Managing summer lawn diseases (07-04-2005)

We encounter most turf diseases in home lawns during the spring when temperatures are moderating and afternoon spring showers are plentiful. However, our summer days can also present several turf disease problems, especially when we encounter several consecutive days of cloudy, wet weather.

Gray Leaf Spot (Piricularia grisea) of St. Augustinegrass can cause severe thinning of home lawns during the heat of summer. This disease is more common during extended periods of hot, humid weather. Although primarily a disease of St. Augustinegrass, it is also found on crabgrass and occasionally on centipedegrass. Infected turf will have irregular gray, dirty-yellow or ash-colored spots with brown, purple, or water-soaked borders on leaf blades. The disease is usually noticed first in shaded areas that remain damp longer. Under heavy disease development, the grass may have a burned or scorched appearance resulting in death or severe spotting of the leaf blades. Fungus spores are carried to sites of new infection by the wind, splashed rain, irrigation water, and mowers. Seldom will this disease kill an entire lawn but can thin it severely enough to be unsightly and encourage weed infestations.

‘Helminthosporium’ Leaf Blights (Bipolaris spp., Curvularia spp., etc.) are probably the most serious summer diseases of bermuda grass and zoysia grass if left unmanaged. Symptoms are round or oblong spots on the leaf blades and are usually brown or purple in color. As the disease infections increase the leaf sheaths turn brown and die. The turf may be badly thinned and even die in patches thus giving an appearance that adequately describes the disease’s other name of “melting out”.

Dollar Spot (Sclerotinia homoecarpa) is a common summer disease found on many turf species, but is most common on Bermudagrass. This disease is found more prevalent on neglected turf or areas that are under moisture or nutritional stress caused by the lack of nitrogen. Humid weather with heavy dew promotes the disease, which begins as small (2-3 inch) circular dead spots throughout the lawn. These small spots can then grow together to form much larger blighted turf. Fungicides are effective in controlling dollar spot, but a good fertilization and watering program will also help in the management of this disease.

Controlling these summer diseases is best accomplished by keeping the turf as healthy as possible by reducing stress conditions:

• Avoid excessive nitrogen fertilization (with the exception of dollar spot).
• Water, when needed, early enough in the day so foliage will not remain wet overnight.
• Mow regularly at proper height with a sharp mower only when the turf is dry.
• Be prepared to apply fungicides according to label recommendations if climatic conditions favor disease proliferation.

Published July 4, 2005

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. wwells@ext.msstate.edu