The distribution of genets and their nuclear composition reveal the dynamics of establishment and survival of Heterobasidion annosum in white fir stands
The structure of Heterobasidiom annosum populations in white fir was studied in 15 mixed conifer sites of central and northern California. Areas selected for the study 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. No significant infections of other tree species were encountered, and the majority of white firs displayed disease symptoms associated with the presence of the pathogen. Each mortality center was characterized by the presence of several fungal genotypes, all belonging to the S ISG. Both homokaryotic and heterokaryolic strains were present in all sites. Eighty-six percent of fungal genotypes were found only within a single tree or stump, while 14 percent had spread to adjacent trees. The two largest genotypes had diameters of 9-2010 m and had colonized 5-9 trees and stumps. 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. In most cases, original infection courts of existing genotypes could he traced to standing trees and not to stumps that became colonized after felling. 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 different genotypes. 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.