| Sclerotinia spp. are destructive and cosmopolitan plant pathogens causing diseases on more than 400 plant species. In the 2005-2006 growing season, widespread occurrence of stem and crown rot of chickpea was observed in the Central and Sacramento Valleys of California. Previously S. minor and S. sclerotiorum were reported to infect chickpea in the US. However, the majority of Californian isolates differed from the reported species in growth rate and oxalic acid production. Isolates were characterized based on presence or absence of ascospore dimorphism, rDNA group I introns and ITS sequences. Most Californian isolates were identified as Sclerotinia trifoliorum. This is the first report of S. trifoliorum infecting chickpea in North America. Since there are no existing molecular markers available to study the population biology of S. trifoliorum, efforts were made to develop microsatellite markers suitable for S. trifoliorum by constructing a microsatellite-enriched library. Thirty-three microsatellite loci were developed, the majority of which were transferable to S. sclerotiorum and to a less extent for S. minor, but not for S. homoeocarpa. This shows that S. homoeocarpa is genetically distant from other members of the genus Sclerotinia.;Microsatellite markers, mycelial compatibility groupings (MCG), rDNA introns and sequence related amplified polymorphism (SRAP) markers were used to study the diversity and genetic structure of a collection of 136 isolates of S. trifoliorum. The 136 isolates displayed high levels of genetic diversity. For example, 116 microsatellite haplotypes were found among the 136 isolates, with most haplotypes consisting of single isolates. No correlation between genotype and geography was found, and related genotypes were distributed across the sampling area, strongly suggesting human-assisted dispersal.;We tested the hypothesis of monocyclic spread in S. trifoliorum by strain typing 57 sclerotial isolates derived from crown infected plants in 17 disease foci. Although the existence of plant-to-plant spread was supported based on chi-square tests of the distribution of MCGs in infected plants, the majority of the isolate-pairs from adjacent plants were of different haplotypes and MCGs, suggesting that plant-to-plant spread is of secondary importance in the epidemic and S. trifoliorum is primarily a monocyclic pathogen of chickpea. |
