Dollar spot is one of our most important but least understood turfgrass diseases. Superintendents spend significant dollars battling the disease and trying to avoid fungicide resistance. Yet for all we know about when the disease occurs and how to control it, we know very little about the biology of this pathogen and how it spreads.

Having a better understanding about the biological processes that affect where dollar spot occurs and how it spreads could ultimately result in the turfgrass manager more effectively managing dollar spot, applying control products only where they are needed and spending less on fungicide applications in the process.

Some of the questions I addressed in my research included: Does dollar spot occur in a pattern? How does that pattern (if it occurs) change over a season? What are the management issues raised by the results? The answers to these questions will lead to a better understanding of this important turfgrass pathogen.

Is there a pattern?

Dollar spot is caused by the fungal pathogen,Sclerotinia homoeocarpa. This pathogen infects both cool-season and warm-season grasses and is somewhat unique among the turfgrass pathogens because it is not known to produce spores of any kind. Without spores to move the pathogen around, it is believed that dollar spot moves from place to place via infected plants transported on equipment or on the bottom of our shoes.

So, to answer some of these questions, a research area was established at the Robert Hancock Turfgrass Research Center at Michigan State University in East Lansing. The study area was 30 feet by 60 feet and was comprised of a grid of 200 sampling locations in 2000 and 888 sampling locations in 2001 and 2002.

Dollar spot epidemics were followed each season from 2000-2002, and the number of dollar spots occurring at each of the sampling locations were counted twice per week. Over the course of the study, over 81,000 dollar spots were counted.

Figure 1A

Once the number and location of the dollar spots were known, the pattern (or lack thereof) the spots was measured. Statistical tools called geostatistics were used to determine if the dollar spots were occurring in a pattern. These tools were originally developed to determine the size and shape of ore bodies and petroleum reserves buried deep underground.

Figure 1B

The primary tool, called a variogram, is used to summarize how the trait one measures (in our case, the number of dollar spots) varies with location in space using a measure called semivariance. This value is plotted to graphically show how similar two locations separated by some distance are to each other. Locations separated by shorter distances tend to be more similar and consequently have a smaller semivariance than locations separated by larger distances. If there are differences in semivariance values between locations separated by short vs. longer distances this indicates that the measured value has a spatial pattern. Our semivariance plots showed that dollar spot did occur in a pattern (Figures 1a and 1b).

How does the pattern change?

Quick Tip

Over the course of a growing season, the number of dollar spots that occur in an area increases and decreases (Figure 2). Presumably, this is due to environmental conditions that affect the appearance of disease, the growth and vigor of the host plant and the virulence of the pathogen.