil, use of a fungicide is not enough. Employ cultural management practices to fully eradicate the disease from the lawn.

Cultural Management

• Allow the grass to grow to 3 inches in height. Don't mow more than a third of the length of the grass blades at any one time. Mowing the lawn short stresses the lawn, encouraging leaf spot. In addition, sharpen your mower's blades at the beginning of each growing season. Dull blades can shred grass, which produce an entryway for fungal spores. Water your lawn in the morning so it can dry during the daytime. Wet grass blades combined with cool nighttime temperatures create a breeding ground for fungal diseases.