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    Description

    The structure of Heterobasidion annosum populations was studied in 15 mixed-conifer sites in central and northern California. Study sites dis­played mortality of white fir trees in enlarging discrete patches (mortality centers). At each site, fungal genotypes were defined by somatic compat­ibility tests. In two sites, further genetic and molecular analyses were per­formed on field genotypes and on homokaryons obtained by dedikaryotization of field heterokaryons. Isolates were found to be colonizing mostly the roots and the bole sapwood of white fir trees, and no significant in­fections of other tree species were observed. Each mortality center was characterized by the presence of several fungal genotypes, all belonging to the S intersterility group. Both homokaryotic and heterokaryotic strains were present in all sites. Multiple genotypes were retrieved in individual trees or stumps. Out of 228 fungal genotypes, 86 percent were found only with­in a single tree or stump, while 14 percent had spread to adjacent trees. The two largest genotypes had diameters of 9 and 10 in., and had colonized five and nine trees, stumps, or both, respectively. The maximum distance be­tween two adjacent trees colonized by the same genotype was 6 m, and a highly significant correlation was found between tree diameter and dis­tance of fungal "vegetative" spread. The largest clones were found in areas characterized by high tree and stump densities, and secondary spread of the fungus was more significant in denser stands. In most cases, original infection courts of existing genotypes could be traced to standing trees and not to stumps. The genetic analysis performed in two mortality cen­ters revealed that most local genotypes had different mating alleles, and thus originated from unrelated basidiospores. In a few cases, the same mating allele was shared by two heterokaryons (n+n genome) or by a homokaryon (n genome) and a heterokaryon. Molecular analysis showed that nuclei bearing the same mating allele were identical, providing evi­dence that the two nuclei forming heterokaryons can act independently in the field and can be shared among isolates, presumably via di-mon mating or by separate matings of different portions of widespread homokaryons.

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    Citation

    Garbelotto, M.; Cobb, F.W.; Bruns, T.D.; Otrosina, William J.; Popenuck, Tina; Slaughter, Garey 1999. Genetic structure of Heterbasidion annosum in white fir mortality centers in California. Phytopathology 89: 546-554.

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