Genetic structure of Heterobasidion annosum in white fir mortality centersin California

The structure of Heterobasidion annosum populations was studied in 15 mixed -conifer sites in central and northern California. Study sites displayed mo rtality nf white fir trees in enlarging discrete patches (mortality centers ). At each site,fungal genotypes were defined by somatic compatibility test s. In two sites, further genetic and molecular analyses were performed on f ield genotypes and on homokaryons obtained by dedikaryotization of field he terokaryons. Isolates were found to be colonizing mostly the roots and the bole sapwood of while fir trees, and no significant infections of other tre e species were observed. Each mortality center was characterized by the pre sence of several fungal genotypes, all belonging to the S intersterilily gr oup. Both homokaryotic and heterokaryotic strains were present in all sites . Multiple genotypes were retrieved in individual trees or stumps. Out of 2 28 fungal genotypes, 86% were found only with in a single tree or stump, wh ile 14% had spread to adjacent trees. The two largest genotypes had diamete rs of 9 and 10 m, and had colonized five and nine trees, stumps, or both, r espectively. The maximum distance between two adjacent trees colonized by t he same genotype was 6 m, and a highly significant correlation was found be tween 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 tra ced to standing trees and not to stumps. The genetic analysis performed in two mortality centers revealed that most local genotypes had different mati ng alleles, and thus originated from unrelated basidiospores. In a few case s, the same mating allele was shared by two heterokaryons (n+n genome) or b y a homokaryon (9 genome) and a heterokaryon. Molecular analysis showed tha t nuclei bearing the same mating allele were identical, providing evidence that the two nuclei forming heterokaryons can act independently in the fiel d and can be shared among isolates, presumably via di-mon mating or by sepa rate matings of different portions of widespread homokaryons.