M. Bess Dicklow is a familiar figure around the lawns and golf courses of Massachusetts. She is extension plant pathologist for the University of Massachusetts at Amherst, and has been at the university for 26 years, so Dicklow is familiar with the killer turfgrass diseases of the region. Yellow patch is not normally a killer, she says, but it can be an eyesore.

The pathogen lives in thatch and can persist over the years, but is susceptible to cultural and chemical controls.

Dicklow says that yellow patch, Rhizoctonia cerealis, is fairly common in the area, but rarely actually kills the grass. Also called cool-season brown patch, it is a different species of fungus than the one that causes brown patch (Rhizoctonia solani). The pathogen is of the genus that also includes brown ring patch and leaf and sheath spot. It is primarily a disease of cool-season grasses, such as creeping bentgrass, Kentucky bluegrass and the ryegrasses, but can also occur on warm-season grasses, says Dicklow, whose background is in plant pathology and plant breeding.

“I have 30 years of experience in plant pathology and none in plant breeding,” she jokes. She has seen every type of turfgrass disease that occurs in Massachusetts.

Optimal temperatures for the yellow patch pathogen are from 50 to 60 degrees, but it can also occur with snow mold in colder periods. It really gets going during extended periods of rainy, cool weather in the spring – usually from late March to early May, depending on weather conditions – but can also pop up in the fall in October or November in Massachusetts.

“Sometimes when it occurs in the fall it throws people for a loop,” Dicklow says, because it is thought of as a springtime turfgrass disease. It favors layers of thatch over .5 inch thick, compacted soils, poor drainage, high nitrogen levels and slow air circulation. The dismal weather of a New England spring, if it lasts for weeks, can cause a significant outbreak. It lives from year to year in the thatch and soil through sclerotia, a tough-shelled survival phase that allows it to survive despite severely low temperatures, desiccation and even fungicide applications.

Yellow patch is a fungus that can cause aesthetic problems in cool-season grasses, but rarely causes death of the host.

What the turfgrass manager sees in the grass are a series of yellowish rings, arcs or patches. These are mainly an aesthetic problem, and on lawns that are not as severely manicured they may go unnoticed. If the weather becomes dry and warm, the disease will generally disappear from sight within a week or so. If, however, damp and cool conditions prevail for an extended period and turf is in poor health, the disease can kill grass.

Correcting compaction problems and improving soil drainage are good long-term practices that will make the turfgrass more resistant to the pathogen. More immediately, the use of slow-release nitrogen fertilizers to reduce nitrogen spikes, and maintaining correct nutritional levels, will make grass less susceptible to the fungus. Succulent and rapidly growing grass is more susceptible to infection.

Reducing the length of time that grasses are wet can also help. Thick thatch layers should be reduced to provide less habitat for the fungus, and trees and shrubs around infected areas should be pruned to increase air circulation and speed drying out of the grass during damp or cold periods.

“You can spread yellow patch with mowers, like most foliar diseases,” Dicklow points out, so mowing equipment should be cleaned before moving into disease-free areas. Some of the above cultural controls should ideally take place in the fall so that infection is not encouraged the following spring, and reducing compaction through aeration and topdressing is an excellent long-term preventative.

“Chemicals are not normally needed for yellow patch,” she says of fungicides. However, if those dismal weather conditions prevail in the spring, aesthetics suffer and the grass can die. Fortunately, there are many fungicides that have proven to be effective against yellow patch.

Dicklow says that the most effective fungicide in her experience is flutolanil (ProStar, Moncut 70-DF). The former is a wettable powder and the latter a dry flowable. All fungicides should be applied to yellow patch in a foliar spray, and one application should be sufficient if good weather conditions return.

Many of the strobilurins, such as azoxystrobin (Heritage), fluoxastrobin (Disarm 480 SC), pyraclostrobin (Insignia) and trifloxystrobin (Compass), are also effective against yellow patch. They can be used interchangeably, but when rotating the use of chemistries to reduce pathogen resistance the strobilurins should be rotated with groups with a different mode of action (FRAC code other than 11), she says.

The DMI fungicides, such as propiconazole (Banner MAXX and many others) and triticonazole (Trinity, Triton), are also effective. Chlorothalonil (Daconil, Echo, Manicure and many others) is too. All should be sprayed on and then watered in to take the fungicide to the thatch layer, but not soaked so that the chemicals are leached into the soil where they would be wasted.

“There are also premixed formulations that are effective,” Dicklow says. All of the above chemicals should be applied at label rates and can be used either in the spring or fall.

Don Dale resides in Altadena, Calif., and is a frequent contributor to Turf. He has covered the green industry for more than 10 years.