This photo St. Augustine grass shows healthy root systems on the left and diseased roots on the right. (Photo by Don Ferrin. Click on photo for downloadable image.)

News Release Distributed 09/13/10

Take-all root rot, a disease caused by a soil-borne fungus, has become a major concern in St. Augustine grass lawns across Louisiana, according to LSU AgCenter experts.

This fungus is frequently found in association with turfgrass roots without causing significant damage, said LSU AgCenter plant pathologist Don Ferrin. However, when the turf is repeatedly subjected to stress, this root disease can become quite destructive.

Experts attribute the recent outbreak of the disease to stresses imposed by severe cold last winter followed by abnormally warm weather this summer. The disease is generally being seen in areas of lawns exposed to full sun, particularly in lawns that have not been properly managed.

“Symptoms of take-all root rot are generally not evident until the roots have already been severely compromised,” Ferrin said.

Initial symptoms are a general yellowing, thinning or drought-stressed appearance of the turf often with only the newest leaves showing signs of life, he said. The overall density of the root system is greatly reduced, and diseased roots are dark-colored and tend to be short and brittle.

“Careful examination of stolons and the bases of the leaf sheaths with a good hand lens usually reveals the presence of black, thread-like fungal strands on their surfaces,” Ferrin said.

The management of take-all root rot relies on the use of proper practices to reduce stress on the turf and to alter the soil to make it more suitable for root growth and less suitable for the pathogen, said LSU AgCenter weed scientist Ron Strahan.

“The first step is to alleviate the stresses that triggered the disease,” Strahan said. “We can’t do much to change the weather, but we can irrigate as needed, aerate the soil to reduce soil compaction and make sure that we are mowing at the proper height, which is about 3 inches for St. Augustine.”

Strahan encourages homeowners to avoid overwatering, keeping the soil moist but not wet. It also is important that the soil pH be adjusted so that it is at the low end of the range recommended for St. Augustine grass, generally a pH of about 6.

“It is important to provide adequate amounts of potassium in addition to nitrogen fertilizer because potassium is an important nutrient needed for disease resistance and water balance in the plant,” Strahan said. “The general recommendation is to supply equal amounts of nitrogen and potassium during the year.”

He advises homeowners to wait until April to begin applying fertilizers because southern turfgrasses will soon be going dormant.

“We are very late in the summer,” Strahan said. “It is not recommended that you fertilize your lawn with nitrogen-containing fertilizer past August 15 in Louisiana. Applying fertilizer this late may lead to increased incidences of another serious disease problem, brown patch.”

The best course of action is to get a soil sample now and apply potash fertilizer – labeled 0-0-60 – according to the results obtained in the sample, he said. Next April, begin fertilizing with nitrogen-containing fertilizers.

“Consider re-applying fertilizer in June and very early August,” Strahan said. “Raise the mowing height to 3 inches and maintain adequate soil moisture. Collectively, these changes in cultural practices should help your St. Augustine grass overcome take-all disease.”

“None of the fungicides that are readily available to homeowners are particularly effective in controlling this disease by themselves once the disease has become established,” Ferrin said. “However, commercial products containing the fungicides azoxystrobin and pyraclostrobin, and to a lesser degree triadimefon and propiconazole – both of which are available to homeowners – may aid in control when they are used as part of an integrated management program.”

Current recommendations are to make two fungicide applications in mid- to late September and again in mid- to late October and one application in mid- to late March, Ferrin said. But additional fungicide applications may be necessary. Each application should be watered in with at least one-fourth inch of water to move the fungicide into the root zone where it is needed to protect the roots.

Rick Bogren