GENETIC-HETEROGENEITY OF LEUCOSTOMA SPECIES IN MICHIGAN PEACH ORCHARDS

The pathogens causing perennial canker of peach, Leucostoma spp., were characterized in Michigan orchards to identify which species and subg roups (cryptic species) were prevalent on a highly susceptible peach c ultivar, Loring, and a less susceptible cultivar, Redhaven. Four hundr ed and three strains of Leucostoma were isolated from cankers in three southwest Michigan orchards where 'Loring' occurred adjacent to 'Redh aven' in side-by-side plots. Based on colony morphology and small nucl ear rDNA size polymorphisms, three cryptic species were detected; 89% of the isolates were identified as L. persoonii phenetic group 1, 10% were L. cincta group 4, and 1% were L. persoonii group 2. Pathogen pro files differed significantly between cankers on small branches of 'lor ing' and 'Redhaven', and between cankers on small branches and trunks. Of 1,232 random pairings among isolates of L. persoonii group 1, 95% were vegetatively incompatible. A minimum of 11 and a maximum of 17 ma ternal lines were identified based upon mitochondrial DNA restriction fragment length polymorphisms among 69 isolates of L. persoonii group I from one split-cultivar block of 72 trees. Vegetative compatibility loci were randomly associated with maternal lines. Evidence of clonali ty was absent in L. persoonii group 1, as no correlation occurred betw een these two sets of independent genetic markers and no genotype was over-represented. The number of cryptic species, the number of materna l lines, and the frequency of incompatibility within maternal lines in dicate that considerable genetic variation exists within the Leucostom a populations in Michigan peach orchards, and that sexual recombinatio n is common.