Features Agronomy
Dollar spot still a mystery but new research shows promise

DOLLAR spot was first discovered in 1925, and it still remains a mystery among turfgrass scientists.

“Even after 85 years, we still don’t have a handle on the basic biology of the pathogen,” Dr. John Kaminski of Penn State University said in December at the 22nd annual Ontario Seed Company/Nutrite Professional Turfgrass Seminar in Waterloo.

The industry’s focus on dollar spot over the past 50 or 60 years has centred around cultural and management practices of the disease, but there is still little understanding of the pathogen’s basic biology which would provide a greater insight into its control.

There may be some good news on the horizon, however, Kaminski speculated. He and several other researchers throughout the United States have pooled their resources and have spearheaded new research into the disease.

“I expect over the next five years you’ll see a lot of new information come out.”

Warm days and cool nights are a natural trigger for dollar spot with the subsequent formation of dew and leaf wetness. Removing dew can reduce the impact of dollar spot. This can be accomplished by various means, including mowing at different times.

Temperatures between 15 and 25 degrees Celsius are an ideal range for active dollar spot, which can be more severe in dry soils.

Kaminski said a study showed watering frequently and light, which goes against what most people believe, indicated less dollar spot in those trial plots. The negative aspects of watering frequently and light outweighed any dollar spot issues that might have been realized, according to one researcher.

Kaminski acknowledged, however, that although the practice might reduce the incidence of dollar spot, it is apt to open the turf up to several other diseases.

Dollar spot is a low nitrogen disease and has more to do with the levels of nitrogen as opposed to its source, whether it’s organic or synthetic.

“That doesn’t mean you go out and add 10 pounds of nitrogen per year per 1,000 square feet, and you’ll eliminate all your dollar spot. You can still have dollar spot problems. The key is when you start to see dollar spot roll in, you give it a little spoon feed or a little boost of nitrogen.”

The jury is still out about whether or not thatch contributes to dollar spot, Kaminski said.

“Thatch management is something I think is very important, but the research on it is very limited.”

Studies suggest dew removal can account for a 53 to 83 per cent reduction in dollar spot. Rolling is one way to knock back dew, and it was always believed that rolling in the morning when dew was most prevalent was perhaps the best strategy. Kaminski said that, since then, it has been discovered that rolling in the absence of dew is resulting in a reduction of dollar spot.

“So there’s something about either the compression or maybe the lack of air that results from that compression.”

Some cultivars are more susceptible to dollar spot than others. NTEP data outlines which ones are most resistant and which are most susceptible.

The bottom line with dollar spot, Kaminski said, is no matter how good a golf course’s integrated pest management program is and how good its cultural practices are, there is no way one can get away with not using fungicides to control spot.

“It’s just become too much of a problem.”

Because the disease is present from May to October, it must be kept under control. Fungicide resistance has impacted control levels. A recent study of about 250 golf courses in the New England area provided important data on resistance levels.

“The level of resistance was way more than I thought,” Kaminski said.

Resistance to dollar spot was found to be a significant problem among the fungicides used. Higher rates can be used to achieve some level of control, but Kaminski said he was still surprised at the overall degree of resistance present in the study.

In some cases, the fairways were worse than the greens, and overall the greens and fairways were worse than the roughs. In most cases, Kaminski said he suspected it had much to do with the spray management strategies the golf courses had adopted over the past several years.

Golf course superintendents who suspect they might have an issue with dollar spot resistance will find they can’t begin basic cultural practices and will have to stick with regular aerification—even if their members don’t want it at the time—because they’ll eventually get burned if they don’t. Kaminski said the number of fungicide applications is important, suggesting the same fungicide can’t be applied over and over again.

“You’re going to develop resistance.”

Tank mixing has been found to generate good control, he said, yet going below label rates is not so good because the level of the pathogen won’t be as effectively controlled, and it will come back stronger.

“We have to figure out a way to improve the level of control based on what we’re doing and not based on what we want to do.”

For foliar diseases such as dollar spot, it’s vital to provide proper coverage. Kaminski said the key to good coverage is selecting the right nozzle. Some nozzle types provide excellent coverage while others fall significantly short.

The air induction nozzle performed best in test studies.

“When you’re going after something that’s a foliar disease, use something like the air induction that’s going to give you good coverage and excellent control.”

Five different nozzles were assessed in the study as well as three different fungicides. It was also found that tank mixing provided better results than those from each individual product.

With dollar spot, the first flush usually occurs in late spring or early summer, and then a second flush often occurs in the fall. In a study conducted in 2006, an application was made in April following the second “true” mowing of the season. The disease wouldn’t start until late May or early June.

Kaminski said abiding by the IPM mandate means that fungicide used to control dollar spot must be curative, but that creates a problem.

“If you wait until dollar spot becomes a curative situation, you’re going to spray more fungicide than you would do on a preventative basis.”

In an early season application, dollar spot is much slower to develop in its initial stages. When it is first seen, putting down a fertilizer can buy another week to 10 days of suppression, and then a normal fungicide for dollar spot management can be applied.

A resurgence of dollar spot in the fall is becoming more commonplace.

“All of a sudden it comes back with a vengeance and no one can figure out why,” Kaminski said, adding resurgence is a bigger problem than many realize.

Compounding the problem is that resurgence occurs at a time of year when budgets are tight and staff are exhausted.

Kaminski suggested that any superintendents who plan to start over with their turf cover can begin by planting less susceptible cultivars and not let thatch get out of control, especially on fairways. He added it’s important to know whether or not there is resistance to the fungicide used.

If a chronic problem with dollar spot exists, Kaminski suggested an early season application and a reduction of wetness. Studies show the efficacy of fungicide is reduced when watered in. Using the proper nozzle is imperative. At the first sign of dollar spot, put down fertilizer to slow the process, he added.