Dry it out

Dollar spot is a tough disease for golf course superintendents to control. Once it shows up on the course, you may never be completely rid of it, but you can manage it with smart practices. We asked Rick Latin, Ph.D., a plant pathology professor at Purdue University who researches disease management on golf courses, to bring us up-to-date with the industry’s response to an annual nemesis.

What is the status of dollar spot thus far in 2015?

Here in the Midwest and perhaps on the east coast, we’ve had our first flush, and I think that those superintendents who are satisfied with their control are those who applied an appropriate fungicide, perhaps in mid-May. Some people were caught off guard, and they are suffering some outbreaks on their fairways. There is still hope for them. They’ll probably apply a little more fungicide, and their recovery won’t be as quick, simply because the outbreak has already occurred.

What conditions help dollar spot thrive?

Mild temperatures of 50 degrees with ample moisture favor dollar spot. A combination of temperature and moisture are ideal for the infection to occur. The moisture can be in the form of precipitation, dew or condensed leaf wetness. It’s the length of time the leaves stay wet at a certain temperature that defines the extent to which infection will occur. If turf leans toward nitrogen deficiency, it may be a little more prone to disease and probably will be a little slower recovering. This situation occurs every single year unless we have a cold, dry spring.

What are some of the trends in combating dollar spot?

There has been a trend among superintendents of applying fungicides really early to control through the spring, but much of the research shows that fungicides applied just before the first flush are most effective.

Practices that reduce the length of the wet period on the leaves are helpful. An old standby that still works is dragging fairways to displace the dew and hasten the drying of the leaves. If you diminish the length of the wet period by two to four hours, the extent of the infection will be much less. Early morning mowing accomplishes the same thing on greens.

A lot of anecdotal evidence and some research suggest that lightweight rolling of turf will limit the amount of dollar spot infection. The mechanism involved is still uncertain, but there’s enough empirical evidence to say this practice can reduce dollar spot, especially on greens.

There’s a lot of interest in biological products—compounds that naturally occur in the environment. These products affect the growth of the pathogens or outcompete the pathogens, reducing disease pressure. The results are somewhat mixed. Fungicides are still very important, but these biological products can mitigate disease pressure.

We’ve also have some new multi-site fungicides added to the arsenal in the past three years, and they are significant. Before that, we only had one fungicide— chlorothalonil—that was a multi-site inhibitor with a low chance of resistance from dollar spot. All others were site specific with an increased chance of resistance. Fluazinam is one of the welcome additions to the multi-site compounds available to turf managers, especially those who are fighting populations that are already resistant to some of the older chemistries. The other two are in a relatively new class called SDHI compounds (Succinate Dehydrogenase Inhibitors). Thus far, there are no reports of resistance to the SDHI class by dollar spot populations, but that’s not to say that it won’t occur.

If controlling moisture is a strategy for managing dollar spot, then when is the best time to water?

I can’t tell you the best time to irrigate, but I can tell you the worst time. It’s the early evening hours. Even in a drought, there is natural condensation or dew at 8 or 9 p.m. If a superintendent begins to irrigate at 6 p.m. and the natural dew period begins at 9 p.m., he or she has essentially extended the dew period another three hours. That can mean the difference between a little bit and a lot of infection. Nighttime and early morning are good, and there are arguments for each, but I don’t think there is any debate about the fact that early evening watering is the worst for dollar spot.