Influence of Host and Geographic Locale on the Distribution of Colletotrichum cereale Lineages

Abstract

Colletotrichum cereale is an ascomycete inhabitant of cool-season Pooideae grasses. The fungus has increased in frequency over the past decade as a destructive pathogen of Poa annua and Agrostis stolonifera turfgrass. Colletotrichum cereale exists as two lineages, designated clades A and B, but little is known about the distribution of these clades in natural environments, or what role these subdivisions may play in the trajectory of disease outbreaks. In this study, our objective was to determine the frequency of C. cereale clades A and B. To rapidly discriminate between the two C. cereale clades, a real-time PCR assay was developed based on the Apn2 gene. A collection of 700 C. cereale pathogens and endophytes from twenty Pooideae grass genera were genotyped. 87% of the collection was identifed as part of clade A, 11.7% as part of clade B, and 1.3% was a mixture. Colletotrichum cereale from turfgrass hosts in North America were most commonly members of clade A (78%). The overabundance of clade A in turfgrass isolates was directly attributable to the dominance of this lineage from southern sampling sites, irrespective of host. In contrast, 111 C. cereale turfgrass isolates collected from northern sampling sites were evenly distributed between clades A and B. Only 28% of C. cereale from A. stolonifera at northern sampling sites were part of clade A. These data show that environmental factors such as geographic location and host identity likely played a role in the distribution of the major C. cereale clades in North American turfgrass.

Peer Review #3 of "Development of a greenhouse-based inoculation protocol for the fungus Colletotrichum cereale pathogenic to annual bluegrass (Poa annua) (v0.1)"

“…C. cereale was re-isolated from inoculated tissue after symptom development, and 3). inoculated plants were tested for the presence of C. cereale using real-time PCR, C. cereale clade-specific assays developed using the Apn2 marker (Beirn, Clarke & Crouch 2014). This assay confirms to the Minimum Information for Publication of Quantitative Real-Time PCR Experiments guideline; complete details describing the assay can be found in Beirn, Clarke & Crouch (2014).…”

“…Clade A predominates in the southern U.S., regardless of host (Beirn, Crouch & Clarke, 2014). In contrast, clade A and clade B isolates have been found in equal frequencies and both lineages can occur on both hosts in the northern U.S.; however, clade A isolates are found more frequently on P. annua and clade B is more common on A. stolonifera (Beirn, Clarke & Crouch, 2014). It is currently not known whether these population-scale differences affect infectivity or the ability of the pathogen to colonize different host species, but this association of specific genotypes in natural populations could explain the difficulties surrounding the development of a consistent, reproducible inoculation protocol for C. cereale.…”

“…However, the use of detached leaf assays only provides information about the process of substrate utilization, and is unlikely to mirror the host-pathogen interaction that take place between C. cereale and intact grass plants. Colletotrichum pathogens of grasses utilize a hemibiotrophic strategy to colonize hosts, which is characterized by an initial stage of quiescent biotrophy, followed by a shift to necrotrophic colonization and degradation of host tissue, resulting in unique waves of differential gene expression taking place during each phase (Crouch & Beirn 2009;O'Connell et al 2012;Crouch et al 2014). Gene expression studies of the hemibiotrophic infection strategy have revealed significantly different expression profiles in…”

“…stolonifera putting greens, the distribution of clades A and B is strongly influenced by geographic and host origin (Beirn, Crouch & Clarke, 2014). Clade A predominates in the southern U.S., regardless of host (Beirn, Crouch & Clarke, 2014).…”

“…stolonifera putting greens, the distribution of clades A and B is strongly influenced by geographic and host origin (Beirn, Crouch & Clarke, 2014). Clade A predominates in the southern U.S., regardless of host (Beirn, Crouch & Clarke, 2014). In contrast, clade A and clade B isolates have been found in equal frequencies and both lineages can occur on both hosts in the northern U.S.; however, clade A isolates are found more frequently on P. annua and clade B is more common on A. stolonifera (Beirn, Clarke & Crouch, 2014).…”