“It really requires a preventative management strategy rather than a curative practice,” Loren Giesler says of necrotic ring spot. This fungal turfgrass disease, like so many others, can be avoided in most cases by growing healthy, stress-free turf. That strategy doesn’t offer much solace to those who have infestations of the disease, however.
Necrotic ring spot is caused by the fungus Ophiosphaerella korrae, an organism that is widespread across the northern tier of states. Its primary host is Kentucky bluegrass, though it may occur infrequently in Poa, and even more rarely in cool-season grasses such as certain ryegrasses and fescues, says Giesler, a professor and extension plant pathologist at the University of Nebraska-Lincoln. He has been working with turfgrass fungal diseases since 1992, part of that time as director of the Plant and Pest Diagnostic Clinic, and notes that necrotic ring spot is more prevalent in residential lawns than professionally managed facilities.
In Nebraska, the disease occurs from May through June, though that will vary across the fungus’ range. It is a soilborne organism that begins to feed on grass roots as the weather and soil warm up, with effects not immediately apparent. The optimum soil temperature for growth is from 68 to 82 degrees, but it becomes active at 50 degrees and is inhibited when soil temperature reaches 86 degrees. Grass plants gradually begin to decline as the season progresses.
“They’re going to appear stressed,” Giesler says, with a loss of vigor and color as in a nutrient deficiency. But, plants begin to die as the weather warms up, leaving small dead patches in the lawn. Often, those dead spots will be circular with a green spot of living turfgrass in the middle, hence the appearance of a ring. These will generally be smaller in size than fairy ring.
The rings will usually appear in parts of the lawn or turfgrass facility where the soil is compacted or where heavy thatch layers or low fertility stresses plants and impacts growth. Drought, or even excess irrigation, can also prompt an infestation if the organism resides in the soil. These infected areas are often the sunny or more exposed parts of the lawn.
Giesler says that the first spring when necrotic ring spot is noticed, grass managers are often left with no option but to treat with fungicides. Long-term management depends on the enactment of a good preventative program, which could start the same year. One of the first steps would be to manage irrigation, avoiding either too much or too little water. During the heat of the year, light, frequent irrigations may be necessary to maintain proper moisture for plants with diseased roots in order to nurse them through the summer.
Since soil compaction or thatch buildup is often the cause of plant stress, a solid aeration program is advised, Giesler says. Long-term, this soil conditioning program would be conducted once per year in the spring or fall. Timing aeration with any overseeding is a good way to integrate management techniques for the gradual decrease of the disease. Hollow-core tines are effective in mixing soil elements and lessening compaction.
Another strategy used to minimize damage to the lawn is to raise mowing height, which allows the grass to increase photosynthesis and become more vigorous, even as the disease takes hold. Soil amendments for pH adjustment are usually not necessary for the control of necrotic ring spot, Giesler says, because the fungus can thrive in a wide pH range. The disease can often be managed without chemicals once grass health is restored. He notes that resistant cultivars of Kentucky bluegrass can be used if a renovation is necessary, and a switch to less-susceptible ryegrass or fescue might be an option under certain circumstances.
Several effective fungicides are available if needed. Giesler says that the usual chemistry recommended for this group of fungi will also control necrotic ring spot, and that list can be found in “Chemical Control of Turfgrass Diseases 2010,” by Paul Vincelli at the University of Kentucky. Those include, in order of efficacy, fenarimol (Rubigan), myclobutanil (Eagle), iprodione (Iprodione Pro, Chipco 26GT, Raven, Lesco 18 Plus), propiconazole (Banner Maxx, Spectator, Savvi), thiophanate-methyl (Cleary’s 3336, Fungo, Systec 1998, T-Storm). The complete publication can be accessed at www.ca.uky.edu/agc/pubs/ppa/ppa1/ppa1.pdf
Giesler points out that these chemicals are generally foliar sprays, which must be watered in to be effective against soilborne fungi such as Ophiosphaerella korrae. “You need to water it in before it dries on the leaves,” he says of the foliar applications. “Fungicides, in general, move up, not down.”
In Nebraska, he advises turf managers to begin applications in mid-April, prior to seeing major symptoms of the disease. A second application could be added 30 days after the first one to ensure good control. It is important to note that you may not see much effect of the fungicide at the time of application, but you should see results later in the season when the disease would show symptoms. Only infected areas and the grass immediately adjacent should be treated, and a backpack sprayer will often do the job. This is a pest that usually creates only localized dead patches, and treatment of wide areas is not necessary.
“You don’t have to manage the whole landscape for the disease, but you may have to manage long-term,” he says. A combination of both cultural and chemical techniques can be employed over a period of years to minimize damage, and ultimately, the use of only preventative strategies may be needed once the soil and turfgrass are restored to health. Resodding infected areas or renovating with more resistant cultivars can reduce the need for further treatment.
Don Dale resides in Altadena, Calif., and is a frequent contributor to Turf. He has covered the green industry for more than 10 years.