Effects of isolates of Clarireedia jacksonii and Clarireedia monteithiana on severity of dollar spot in turfgrasses by host type

Dollar spot, caused by four Clarireedia species (formerly Sclerotinia homoeocarpa), is a devastating disease that affects many turfgrass species throughout North America. Proper classification and genetic diversity of Clarireedia spp. was debated for the past several decades until recently when the organism was reclassified in 2018. Previous studies have shown that there is little genetic diversity except for isolates obtained from warm season hosts in Florida and the southern United States. Our research confirms the presence of isolates obtained from cool-season (C3) turfgrass species which are consistent with the recently classified species Clarireedia jacksonii; and a second, distinct sub-group of isolates obtained solely from warm-season (C4) turfgrass species, which are consistent with the recently classified species Clarireedia monteithiana. Additionally, we have documented the coexistence of these two species throughout the transition zone of the United States, extending as far north as Virginia, with both species present among adjacent stands of C3 and C4 turfgrass species in the same locale. Despite previous documentation of genetically distinct isolates occurring in the United States, there have been no reports on the variability in growth of the two named species over a range of temperatures, nor have these species been evaluated for their ability to infect both C3 and C4 turfgrass species. The effects of temperature on the lack of in vitro isolate growth for both C. jacksonii and C. monteithiana isolates were most noticeable at 10 and 35 °C, where all isolates exhibited little to no growth after 60 h. Cross inoculation experiments showed that both species are capable of infecting and inciting disease on both the preferred and alternative host, and that C. jacksonii isolates are more virulent on both C3 and C4 host grasses than C. monteithiana isolates. The results of this study will have implications for dollar spot management strategies on bermudagrass as well as an increased need for turfgrass breeders to develop dollar spot resistance cultivars of bermudagrass, particularly for those used in climates extending north of the transition zone.