The extended periods of hot humid weather we have recently experienced due to frequent afternoon showers has increased the incidence of gray leaf spot on many southern lawns. Gray leaf spot (Pyricularia grisea) is a fungal turf disease primarily attacking St. Augustine lawns, but centipede lawns are also susceptible. The fungus causes irregular gray or dirty-yellow spots with brown, purple, or water soaked borders on leaf blades. In areas of heavy disease development, the grass blades may have a burned or scorched appearance similar to pouring scalding hot water across the lawn that results in death or spotting of leaf blades. Shaded areas with poor air circulation that stay wet longer are usually the first to be attacked. The fungus thrives when air temperatures are above 70 degrees.
Gray leaf spot is best managed by 1) Avoiding high nitrogen fertilization, especially water-soluble nitrogen during summer wet periods, 2) Watering properly so foliage doesn’t stay wet for extended periods. If irrigation is necessary do it early in the day so leaves will dry before nightfall. 3) Mowing when leaves are dry and maintaining optimum height, 4) Applying repeated applications of appropriate fungicides during warm wet periods.
For a current list of labeled fungicides for homeowner use consult the homeowner fungicide database at http://www.extensionplantclinics.msstate.edu.
Dr. Wayne Wells is an Extension Professor and Turfgrass Specialist. His mailing address is Department of Plant and Soil Sciences, Mail Stop 9555, Mississippi State, MS 39762. firstname.lastname@example.org