Clarireedia: A new fungal genus comprising four pathogenic species responsible for dollar spot disease of turfgrass

Dollar spot is one of the most destructive and economically important fungal diseases of amenity turfgrasses. The causal agent was first described in 1937 as the ascomycete Sclerotinia homoeocarpa. However, the genus-level taxonomic placement of this fungus has been the subject of an ongoing debate for over 75 y. Existing morphological and rDNA sequence evidence indicates that this organism is more appropriately placed in the family Rutstroemiaceae rather than the Sclerotiniaceae. Here we use DNA sequence data from samples of the dollar spot fungus and other members of the Rutstroemiaceae (e.g. Rutstroemia, Lanzia, Lambertella) collected throughout the world to determine the generic identity of the turfgrass dollar spot pathogen. Phylogenetic evidence from three nucleotide sequence markers (CaM, ITS and Mcm7; 1810-bp) confirmed that S. homoeocarpa is not a species of Sclerotinia; nor is it a member of any known genus in the Rutstroemiaceae. These data support the establishment of a new genus, which we describe here as Clarireedia gen. nov. The type species for the genus, Clarireedia homoeocarpa comb. nov., is described to accommodate the dollar spot fungus, and a neotype is designated. Three new species in this clade, Clarireedia bennettii sp. nov., Clarireedia jacksonii sp. nov., and Clarireedia monteithiana sp. nov. that also cause dollar spot disease are described. Clarireedia homoeocarpa and C. bennettii occur primarily on Festuca rubra (C3 grass) hosts and appear to be restricted to the United Kingdom. Clarireedia jacksonii and C. monteithiana occur on a variety of C3 and C4 grass hosts, respectively, and appear to be globally distributed. This resolved taxonomy puts to rest a major controversy amongst plant pathologists and provides a foundation for better understanding the nature and biology of these destructive pathogens.

Characterization and distribution of mating-type genes of the turfgrass pathogen Sclerotinia homoeocarpa on a global scale

Sclerotinia homoeocarpa F.T. Bennett is a filamentous member of Ascomycota that causes dollar spot, the most economically important disease of turfgrass worldwide. We sequenced and characterized the mating-type (MAT) locus of four recently-collected contemporary strains causing dollar spot, four historical type strains used to describe the fungus, and three species of Rutstroemiaceae. Moreover, we developed a multiplex PCR assay to screen 1019 contemporary isolates for mating-type. The organization of the MAT loci of all strains examined could be classified into one of four categories: (1) putatively heterothallic, as exemplified by all contemporary strains and three of four historical type strains; (2) putatively heterothallic with a deleted putative gene in the MAT1-2 idiomorph, as detected in strains from two recently-collected populations in the United Kingdom that show more similarity to historical strains; (3) putatively homothallic with close physical linkage between MAT1-1-1 and MAT1-2-1, as found in one historical type strain of S. homoeocarpa and two strains of Rutstroemia cuniculi; and (4) an unresolved but apparently homothallic organization in which strains contained both MAT1-1-1 and MAT1-2-1 but linkage between these genes and between the two flanking genes could not be confirmed, as identified in R. paludosa and Poculum henningsianum. In contemporary S. homoeocarpa populations there was no significant difference in the frequency of the two mating types in clone-corrected samples when analyzed on regional and local scales, suggesting sex may be possible in this pathogen. However, two isolates from Italy and twenty from California were heterokaryotic for both complete heterothallic MAT idiomorphs. Results from this study contribute to knowledge about mating systems in filamentous fungi and enhance our understanding of the evolution and biology of an important plant pathogen.

Sclerotinia homoeocarpa overwinters in turfgrass and is present in commercial seed

Dollar spot is the most economically important disease of amenity turfgrasses in the United States, yet little is known about the source of primary inoculum for this disease. With the exception of a few isolates from the United Kingdom, Sclerotinia homoeocarpa, the causal agent of dollar spot, does not produce spores. Consequently, it was assumed that overwintering of this organism in soil, thatch, and plant debris provides primary inoculum for dollar spot epidemics. Overwintering of S. homoeocarpa in roots and shoots of symptomatic and asymptomatic creeping bentgrass turfgrass was quantified over the course of a three-year field experiment. Roots did not consistently harbor S. homoeocarpa, whereas S. homoeocarpa was isolated from 30% of symptomatic shoots and 10% of asymptomatic shoots in the spring of two out of three years. The presence of stroma-like pathogen material on leaf blades was associated with an increase in S. homoeocarpa isolation and colony diameter at 48 hpi. Commercial seed has also been hypothesized to be a potential source of initial inoculum for S. homoeocarpa. Two or more commercial seed lots of six creeping bentgrass cultivars were tested for contamination with S. homoeocarpa using culture-based and molecular detection methods. A viable, pathogenic isolate of S. homoeocarpa was isolated from one commercial seed lot and contamination of this lot was confirmed with nested PCR using S. homoeocarpa specific primers. A sensitive nested PCR assay detected S. homoeocarpa contamination in eight of twelve (75%) commercial seed lots. Seed source, but not cultivar or resistance to dollar spot, influenced contamination by S. homoeocarpa. Overall, this research suggests that seeds are a potential source of initial inoculum for dollar spot epidemics and presents the need for further research in this area.

RNA-Seq analysis of the Sclerotinia homoeocarpa--creeping bentgrass pathosystem

Sclerotinia homoeocarpa causes dollar spot disease, the predominate disease on highly-maintained turfgrass. Currently, there are major gaps in our understanding of the molecular interactions between S. homoeocarpa and creeping bentgrass. In this study, 454 sequencing technology was used in the de novo assembly of S. homoeocarpa and creeping bentgrass transcriptomes. Transcript sequence data obtained using Illumina's first generation sequencing-by-synthesis (SBS) were mapped to the transcriptome assemblies to estimate transcript representation in different SBS libraries. SBS libraries included a S. homoeocarpa culture control, a creeping bentgrass uninoculated control, and a library for creeping bentgrass inoculated with S. homoeocarpa and incubated for 96 h. A Fisher's exact test was performed to determine transcripts that were significantly different during creeping bentgrass infection with S. homoeocarpa. Fungal transcripts of interest included glycosyl hydrolases, proteases, and ABC transporters. Of particular interest were the large number of glycosyl hydrolase transcripts that target a wide range of plant cell wall compounds, corroborating the suggested wide host range and saprophytic abilities of S. homoeocarpa. Several of the multidrug resistance ABC transporters may be important for resistance to both fungicides and plant defense compounds. Creeping bentgrass transcripts of interest included germins, ubiquitin transcripts involved in proteasome degradation, and cinnamoyl reductase, which is involved in lignin production. This analysis provides an extensive overview of the S. homoeocarpa-turfgrass pathosystem and provides a starting point for the characterization of potential virulence factors and host defense responses. In particular, determination of important host defense responses may assist in the development of highly resistant creeping bentgrass varieties.

Evidence for morphological, vegetative, genetic, and mating-type diversity in Sclerotinia homoeocarpa

Morphology, vegetative compatibility groups, and molecular characteristics were compared among 47 isolates of the dollar spot pathogen Sclerotinia homoeocarpa. Isolates were collected from cool- and warm-season turfgrasses in Florida and the northern United States. Mycelial pigment accumulation, substratal stromata formation, and symptom development were used to separate the collection into two distinct morphological types: a common-type (C-type) and a Floridian-type (F-type). Phylogenetic relationships estimated from ITS sequences supported the morphological typing. Identification and characterization of the S. homoeocarpa mating-type locus revealed an idiomorphic organization for both C- and F-types with nearly equal frequencies of each mating types present in both groups. These findings suggest heterothallic control of mating and indicate potential for outcrossing in both groups. Dollar spot disease of turfgrass in Florida is caused by two distinct morphological types of S. homoeocarpa which may be cryptic species. These findings could have implications for disease management.