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Pathogen Biology

Armillaria is in the phylum Basidiomycota. Basidiospores are produced in mushrooms, specifically on basidia that line the gills on the underside of the cap (see Symptoms and Signs for more information on mushrooms).

Armillaria species have an unusual nuclear cycle that is not yet completely understood. According to most studies, they differ from related members of Basidiomycota in that the mycelia, after mating, are predominantly diploid (2n) rather than heterokaryotic (different haploid nuclei sharing a mycelium).

Sexual Reproduction

Sexual compatibility among single-basidiospore isolates is regulated by two mating-type loci, each with multiple alleles in the population. Isolates must have different alleles at both loci to be sexually compatible. If they are, the hyphae can fuse, then nuclei fuse shortly afterwards to establish a diploid mycelium. Such a mycelium is the dominant phase that one finds growing in wood, growing through soil as rhizomorphs, and killing trees. Eventually, whcn conditions are suitable, mushrooms are produced. There may be, in some species, some intermediate nuclear changes, but ultimately, in special cells (basidia) lining the gills of the mushroom, meiosis results in the formation of four haploid nuclei. Each nucleus enters a developing basidiospore, completing the sexual cycle.

Asexual Reproduction

Armillaria species do not produce asexual spores, but they are able to disperse locally and colonize new trees. They do this either by growing as mycelium through root contacts or root grafts between two trees, or by growing through the soil as rhizomorphs (1-5 mm in diameter) to a nearby tree (Figure 7).

Figure 7

Species

Although many species of Armillaria had been described over the years, their characteristics and differences were uncertain. Plant pathologists recognized there was variability in the pathogen, but difficulty in resolving that variability resulted in lumping the pathogen under the species A. mellea. This changed after it was demonstrated that there were intersterile groups, or biological species, of Armillaria. Within a group, mating followed a normal mating system, but generally no mating occurred between the groups. Gradually, often subtle differences between the groups in host preference, aggressiveness, geography, mushroom morphology, and DNA were demonstrated. Most of the intersterility groups have now been linked to previously described species or have been described as new species.

Today the genus Armillaria is thought to contain about 40 species. Some occur in multiple continents and some are relatively restricted to a region of a continent. About nine species are recognized in North America. Two important pathogens, A. ostoyae (usually on conifers) and A. mellea (in the strict sense; usually on angiosperms) are probably circumboreal in distribution, meaning they occur around the northern hemisphere.