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Features - disease

Whether dealing with dollar spot preventatively, or curatively, understanding what makes this pathogen tick is your most effective weapon.

March 18, 2016

You’re walking the course and notice some straw-colored spots, about an inch or two in diameter, on a putting green. Upon closer inspection, it’s dollar spot. Have you missed your window to stave off the foliar disease throughout the rest of your course? And when was your window to avoid this mess, altogether?

Dollar spot infects turfgrass any time environmental conditions are conducive for the pathogen to develop, says Matt Giese, Syngenta’s Midwest field technical manager. If environmental conditions are not favorable, the likelihood of a dollar spot epidemic to occur is low.

“To understand the best timing for fungicidal control of dollar spot, it’s helpful to know what factors trigger disease development so applications may be timed prior to these events,” Giese says. “Temperature and leaf wetness are important aspects in dollar spot development. Night time temperatures in the range of 60 to 85(F) degrees and greater provide suitable conditions for development, coupled with extended periods of leaf wetness or heavy dews that persist well past daybreak.”

Low soil moisture, excessive thatch and low nitrogen fertility are stressors that encourage disease development on susceptible turf.

“These are just a few components superintendents should be aware of to prepare for preventive fungicide applications by watching weather forecasts, scouting indicator areas and monitoring cultural practices on the golf course to help predict when and where the risk of disease outbreaks is greatest,” he adds.

Dollar spot is caused by a fungus that overwinters as mycelia in plant tissue or soil. The growth of the fungus begins when temperatures are around 60.

Dollar spot fungus does not produce any spores. Instead it forms a stroma, which is a dense mass of fungal hyphae, says Nancy Dykema, research assistant at Michigan State University.

“For the stroma to infect and cause disease, it has to increase in size,” Dykema says. “Early season fungicide applications injure the stroma, setting it back and delaying the time before it can reach the size to cause disease. This is the reason early fungicide application can delay the development of dollar spot.”

Timing is important, of course. Dykema has read reports of early season preventive fungicide applications resulting in reduction of dollar spot two to three months later.

“Some of the early applications are timed based on growing degree day (GDD) models, while others are calendar dependent or based on the growth of the turfgrass,” she says. “Degree day models typically suggest applications be made between 150 to 200 GDD for early season preventive dollar spot control.”

Some turf managers time their early season applications to coincide with the onset of true mowing, such as after the second true mowing.

“This would be the mowing that occurs once the growth of the turf has started and it’s not just dead plant debris that’s being collected in the baskets,” Dykema adds. “The ideal timing of the early fungicide applications is mid to late spring, which, in Michigan is in April or May. Since the exact ideal timing hasn’t been identified yet, the best results are typically gained by making two consecutive systemic fungicide applications ... one in late April or early May, followed by one 30 days later.”

But applying too early is a waste of valuable resources, so timing is critical.

Dollar spot development is limited when night time temperatures fall below the suggested range above, so applying below 50 degrees is not typically considered a prudent use of fungicide, Giese says. However, research has shown that early spring applications when plant growth resumes, but before disease symptoms are present, can delay or even minimize dollar spot development during favorable disease conditions, he says.

Fortunately, applying too early won’t damage the turf, so keep an eye on the forecast.

Applying a fungicide treatment too early before the disease develops can result in fungicide degradation or removal from foliage resulting in limited or no protection when the disease actually does develop.

Curative treatment. The image shows the results gained after 14 days from a treatment once dollar spot has gained a foothold in turf.
© Syngenta
In contrast to the top image, dollar spot emergence on an untreated plot of turf.
© Syngenta
The turf plot represents the results of preventative treatment for dollar spot.
© Syngenta
Dollar spot allowed to run wild on an untreated turf plot.
© Syngenta

“A more reasonable approach focuses on forecasted favorable weather conditions and timing fungicide applications before those conditions are present,” Giese says. “Best results for dollar spot control are achieved with a sound agronomic program that includes both cultural and preventive fungicide treatments.”

Dykema, who works closely with Dr. Joseph Vargas at Michigan State, says there seems to be a “significant window of opportunity” for timing of early sprays to reduce dollar spot later in the season. However, making applications too early in the season prior to the turfgrass reaching its regular growth pattern is likely not very effective in delaying dollar spot epidemics and would, rather, be considered a waste of resources. Likewise, waiting until temperatures have warmed enough that the turf has been actively growing and has been mowed regularly for a month or more is less apt to delay the dollar spot epidemic much beyond the typical interval expected for the fungicide. Neither applying too early nor too late to delay the onset of dollar spot has any negative implications on the health of the turf as long as label directions are followed, she adds.

Unfortunately, you missed your window to treat dollar spot preventatively. What now?

“If it is too late and dollar spot has begun to develop, you will need to use a systemic fungicide to stop the progression of the disease,” Dykema says. “Unfortunately, folklore, which there is still too much of in turfgrass science, tells us to use a contact to knock down the disease. I have no idea what the term ‘knock down’ means. The fungus is already inside the plant and, unless stopped, will continue to destroy tissue.

“It would be analogous to the doctor giving me antibiotic pills to control the pneumonia in my lungs, but instead of putting them in my mouth so the antibiotic can enter my body and kill bacteria in my lungs, I rub them on my chest,” she continued. “Do you think that is going to kill the bacteria in my lungs? Not any more than a contact fungicide applied to the outside of the plant is going to kill the fungus inside the plant.”

Giese recommends systemic fungicides in conjunction with contact for best results once dollar spot has reared its ugly head.

“If dollar spot symptoms are already detected, then fungicide treatment at this stage is considered curative,” he says. “The disease is in an advanced developmental stage and typically requires higher rates and shorter intervals of fungicide treatments.”

Combinations of contact and systemic fungicides help arrest foliar disease activity as well as preventing new infections, Giese adds.

Curative applications do require additional patience for symptoms to dissipate. Fungicides need time to control the pathogen, especially with systemic fungicides that must be absorbed into the plant, and time for the plant to recover from the infection so it can grow out of the existing disease scar. Because of these reasons, preventive applications are preferred when patience isn’t an option.

Dollar spot is one of the most common foliar diseases across much of the United States. Dykema says the Northeast and upper Midwest have been hit the hardest by dollar spot the past couple of years, mainly due to the milder-than-normal summers.

“Dollar spot thrives in moderate daytime temperatures, cool nights and high humidity which is what we have experienced the past couple of summers,” Dykema says. “In fact, in warmer summers in Michigan, we do not see severe outbreaks of dollar spot until August when the night time temperatures begin to drop. In recent years, the severe outbreaks have started in June and continued through the summer into September.”

While expectations for 2016 are anyone’s guess, those who don’t learn history are doomed to repeat it.

“If 2015 taught us anything, we learned that weather can be ever-changing and hard to predict no matter where in the country you are located,” Giese says. “The key to avoiding dollar spot difficulties is to have a solid agronomic program heading into the season and to be prepared to adjust at any moment.”

Look to the “disease triangle” to avoid dollar spot ... and others.

“For disease to develop, there needs to be a pathogen present, a susceptible host and favorable environmental conditions,” Giese says. “All three must happen together for dollar spot to occur, if one or two legs of the proverbial stool are missing, the stool falls over. No disease.”

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