Worldwide Population Biology of Sclerotinia homoeocarpa from Common Turfgrass Hosts and Mating Systems of the Pathogen and Closely Related Fungi

Sclerotinia homoeocarpa F.T. Bennett causes dollar spot, the most economically important disease of turfgrass throughout the world. Reasons that managing this pathogen is a persistent challenge include its broad host range and the widespread development of fungicide resistance. New or improved options to manage dollar spot are needed, but basic information on this pathosystem is lacking. Here we present an analysis of the mating type genes and the population genetic structure of S. homoeocarpa to improve knowledge of the population biology of this fungus.;There is conflict in the literature between reports of fertile sexual reproduction of S. homoeocarpa obtained from the United Kingdom and the apparent sterility of S. homoeocarpa obtained worldwide. We sequenced and characterized the mating-type (MAT) locus of several recently-collected 'modern' strains causing dollar spot, historical type strains used to describe the fungus, and selected close relatives of the pathogen, and also developed a multiplex PCR assay to screen 1,019 modern isolates for mating-type. The modern strains have a heterothallic MAT locus organization, suggesting these strains are required to encounter an individual of the opposite mating type to sexually reproduce. However, while three of four historical type strains were previously reported to undergo sexual reproduction from single-ascospore cultures, we found that only one of the four had a homothallic MAT locus organization. Some isolates collected from southern California and Italy contained both MAT idiomorphs, providing additional evidence that S. homoeocarpa can maintain stable heterokaryons in nature. Clone-corrected mating-type distributions showed few deviations from an equal ratio when analyzed on a sample location or regional scale, but the interpretation of that result is unclear due to low sample sizes.;Sclerotinia homoeocarpa is endemic in most regions of the world and is reported to have distinct populations on C3 or C4 photosynthetic grass hosts, but there is little population-level evidence available to support suggested modes of dispersal or host specialization. Here, we developed 14 microsatellite loci for S. homoeocarpa and used them to investigate population genetic structure, migration, and recombination among 1,170 isolates of the pathogen obtained from 13 and 1 grass and sedge genera, respectively, from a total of 79 locations distributed among 5 continents and Oceania. We found hierarchical population structure with a strong distinction between isolates typically obtained from C3 or C4 grass hosts, and within the C4 clade weak but significant structure between mating-types and between isolates obtained from Cynodon or Paspalum spp. A total of 42% of 634 C3 clade isolates were represented by one of two haplotypes, each of which was sampled in all but one region in our sample. Weak geographic structure within C4 clade isolates was found at the regional level. Analysis of clone-corrected mating-type distributions showed few deviations from an equal ratio, but linkage equilibrium was detected at several C3 clade locations and select C4 locations.;In the first large-scale population genetic study of S. homoeocarpa , we provide strong evidence that this fungus has undergone long range dissemination followed by clonal amplification. Results from this work have improved our understanding of the basic biology of this fungus and could lead to methods to better manage dollar spot disease of turfgrass.