Compatibility Among Host-Specialized Isolates ofHeterobasidion annosumfrom Western North America

Abstract

Host-specialization of isolates of Heterobasidion annosum from California was previously demonstrated by inoculating Pinus ponderosa and Abies concolor (white fir) seedlings in the greenhouse. In the present study, 18 of these isolates were tested for sexual compatibility in heterokaryon-homokaryon pairings with six S and four P tester strains of H. annosum from Europe representing two intersterility groups. Compatibility was determined by the presence of clamp connections in subcultures from the homokaryotic tester mycelia taken 5 wk after pairing. The nine isolates specialized to fir seedlings were compatible with S testers, and the nine isolates specialized to pine seedlings were compatible with P testers. Some P-compatible isolates formed clamps with a few S testers. Heterokaryotic isolates and homokaryotic (single-basidiospore) strains from another 18 trees and stumps in Pacific Coast states also fell into the S and P groups based on compatibility; those from diseased pine trees were P-compatible and those from other host genera were S-compatible. However, compatibility of isolates and strains from stumps with S and P testers was not consistent with the host genus, indicating that stump colonization can be non-host specific. When North American strains were paired among themselves or with heterokarytoic isolates, the two compatibility groups were less evident than when European strains were used. Reports thus far suggest that both the S-compatible and P-compatible groups are widespread in western North America, but only the P group has been reported in eastern North America.