Analyses of RAPD data for detection of host specialization in Sclerotinia homoeocarpa

UPGMA analysis, principal component analysis, genetic diversity analysis and genetic distance analysis of RAPD data were used to assess the extent of host specialization in 50 isolates of S. homoeocarpa from five turfgrass hosts. In UPGMA analysis and principal component analysis, the occurrence of host specialization was not readily apparent based on visual inspection. Genetic diversity analysis showed significant differentiation among isolates from different host species (GST1⁄4 0·34, P <0·001). The strongest evidence for some degree of host specialization came from the statistical analysis of genetic distances among isolates. By grouping pairwise genetic distances between isolates based on their host species, and analysing for average distance within the same host species and among different host species, it was found that the average distance within species was less than among species (P< 0·0001). An analysis of molecular variance of the genetic distances among isolates found that 32·3% of the total variation was attributable to host species. It is concluded that these isolates of S. homoeocarpa showed a weak level of host specialization, which was not readily apparent by UPGMA or principal component analyses, but was revealed by genetic diversity analysis and statistical analysis of genetic distances among isolates. Inoculation tests on different host species and tests using a greater number of isolates are required to confirm the extent of specialization.