Compatibility Among Host - Specialized Isolates of Heterobasidion annosum from Western North America

Harrington, T. C., Worrall, J. J., and Rizzo, D. M. 1989. Compatibility among host-specialized isolates of Heterobasidion annosum from western North America. Phytopathology 79:290-296. Host-specialization of isolates of Heterobasidion annosum from from another 18 trees and stumps in Pacific Coast states also fell into the S California was previously demonstrated by inoculating Pinus ponderosa and P groups based on compatibility; those from diseased pine trees were and Abies concolor (white fir) seedlings in the greenhouse. In the present P-compatible and those from other host genera were S-compatible. study, 18 of these isolates were tested for sexual compatibility in heteroHowever, compatibility of isolates and strains from stumps with S and P karyon-homokaryon pairings with six S and four P tester strains of H. testers was not consistent with the host genus, indicating that stump annosum from Europe representing two intersterility groups. colonization can be non-host specific. When North American strains were Compatibility was determined by the presence of clamp connections in paired among themselves or with heterokaryotic isolates, the two subcultures from the homokaryotic tester mycelia taken 5 wk after pairing, compatibility groups were less evident than when European strains were The nine isolates specialized to fir seedlings were compatible with S testers, used. Reports thus far suggest that both the S-compatible and Pand the nine isolates specialized to pine seedlings were compatible with P compatible groups are widespread in western North America, but only the testers. Some P-compatible isolates formed clamps with a few S testers. P group has been reported in eastern North America. Heterokaryotic isolates and homokaryotic (single-basidiospore) strains Additional keywords: Annosum root rot, fungal genetics, host specialization. Heterobasidion annosum (Fr.) Bref. (syn. Fomes annosus (Fr.) hyphae (6). Sexual compatibility among isolates can be tested by Cke.) causes an important root and butt rot of conifers throughout pairing primary (homokaryotic) mycelia against either primary or the northern temperate zone. Although the host range of the secondary (heterokaryotic) mycelia and examining hyphae of the fungus is broad, including some hardwood species, mortality is homokaryon for clamp connections. In pairings among homomost significant on pine (Pinus) species, in which the fungus is karyons derived from single basidiospores (herein referred to as capable of spreading rapidly in the cambium and phloem tissues in strains), Korhonen found intersterility between two Finnish popuadvance of sapwood colonization and girdles the lower stem (4). In lations of H. annosum, and designated the two populations as S contrast, colonization of most other conifer hosts, such as species and P types. Presumably heterokaryotic isolates from decay of fir (Abies) and spruce (Picea), is frequently restricted to (herein referred to as isolates) in a number of hosts around the heartwood tissues or the oldest growth rings of the sapwood in the world were tested for compatibility with homokaryotic testers of largest lateral roots and at the base of the tree. Losses due to decay the S and P groups by looking for changes in cultural morphology of the butt log in these non-pine species can be substantial, and of the testers after pairing. Most of the isolates from Pinus species mortality may occur, though not as rapidly as in pine species. were compatible with P tester strains, and most isolates from Differences in host colonization patterns appear to be hostNorway spruce (Picea abies (L.) Karst.) were compatible with S mediated (10). In addition to pathogenic colonization, saprophytic tester strains. Of the 10 isolates tested from North America, all colonization may result from deposition of basidiospores onto were from pines and compatible with P tester strains. Chase (1) freshly cut stump tops (8). Clones of the fungus can spread from examined a number of North American homokaryotic strains of root systems of colonized stumps or diseased trees to those of H. annosum, and nearly all were compatible with either S or P adjacent trees via root contacts or grafts (2,8). strains from Finland. Most of the North American strains Host preferences of isolates of H. annosum from pines and white compatible with P testers were from species of pine, and most of fir (Abies concolor (Gord. & Glend.) Lindl.) were demonstrated in the strains compatible with S testers were from non-pine species. a seedling inoculation study. In two experiments, Worrall et al (10) In our initial pairings among California isolates and strains, a found that isolates from Pinus spp. infected and killed more high frequency of compatibility was found between strains from fir ponderosa pine (P. ponderosa Laws.) seedlings than fir seedlings; and pine trees. Although we tentatively concluded that the two isolates from diseased fir trees, in contrast, infected and killed intersterility groups found in Europe were not homologous with about the same proportion of pine and fir seedlings. A preliminary the two host-specialized groups in California, Chase's (1,3) results study of allozyme variation among California fir and pine isolates presented an alternative interpretation. For compatibility, two also suggested differences between the two groups (7). Indirect strains must be heterogenic at the mating type locus, as evidence for host specialization had been obtained in earlier demonstrated by Korhonen (6), and must share a particular allele mating experiments by Korhonen (6). at one or more of five previously unknown compatibility loci. The pathogen is heterothallic over most of its range, and mated, Alleles at two of these loci (the S and P loci) appeared to correlate secondary mycelia form clamp connections on at least some of the with the capacity of the Finnish tester strains to differentiate the S and P intersterility groups. With Chase's genetic model and Finnish tester strains, we reexamined compatibility among the _____________________________________________ isolates from California that were demonstrated to be host© 1989 The American Phytopathological Society specialized (10).