Centipede grass horrors: Why is my grass dying?

By Charlie Spencer 

Master Gardener

It is that time of year when the Master Gardener information phone starts to ring off the hook. Homeowners who have centipede grass in their yards realize that there is something wrong with the grass. It is dying in places, usually in a circular pattern.

Centipede, as well as the other three warm season grasses, is susceptible to a disease referred to as “large patch.” It is a difficult disease to control and can usually only be controlled in late August through October.

Matt Martin, area turf specialist with N.C. State University, has prepared an informational article on the cause and corrective actions concerning large patch. This article will be in two parts:

Large patch is one of the most damaging diseases of warm-season turfgrasses in North Carolina. In turfgrass, it is caused by the fungus Rhizoctonia solani. All commonly cultivated turfgrasses in North Carolina are affected by this disease, but differences in susceptibility exist within the cultivars of the various turfgrass species. 

In warm season turfgrass, St. Augustine, zoysiagrass, and centipedegrass are most susceptible to this disease, while common and hybrid bermudagrass is rarely damaged. Certain species of Rhizoctonia are capable of attacking turfgrass plants from seedling stage to mature plants and are pathogenic over a wide range of environmental conditions. 

Large patch is a soil born disease that will survive from year to year in soil surrounding areas that have been affected by the disease. The disease is present in almost all warm season turf everywhere. However, when conditions for the disease are favorable for development, the environment and the turfgrass health depend on whether it spreads into a pest problem for the turfgrass.

Symptoms

Large patch symptoms vary, depending primarily on turfgrass species and mowing height. Generally, the disease will begin in the late summer when soil temperatures begin to fall below 70 degrees. This will begin in southeastern North Carolina in August and September. 

Disease activity and development will continue throughout the fall until the turfgrass goes into dormancy. At this point, the disease loses the green leaf tissue needed for continued damage. However, the disease will remain in the thatch and soil areas that have been affected and will, given the right environmental conditions, continue to damage the turfgrass in the spring after green-up. The degree of turfgrass injury depends largely on susceptibility of the cultivar, management practices and weather condition.

Large patch first appears in lawns as small, circular, brown areas several inches in diameter, which quickly increase to 3-6 feet across. These areas often grow together, forming irregular patches of brown, blighted turf up to 20 feet in diameter. 

The foliage of high-cut St. Augustinegrass or centipedegrass turf often wilts and collapses, giving the blighted patches a sunken appearance. Damaged turf usually recovers when conditions no longer favor the spread of disease. Regrowth of the turf usually starts in the center of the blighted area, forming a rung or frog-eyed pattern. 

Weeds frequently invade turf damaged by large patch. Damage to individual grass plants is usually confined to foliage. Leaves attacked by the large patch fungus, Rhizoctonia solani, first become water-soaked and wilted, finally turning brown. On broad-leafed turfgrasses like St. Augustinegrass, distinct tan-colored leaf spots surrounded by a water-soaked margin are sometimes seen. 

Green plants with the affected turf have grayish-colored leaf spots that are long, irregularly shaped and surrounded by a dark brown margin. If the crowns or stolons are invaded, large areas of turf may be killed.

Factors favoring large patch

Large Patch can develop when air temperatures are between 50-86 degrees and soil temperatures fall below 70 degrees. The disease usually develops on lawns during periods of wet, overcast weather in early spring or fall. Damage is often heaviest after several days of showers with temperatures of 60-70 degrees.

Poorly drained soils, thick thatch and night irrigation lengthen the period of leaf wetness and promote greater infection. High levels of nitrogen and low levels of phosphorous or potassium may contribute to increased disease severity. Mowing with a dull mower blade damages leaf blade tips and causes excessive wounding that enhances infection through those frayed blade tips.

Send your gardening questions or comments to: Brunswick County Master Gardener Column, P.O. Box 109, Bolivia, NC 28422, or call 253-2610. Enclose a self-addressed stamped envelope if requesting information or a reply. Answers may be printed in this column.