Armillaria Root Rot in Eucalypt Forests: Aggravated Endemic Disease

Abstract

Species of the woody root rot fungus Armillaria are indigenous in cool temperate rain forest, mixed forest, and wet and dry sclerophyll eucalypt forests in Australia. Four species have been described or identified from southeastern Australia: A. luteobubalina Watling and Kile, A. fumosa Kile and Watling, A. hinnulea Kile and Watling, and A. novae-zelandiae (Stevenson) Boesewinkel. The latter species was first described from New Zealand, and A. hinnulea also occurs in that country. Armillaria novae-zelandiae and A. hinnulea occur in wet forests (rain forest, mixed forest, and wet sclerophyll communities), while A. luteobubalina and A. fumosa are found mainly in dry sclerophyll forests. Armillaria luteobubalina is so far the only species known to behave as a primary pathogen in native forests. While the fungus has an extensive geographical distribution in southeastern Australia, damage is most severe in selectively logged forests in the central highlands of Victoria, where it is estimated that approximately 3-5% of the forest area is moderately to severely affect~. The fungus kills all species of eucalypts and a wide range of the under- story trees and shrubs present in the forests. Most infections occur in small (0.1-1.0 ha), well-defined patches, but larger (up to 20-30 ha), more diffuse infections also occur. Evidence of primary pathogenicity includes (a) constant association of the fungus with disease; (b) the pattern of disease development within stands (the fungus spreads by root contact from infected food bases); (c) correlation between root infection and symptom development in large trees; (d) evidence of host resistance to infection; and (e) pathogenicity in pot and field trials. There is no evidence that climatic stress or other pests or pathogens initiate disease. Within the forest, the fungus has a discontinuous distribution. Studies of genotypes of the fungus (identified by analyses of mating alleles, since Armillaria sp. are bifactorial heterothallic, or intraspecific antagonism), suggest that A. luteobubalina consists of a community of genetically distinct mycelia. Individual genotypes may contract, expand, or coalesce, depending on circumstance. The development and status of the community depends on the two processes of new basidiospore infection and local spread by vegetative growth through root systems. Similar patterns of genotype distribution and clonal development were evident in logged and unlogged forest. It is concluded that root rot caused by the fungus is endemic in these forests but that logging has aggravated the disease.