In the past week, frustrated homeowners visiting the UF/IFAS Duval County Extension Office have brought in several samples of stressed and thinning turf. These samples have included zoysiagrass, bermudagrass, but mainly St. Augustinegrass. It happens every year in Northeast Florida.
With this year's drier weather, the problem was somewhat delayed. Recently, with more predictable rainfall, summertime heat and humidity, it has returned.
While chinch bugs might be a reasonable answer, we see far more instances of a turf disease called take-all root rot.
Take-all root rot (Gaeumannomyces graminis var. graminis) is caused by a fungus that is always present in the soil but really makes itself known in our landscapes during the summer rainy season. In many of the cases brought to the Extension office, the turf has also been stressed by overwatering.
When homeowners bring in a sample, the most common complaint is that they have had to re-sod multiple times and the same patch of lawn continues to struggle. Several homeowners have even completely resodded, replacing their St. Augustine with a different turf species such as zoysia, only to have the symptoms return.
The first symptom of take-all root rot is often yellowing turf that appears in irregular patterns. These yellow patches soon begin to thin and eventually become bare. As the condition progresses, the bare patches grow larger. Turf can quickly go from thick and green to thin and patchy. By the time these aboveground symptoms become noticeable, significant damage to the root system has usually already occurred.
If you pull up one of the remaining patches of turf, you may notice that the roots have become short, black, and rotten. The outer layer of the roots often sloughs off between your fingertips.
Because the aboveground symptoms can be confused with other problems such as chinch bugs, drought stress, or webworms, proper diagnosis is important. Examining underground plant parts with a microscope is the best way to confirm this disease. Roots, rhizomes, and stolons may simply appear dirty to the naked eye. However, under magnification, the telltale signs of take-all root rot become visible: masses of dark fungal strands emerging from these plant tissues. These strands indicate that the fungus has colonized the root system.
If you suspect take-all root rot, you can submit a sample to the University of Florida Plant Diagnostic Center: https://plantpath.ifas.ufl.edu/extension/plant-diagnostic-center/.
Like any plant disease, prevention is much easier than trying to control it after symptoms appear. Take-all root rot takes advantage of stressed turf. Cultural practices that promote healthy turfgrass can go a long way toward preventing this disease. These practices include proper irrigation, fertilization, weed management, and mowing at the correct height.
After homeowners learn that their lawn has a disease, they often respond, "But I have a lawn service." However, notice that some of the most important management practices, particularly irrigation and mowing height, remain under the homeowner's control. Fungicides alone will not solve the problem.
Poor irrigation practices are among the most common contributors to lawn disease. To avoid overwatering or underwatering, calibrate your irrigation system, make sure the rain sensor is functioning properly, and water only when needed. Most turf requires no more than 1/2 to 3/4 inch of water per application, up to twice per week, including rainfall.
An afternoon thunderstorm can often provide a week's worth of irrigation. During the rainy season, you may need to reduce irrigation or turn your system off entirely. The longer turf leaves remain wet, the more favorable conditions become for take-all root rot. If irrigation is needed, always water in the early morning.
Fertilization practices also play a role. Excessive nitrogen can create conditions favorable for disease development. Applying slow-release nitrogen and potassium sources is recommended. Fertilizers containing equal amounts of nitrogen and potassium, such as 16-0-16, are preferred. In addition, monitor soil pH and maintain it between 5.5 and 6.5, with a target of about 6.0. Soil pH above 6.5 has been associated with increased severity of take-all root rot.
Even when turf herbicides are used properly, they can cause mild stress to the lawn. Effective weed management includes spot-treating weeds whenever practical and always applying products according to label directions.
Mowing turf at the correct height allows leaf blades to capture the sunlight needed for photosynthesis, the process by which plants produce their own food. Mowing too low reduces the turf's ability to photosynthesize and can increase stress.
Recommended mowing heights vary by turf species:
St. Augustinegrass: 2.5 inches for dwarf varieties and 4 inches for standard varieties
Bermudagrass: 0.5 to 1.5 inches
If you have experienced take-all root rot in previous years, consider planning preventive fungicide applications now and again next spring, along with implementing proper turf management practices. Continue applications according to label directions while weather conditions remain favorable for disease development. While fungicides can help suppress the disease, they are generally much more effective as preventive treatments than as curative treatments.
For a list of fungicides recommended for take-all root rot, visit: https://edis.ifas.ufl.edu/publication/LH079.
If you have already seen signs of take-all root rot this year, liquid micronutrient applications may help improve turf appearance while roots recover. Small, frequent applications may be necessary when root systems are severely damaged.
By following sound cultural practices such as proper irrigation, fertilization, weed management, and mowing at the correct height, you can reduce the risk of take-all root rot and help keep your lawn healthy throughout the growing season.
Larry Figart is an urban forestry extension agent with the University of Florida/IFAS.
This article originally appeared on Florida Times-Union: Is 'take-all root rot' killing your Florida lawn? What you should know