New Insights to the Population Biology of the Dollar Spot Pathogen:

Implications of Host Adaptation

By Wakar Uddin, Ph.D. and Brian Aynardi, Ph.D.

Symptoms of dollar spot are readily recognizable on low-cut turf, such as golf course tees, fairways, and putting greens, producing cream-colored necrotic lesions which result in necrotic spots of 2–5cm (approx. 0.5 –2 inches) in diameter. The spots reduce turf quality in both aesthetic value and playability. As the disease progresses the individual necrotic spots coalesce to cause large areas of high amenity turf to become blighted and form depressed or sunken areas that affect ball roll and green speeds. In higher cut turf such as in residential lawns and sports fields, the necrotic areas are typically much larger, extending to several inches in diameter.

Nearly all turfgrass species used in golf courses, athletic fields, residential lawns, business parks, and sod-production areas are susceptible to dollar spot disease. The identity of the pathogen that causes dollar spot across the United States has been debated for decades, essentially since the disease was first described. Advancements in fungal genetics in the 1990s and early 2000s used molecular methods to determine that the pathogen causing dollar spot, Sclerotinia homoeocarpa, was not a member of the Sclerotiniaceae family, but rather the Rutstroemiaceae family. In 2018, published research using isolates collected from around the world studied various molecular markers to split what was once thought to be a single species into four species under a new genus, Clarireedia. Only two of the four species are predominately found in the United States, but also have global distribution; Clarireedia jacksonii occurring on cool season (C3) grasses and Clarireedia monteithiana occurring on warm season (C4) grasses.

While looking at the taxonomic issues facing S. homoeocarpa, we also concurrently focused on the genetic diversity among isolates infecting cool and warm-season turfgrass species. Previously, research studies have shown that there is little genetic diversity in the pathogen population except for those infecting warm season grass hosts in Florida and the southern United States. After collecting isolates throughout the transition zone of the United States from 2011–2016, and also examining isolates from our pools of collections dating back to the 1970s, our research was able to clearly demonstrate that there are two genetically distinct types of the pathogen isolates causing dollar spot in North America: Type I, consisting primarily of isolates collected from cool-season turfgrass species, and Type II, a distinct subgroup of isolates solely from warm-season turfgrass species. While this finding is consistent with recent literature, we have documented the coexistence of these two isolate types throughout the transition zone of the United States, extending as far north as Virginia, with both isolate types present among adjacent stands of cool and warm-season turfgrass species in the same locale. There have been no reports on the variability in growth of the isolate types over a range of temperatures, nor have the isolate types been previously evaluated for their ability to infect both cool and warm-season turfgrass species.

Figure 1: Coalescing of necrotic spot causing larger dead areas on bentgrass fairway (on left side).

This work addressed important questions concerning the growth of Type I and Type II isolates of Clarireedia spp. over a range of temperatures consistent with field conditions in various geographic regions. Results from the in vitro growth studies showed minimal growth at 10°C (50°F), significant growth from 20–30°C (68–86°F), and a near complete lack of growth at 35°C (95°F). Further, our study evaluated the ability of both isolate types to incite disease on both cool and warm-season turfgrass species, which is an important insight into the biology of the pathogen and may have deep implications in disease management. Results from the crossinoculation experiments indicate that both isolate types are capable of inciting disease on both cool and warm-season turfgrass species. Our results suggest that Type I isolates are more virulent than Type II isolates at 25 (77°F) and 30°C. This finding may explain why cool-season turfgrass is subjected to more severe outbreaks of dollar spot, excluding the extended duration of optimal environmental conditions in areas where cool-season turfgrass species thrive.

Biologically, the temperatures we studied in the 20 to 30°C range are frequently observed in the mid-Atlantic and Northeastern United States during early to mid-summer and again late-summer to early-fall, when dollar spot is widely found on turfgrass. Our data support the near complete lack of growth from all isolates at 35°C. These results may explain why dollar spot has a binomial disease distribution during the growing season in the United States, particularly in the South (approximate latitude from 24 to 37°N). Isolates from this study that did not grow at 35°C in the controlled environment chamber resumed growth when exposed to decreased temperature of 22°C (72°F). We infer this is why a second dollar spot epidemic is readily occurring throughout the United States.

From the standpoint of a disease management strategy, these results present a potential problem for using bermudagrass in northern climates, especially in states at the northern tier of the transition zone. In our study, Type I isolates were more virulent than Type II isolates on both cool and warm season grasses. The movement of bermudagrass further north from Virginia may result in increased disease severity of dollar spot on warm-season species. This should be of significant interest to turfgrass breeders developing bermudagrass cultivars for use in northern locations. In the 2013–14 Progress Report published by the National Turfgrass Evaluation Program (NTEP), the only site where dollar spot data were collected on bermudagrass cultivars was in Jay, FL (NTEP, 2014). Expanding the locations of evaluations of dollar spot data collection sites should be included in future reports, particularly in transition zone states. This will serve as useful information for golf course superintendents and turfgrass managers during consideration of turfgrass species selection. In chemical control management strategy, to our knowledge, there were no studies comparatively examining the effect of fungicides on Type I versus Type II isolates. Therefore, it is likely that the same fungicide classes provide acceptable field control at this time work on both populations, with the exception of regularly occurring resistant populations. Optimal classes for dollar spot control include the SDHIs (succinate dehydrogenase inhibitors), DMIs (demethylation inhibitors), chloronitriles, dicarboximides, dinitroanilines, and methyl benzimidazole carbamates (MBCs).

Dr. Wakar Uddin is a professor of Plant Pathology and Dr. Brian Aynardi is former graduate research assistant in the Department of Plant Pathology and Environmental Microbiology at Penn State University, University Park, PA 16802.