By: Lane Tredway
Associate Professor and Extension Specialist
Department of Plant Pathology
North Carolina State University
Dollar spot, caused by the fungus Sclerotinia homoeocarpa, is the most important turfgrass disease worldwide. Dollar spot develops all over the world, and can infect every species of grass that is grown for turf. According to some estimates, 40% of fungicide applications made to turf are for the control of this single disease. Unfortunately, this fungus develops fungicide resistance very quickly, and this severely limits the number of tools that superintendents have at their disposal to control dollar spot.
Despite its importance, very little is known about dollar spot or the fungus that causes it. We don’t know how the fungus spreads, where it survives when it’s not causing disease, the specific conditions that trigger its development, or how it develops resistance to fungicides so quickly. Heck, we don’t even know what the correct Latin name is for the Dollar Spot pathogen. We know that it isn’t really a Sclerotinia, but we don’t know what else to call it!
To fill these gaps in our knowledge, my research program has begun to focus almost exclusively on Dollar Spot, and in particular, the development of fungicide resistance. First, we need to understand how this pathogen spreads, survives, and reproduces. So for the last few years, we’ve been traveling the world to sample dollar spot populations from different turf species for genetic analysis. So far, we’ve amassed over 3000 isolates from the US, Dominican Republic, Argentina, Chile, United Kingdom, and Japan. Collecting trips to South Africa and Southeast Asia are in the plans for next year.
A few years ago, I had the opportunity to visit with Les Jeremiah on Lanai, along with Dr. Micah Woods from the Asian Turfgrass Center. We toured The Experience at Koele and The Challenge at Manele and collected samples of dollar spot from Creeping Bentgrass, Hilograss, and Seashore Paspalum. So far, we’ve learned a lot about the Dollar Spot pathogen from our work with these Hawaiian isolates.
We were very surprised to find that the isolates we obtained from Creeping Bentgrass in Hawaii are nearly identical to those that we obtained from cool-season grasses all across the continental United States. Similarly, the isolates we obtained from Seashore Paspalum and Hilograss in Hawaii are nearly identical to our Bermudagrass isolates from North Carolina. Since then, we’ve found similar trends in other parts of the world such as Europe and South America. These results tell us that populations of this pathogen are very uniform, that the pathogen spreads long distances quite readily, and that techniques we develop to prevent the development of fungicide resistance may be useful in many parts of the world.
We have found that isolates from Cool-Season grasses are different genetically from those that infect Warm-Season grasses, although some cross-infection can occur in locations like The Challenge at Manele where Warm and Cool-Season grasses are growing in close proximity. In the long term, we may be able to take advantage of this host specificity to develop Creeping Bentgrass or Seashore Paspalum varieties with improved resistance to Dollar Spot.
Finally, we’ve also detected evidence of genetic recombination in populations of the Dollar Spot pathogen. Genetic recombination can either sexually or asexually in fungi, and this is one way that genetic diversity evolves in fungal populations. This may help to explain how the Dollar Spot pathogen develops resistance to fungicides so quickly.
We are just starting to learn about the Dollar Spot pathogen, how it spreads, where it survives, and how it develops resistance to fungicides. This is a long term endeavor, but we are committed to developing practical solutions to control this major turf disease. We appreciate the hospitality that Les Jeremiah and everyone else in Hawaii provided us during our collection trip!