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Abstract
Isozyme analysis was used to study the variation among isolates of Leucostoma causing canker on fruit trees. Seventy-seven isolates including 17 from teleomorphs were obtained from six Prunus species and Malus domestica in North America. Thirty-one alleles were resolved at eight putative loci and the distance and maximum parsimony methods of analysis found the two species of Leucostoma, L. persoonii and L. cincta, to be distinct. The two species had no similarity as measured by the coefficient of Jaccard. Parsimony analysis separated the species into two clades with a basal branch supported by 99.8% of the boot-strapped data sets. Fifty six isolates of Leucostoma clustered into six phenetic groups. Isolates corresponding to L. persoonii were clustered into three distinct groups, PG1, PG2, and PG3. Those corresponding to L. cincta were clustered into two similar groups on Prunus spp., PG4 and PG5, and a third group on Malus, PG6. Isozyme polymorphism was low within each of the three phenetic groups of L. persoonii, including PG1 that was geographically widespread on many host species. PG1 was distinct in culture morphology, having a lobate colony margin. Polymorphism was low within L. cincta PG6 on Malus but high within PG4 found on Prunus spp. Isolates from Malus were distinct in cultural characteristics and clustered in a separate branch from the other members of the L. cincia clade in 89% of the bootstrap trees. Leucostoma persoonii isolates were all able to grow at 33 C, a temperature at which growth ceased in L. cincta isolates. Isozyme polymorphism was evident among seven ascospore progeny of an L. cincta perithecium from Prunus, resolving the segregation of alleles during meiosis. Polymorphism was not evident among ascospore progeny or among 13 vegetative compatibility groups of L. persoonii. Two phenetic groups of L. persoonii and two of L. cincta were evaluated for relative virulence on inoculated peach trees. Each group contained some isolates of high virulence, except PG6 of L. cincta from Malus.