Million dollar question

It’s sometimes called “million dollar spot” when poor maintenance practices lead to a severe outbreak on greens. As a result, considerable cost is required to rectify dollar spot outbreaks and to keep paying golfers happy with playing conditions.

In Wisconsin, for instance, it has been reported that golf course superintendents typically spend 60 to 75 percent of their chemical budgets spraying for dollar spot. Caused by the fungal pathogen Sclerotinia homoeocarpa, it impacts the majority of turfgrass species, and is active under a wide range of temperatures. Dollar spot is the most common turfgrass disease in North America, with the exception of the Pacific Northwest and western Canada.

So what’s in store for superintendents this growing season?

Given the abnormally warm, long autumn most of the country experienced followed by a bitterly cold and snowy winter for most of the upper portion of the U.S. and even some parts of the South, industry experts say so many scenarios are involved it’s difficult to predict dollar spot severity.

“Extreme late fall and winter weather might reduce mycelia density on the infected tissues and thatch and theoretically leave certain regions with reduced dollar spot pressure in spring,” says Dr. Geunhwa Jung, associate professor of turfgrass pathology/breeding at the Stockbridge School of Agriculture at the University of Massachusetts.

Seasonable warm temperatures this spring and ample moisture will favor dollar spot appearing early, but dry conditions and cooler temperatures will delay infection. “If late fall dollar spot infections were prevalent, it is likely that dollar spot severity could be higher and observed early if conditions are favorable,” Jung says.

North Carolina State turfgrass pathologist Dr. James Kerns advocates going into winter “clean” because the fungus overwinters readily in foliage. If superintendents allow the disease to develop and infection centers to sit overwinter it provides “an excellent source of inoculum for epidemics” the following year. “Moreover, for those that experience snow mold pressure, applications in the fall for dollar spot will help limit snow mold development as well,” he adds.

As far as winter temperatures, the dollar spot fungus can survive readily in foliage and most likely thatch in extreme weather conditions, Kerns says. “We examined this in a three-year study in Wisconsin and we were always able to find the fungus even after the harshest winter,” he says. “As an example, we can store the dollar spot fungus on filter paper at -112 Fahrenheit and it will survive if it has had an opportunity to dry down.”

Spring weather is crucial to dollar spot outbreaks, and if a region has a mild, humid spring the disease will readily develop. Superintendents should watch RH values and use temperature only as a guide. “We know dollar spot can develop from temperatures ranging from 50-95 F if relative humidity is sufficient,” Kern says.

If a region experienced a late fall with an extended time for dollar spot activity (it stayed warm later than normal) there could be two possible dollar spot scenarios.

“One, it gave you more time to clean up disease if present and go into winter in a healthy condition,” says Todd Hicks, program coordinator of turfgrass pathology in the department of plant pathology at The Ohio State University. “Or, two, if there was dollar spot and you did not manage the disease, the turf would be in poorer condition going into winter and with disease that will be more difficult to manage the next year.”

A warmer than normal spring with moisture and high humidity will enhance early dollar spot development that could be severe, adds Joseph Rimelspach, program specialist of turfgrass pathology in the department of plant pathology at The Ohio State University.

However, it’s unknown the effect a severe winter will have on dollar spot severity, making an accurate prediction for the 2015 season difficult, says Dr. John Inguagiato, assistant professor, turfgrass pathology at the University of Connecticut’s Department of Plant Science and Landscape Architecture. “There is still a lot we don't know about the biology of this pathogen and how overwintering is affected,” he says. “However, my opinion is that the winter will have little to do with the severity of dollar spot epidemics this spring and summer.”

Researchers consider dollar spot epidemics to be polycyclic, meaning as long as favorable conditions for disease exist, the pathogen will infect its host and quickly grow its population. “Therefore, the severity of dollar spot epidemics this upcoming season will most likely have more to do with temperature and humidity this spring than anything that has happened over the winter,” Inguagiato says.

Adam Moeller, agronomist for the USGA Green Section’s Northeast Region, is optimistic despite the crushing amount of snow and cold his region received during the second half of Winter 2014-15. “I don't think a distinct correlation between a late-fall or extreme winter weather and dollar spot problems will be noticeable in the Northeast region,” he says. “Superintendents did not express these concerns last season following one of the most severe winters in the past 30-plus years.” However, he adds, warm, humid spring weather is likely to bring on dollar spot activity.

Spring weather has a more pronounced impact on the pathogen’s severity than winter or fall conditions, says Dr. Paul Koch, assistant professor in the department of plant pathology at the University of Wisconsin-Madison’s Molecular and Environmental Toxicology Center. Warm and wet conditions in the spring encourage faster fungus growth, which leads to more pronounced symptom development during the summer. “However, the dollar spot fungus grows quickly, and dollar spot symptoms can develop rapidly in the summer even without conducive conditions in the spring,” he adds.

Superintendents should take into account their grass species. For example, creeping bentgrass is the most susceptible species to dollar spot, Jung says. Superintendents should also closely monitor “indicator” or “hot-spot” areas in which dollar spot historically shows up first. These spots are likely to have extended leaf wetness from poor drainage/more morning shade or lack of air movement.

Dollar spot severity should be “objectively” assessed if an outbreak occurs, taking into account the entire scope of the course, Moeller says. In many cases, a very small outbreak does not warrant an extra fungicide application outside of a traditional preventative program. Superintendents should increase efforts to manage the disease culturally via dragging fairways to remove dew on days when mowing is not performed and increase nitrogen inputs.

Inguagiato suggests checking sprayer output to ensure fungicides are applied at the proper rate, in a carrier volume of one to two gallons per 1,000 square feet, with flat fan nozzles that produce a medium to coarse droplet size.“If dollar spot damage is evident, consider using a water-soluble nitrogen source to grow out of damage,” he says. “When outbreaks occur we often find it necessary to temporarily shorten fungicide re-application intervals to arrest further disease development.”

Early spring fungicide applications, applied after second or third “true” mowing events, is another strategy. “Early spring applications have shown some positive results for delaying dollar spot outbreaks in early summer (May and June) and have the potential to reduce the number of overall applications,” Jung says. “If renovating, choose a cultivar that has displayed good dollar spot tolerance.”

If dollar spot infection occurs and curative action is required, Jung says to tank-mix contact and systemic fungicides to stop infection and maintain a regular (14 to 21 days) application interval following initial curative application. However, he adds selection of the proper fungicide classes is critical since resistance to three fungicide classes: DMI (demethylation inhibitors) such as (metconazole, myclobutanil, propiconazole, tebuconazole, triadimefon, triticonazole and tebuconazole), dicarboximide (iprodione and vinclozolin), and benzimidazole (thiophanate-methyl) has been reported on golf courses. Multi-site fungicides (chlorothalonil and fluazinam) are good options to partner with single-site fungicides under high disease pressure. “Resistance has not been reported to the SDHI (succinate dehydrogenase inhibitors) fungicide class (boscalid, fluxapyroxad, and penthiopyrad) and this class provides very good dollar spot control,” he says.

Research from multiple universities indicates making an early-season fungicide application well before symptoms typically appear can have long-lasting and positive impacts on disease control.

“For example, some of our research here at Wisconsin showed less dollar spot three months following an early season application on May 1 relative to a non-treated plot,” Koch says. “Fungicides with long-lasting dollar spot protection, such as Xzemplar, Emerald, and Banner MAXX, should be used for these early-season applications. In addition to early-season applications, superintendents can watch for initial dollar spot symptoms developing in their highest risk 'indicator areas' to inform them when applications for the entire course can be scheduled.”