Blame it on the weatherman

You’ve sprayed for dollar spot control and, in a perfect world, now you sit back and watch the results take place as the label suggests. This is anything but a perfect world when it comes to Mother Nature, however. What affect does the weather have on the efficacy of that costly fungicide?

Dr. Paul Koch, associate researcher and turfgrass diagnostic lab manager at the University of Wisconsin-Madison, has recently conducted studies that look at the effect of snow cover on the persistence of iprodione and chlorothalonil and the effect of temperature on these same two fungicides.

From the snow-cover study, Koch’s team found snow cover itself didn’t impact the persistence of either fungicide.

“This suggests that sunlight – or photodegradation – doesn’t impact fungicide persistence significantly on turfgrass leaf blades, at least in a winter environment,” Koch says. “What we did find was that winter rainfall or snowmelt events did lead to a fairly rapid reduction in fungicide concentration.

In addition, temperature did appear to impact fungicide persistence, as temperatures rose out in the field the fungicides degraded at a faster rate, Koch adds. “This was observed even on non-snow covered plots, so it wasn’t just a factor of melting snow,” he says.

These results led researchers to look more directly at temperature. Koch sprayed iprodione and chlorothalonil on fairway-height bentgrass, then sampled cores from each plot and placed them in growth chambers set at 10, 20 and 30 degrees C (50, 68 and 86 degrees F) and tested the fungicide concentration weekly over a five-week span.

“We found both fungicides degraded fastest at 30 (degrees C), slightly slower at 20, and slower still at 10,” he says. “The degradation rate was linear for chlorothalonil, suggesting potentially that microbial degradation plays the key role in degrading that fungicide.”

The degradation rate dropped much faster between seven and 14 days after application with iprodione, and barely dropped at all at 10C. So Koch is hypothesizing that plant metabolism may be the key degradative agent with that fungicide since it is absorbed into the leaf blade, while chlorothalonil, as a contact, stays on the outer leaf surface.

Additionally, at every time point they tested leaf blades from the growth chamber, they also took samples from the initial field plot sprayed and found that with both fungicides they could not detect any fungicide 14 days after the application.

“This was a much faster rate of degradation than we saw even from the plants in the 30 (degree C) growth chamber, and the temperatures were much cooler than 30 (degrees) out in the field,” Koch says. “We hypothesized that the reason for this rapid loss was due to removal by mowing, and that in most situations – at least on fairways – all the fungicide will be removed from the plant within 14 days of application due to mowing.”

Dollar spot is easily identifiable because of its small, tan circular patches.

One way to extend protection, Koch suggests, is to use plant growth regulators (PGRs) so less leaf tissue is removed by mowing – pointing to other studies that have found using plant growth regulators has led to extended durations of disease control.

“I want to stress that just because the fungicide is no longer present on the leaf blade it doesn’t mean a fungicide cannot control a disease for 21 or 28 days,” Koch says. “Fungicides do likely suppress disease by other means besides just residing on the leaf blade, and it likely relates to a fungicide’s ability to knock the fungus back out of the turfgrass canopy, which then takes time to grow and reinfect the turfgrass plant.”

Mike Salinetti, golf course superintendent at Berkshire Hills Country Club in Pittsfield, Mass., has seen dollar spot occur in all types of weather, but it most commonly impacts his course – located in the state’s western mountains – during moist, humid summer nights when the temperatures remain above 60 degrees F.

“This past summer was, by far, the worst for me trying to control dollar spot,” Salinetti says. “Typically, we are a very windy course, with good air movement. That was not the case this past summer. I’ve never had any problems on greens because they get sprayed on a 10-to-14-day schedule with excellent fungicides and spoon-fed fertilizer.”

With 34 acres of fairways and tees, Salinetti needs to get the most he can out of each spray.

“Last year, I was only getting eight days control at some points during the summer, spraying good fungicides at the label rate” he says. “Dollar spot was the only disease I had on fairways all year and [was] very difficult to control during hot humid weather, especially after we got a little precipitation.”

Salinetti believes the best way to counter the effects of weather on battling dollar spot is to have the proper amount of nitrogen feeding the turf throughout the year.

Koch sprayed iprodione and chlorothalonil on fairway-height bentgrass, then sampled cores from each plot and tested the fungicide concentration weekly over a five-week span.

“The past two years I looked at some cost-saving approaches by lowering my nitrogen levels on fairways and tees from 3lbs. N/m/year to 1.5lbs. N/m/year,” he says. “I’m a huge fan of polymer-coated slow-release Polyon fertilizer for fairways and tees. My first two years at BHCC I chose the 3-pound rate of nitrogen [and] never had any dollar spot issues, even when the weather was appropriate. Then, once I cut back to the 1.5-pound rate, the dollar spot has gotten worse and worse each year.

“Next season I’ll be going back to the 3-pound rate, and if I was a betting man, I’d bet my dollar spot gets cleaned up and hopefully gone,” Salinetti adds. “I feel proper nitrogen amounts will aid in dollar spot control, far better than fungicides.”

Koch’s research has shown fungicides will degrade faster at higher temperatures, but he’s not positive on wet vs. dry. His feeling, however, is that fungicides will degrade faster in wet conditions, which he has seen with drops in concentration following winter rainfall and snowmelt events.

“Countering these effects aren’t easy unless you can control the weather,” he joked. “But probably the most effective way is to shorten the reapplication interval and/or increase the rate of fungicide application.

“In addition to pathogens being more aggressive and plants more susceptible during hot, humid conditions, I think superintendents should also be thinking that their fungicides are degrading more rapidly... and this more rapid degradation could be leading to disease breakthroughs,” Koch adds. “I think PGRs can also extend the duration of control as well, as I mentioned before.”

Megan Kennelly, associate professor in the department of plant pathology at Kansas State University, has been studying dollar spot since 2006. She says there is myriad research on the disease, including: efficacy of fungicides and fungicide programs, forecasting models to help optimize the timing of applications, studies of which nozzles lead to best performance, and screening the causal fungus for resistance to fungicides. Researchers also are evaluating the susceptibility of different cultivars, she adds.

Kennelly says her area only sees a break during the winter and height of summer. “In Kansas, we most commonly see dollar spot in late spring/early summer, like late May through July,” she says. “Then the disease usually decreases significantly during the hottest part of summer. Dollar spot is most severe again in late August through September, sometimes continuing through October. The disease really takes off when we get dewy mornings in late summer/early fall after the heat is done.”

A superintendent has to be on alert from spring to fall. Fortunately, dollar spot isn’t too difficult to spot, according to Salinetti.

“To the superintendent’s eye, dollar spot is very easy to recognize,” he says. “A morning tour of the course in the dog days of summer may reveal white mycelium with straw brown leaf blades underneath. Left untouched, it can spread quite rapidly, especially under moist, humid conditions.”

Koch agrees dollar spot is one of the most easily identifiable diseases in the field because no other disease produces such small, tan circular patches. That said, a superintendent can’t jump to conclusions. “In certain cases the disease foci can blend together and become difficult to ID, and almost look similar to drought or anthracnose,” he says. “In that case, pick out some affected leaf blades and look for bleached-colored lesions with a reddish brown border. These lesions don’t have to be circular; sometimes they are encompassing the entire width of the leaf blade.”

Fortunately, greens aren’t often the diseases target.“It’s usually most common on fairways or tee boxes, as opposed to putting greens,” Koch says. “This might have something to do with a more dense turf canopy on these locations that can hold more moisture relative to the very short height of cut and often sandy conditions on a putting green. But more likely it’s related to the more intensive management regime on putting greens compared to fairways or tee boxes.”

While Koch acknowledges that dollar spot is possibly the most-studied disease in turf, there has not been a lot of research targeting weather’s impact on fungicide efficacy and persistence. In conjunction with his current research, superintendents are going to have to trust their own experience in the field - apply, watch, react.

Rob Thomas is a Cleveland-based freelance writer and frequent GCI contributor.