The fungal disease known as melting out is well-documented in cool-season grasses, but it can also be a killer of warm-season species. In fact, Maria Tomaso-Peterson has found that in the heat of the South, the fungi that incite this disease can be persistent and virtually impossible to get rid of once established.
Tomaso-Peterson is an assistant research professor in turfgrass pathology at Mississippi State University, and has studied melting out and other fungal diseases for the past five years. In fact, she calls the diseases known as leaf spot and melting out “a complex” that can occur at the same turfgrass facility and be very difficult to manage.
“It’s an interesting group of fungi. It’s not just one species,” she says. The species Bipolaris cynodontis, B. sorokiniana and B. spicifera may be present all at once or sporadically throughout the growing season. Melting out is a summer disease, occurring in Mississippi bermudagrass in July as temperatures soar and lawns begin drying out. Infected residential lawns will begin thinning out and golf course fairways can take on a maroon or purple hue before beginning to die off.
The fungi attack the leaves of the plants, and the leaves begin to fall off, Tomaso-Peterson says. Plants die as the fungi then begin to attack the crowns and roots. The grass first appears patchy, and thinning out may result as the disease progresses. In northern states, Kentucky bluegrass and fescue are the primary targets, and in the South, bermudagrass and zoysiagrass bear the brunt, with St. Augustinegrass and centipedegrass not being as susceptible. Early vigilance is critical, because from the time the first leaf spot appear to the time that large, irregular patches of turf die off may only be a few days.
“There are several cultural practices that can be applied with both warm-season and cool-season grasses,” she says, and the most important is in the selection of varieties or cultivars prior to planting or renovating a facility. There are many bluegrass varieties that have a range of resistance (a good list is at the Iowa State University Web site, http://www.extension.iastate.edu/Publications/SUL13.pdf), and it may be beneficial to use a mix of varieties, some of which have resistance and some of which have other beneficial traits. Tomaso-Peterson points out that in her field trials, she has also determined that the improved seeded bermudagrass varieties, such as Riviera, Princess-77 and Yukon, showed some tolerance of these fungi. Common bermudagrass is less tolerant, and additional research is necessary to determine field tolerance of hybrid bermudagrasses.
She notes that as with all disease prevention and management, the growing of healthy grass is essential in the attempt to control melting out, whether in warm or cool climates. A good fertility program with adequate phosphorus and potassium content is advised. Watch nitrogen spikes that can give growth spurts and promote the disease; slow-release nitrogen sources often provide the best nutritional conditions. She says that in her trials, the above three tolerant seeded bermudagrass varieties also showed satisfactory tolerance, even under high nitrogen. When grass is unhealthy and melting out is active, mowing height can be increased to allow the plants to withstand the effects of the disease.
Irrigation management is another important tool. Tomaso-Peterson says that these fungi will be encouraged when the grass is stressed, whether it is by too much water or too little. Care should be taken to water deeply and infrequently and during times of the day when dew is naturally present to reduce the period of time when the leaves are wet. Grass that is most susceptible is that growing either under shade, where it dries out slowly, or on the tops of exposed ridges where it dries out most quickly.
Thatch management is also crucial, because that is where the melting out fungi live and develop during the course of the year. Tomaso-Peterson says she has observed that .5 inch of thatch is healthy for turfgrass, but a layer over an inch deep sets up an unhealthy situation that encourages leaf spot and melting out. Dethatching, aeration and topdressing are good ways to manage grass health and minimize fungal disease.
“There’s a very good group of fungicides available to control the disease,” she adds, and these fungicides should be employed in a preventative manner once melting out has become a recurring problem. She has not seen a facility rid itself entirely of the fungi once they are firmly established, but the disease can be minimized with good turf management and fungicide applications. She recommends early spring preventative treatments and then regular follow-up treatments throughout the summer, rotating the fungicide chemistries used in order to avoid resistance issues.
The list of effective fungicides is long, and a good place to find such a list is at a North Carolina State Web site: http://www.turffiles.ncsu.edu/Diseases/Melting_Out.aspx. Tomaso-Peterson says that based on her trials, the following is one good course of action to take.
Start with the strobilurins, such as azoxystrobin (Heritage) or pyraclostrobin (Insignia), in April as a preventative measure in a turfgrass sward with a history of leaf spot and/or melting out. These may be applied as foliar sprays or granular applications, according to the label, and more than one application may be needed throughout the growing season.
Once summer starts, a 14 to 21-day application schedule may be warranted if the disease persists. Tomaso-Peterson has observed that a dicarboximide, such as iprodione (26GT, Iprodione Pro) or vinclozolin (Curalan, Touche), or a protectant, such as mancozeb (Fore), is a good follow-up fungicide that has a different mode of action. Again, multiple applications may be necessary. These are all spot or area sprayings, however, applying to those areas of the lawn or facility where the disease is evident. Treatment of entire fairways or sports facilities may not be necessary. Golf course superintendents on infested courses will often spray the collars of greens to prevent spread of disease from fairways onto the greens.
Finally, as summer progresses, she might follow those treatments with an application of chlorothalonil (Daconil), a protectant fungicide that targets the fungi in several sites. Severely infested areas may require a fungicide program until the grass goes dormant, an important measure in reducing populations going into winter. In Gulf Coast states where there is a brief winter dormancy, it may be necessary to apply fungicides during that period in order to reduce leaf spot where it can be a precursor to melting out.
Don Dale resides in Altadena, Calif., and is a frequent contributor to Turf. He has covered the green industry for more than 10 years.