Studies on the biology of Armillaria in New Zealand

Abstract

A detailed study was carried out on cultural differences between Armillaria novae-zelandiae and A. limonea, suitability of various tree species as food bases for the fungi, the pathogenicity of the fungi on Pinus radiata, Eucalyptus delegatensis, E. regnans, E. fastigata and E. saligna and the clonal situation in New Zealand forests. The growth of 20 isolates of A. novae-zelandiae and A. limonea under continuous light and dark was observed on malt extract agar, dextrose asparagine agar, prune agar and Hagems agar. Negligible differences occurred between the isolates within each species with respect to any particular treatment (light or dark, media used). However differences between the two species were always present when grown under continuous light e.g. A. novae-zelandiae failed to produce rhizomorphs whereas A. limonea merely suffered a reduction in number and rate of elongation; on prune agar and especially on dextrose-asparagine agar, prominent vertical hyphal fascicles occurred on cultures of A. novae-zelandiae; on malt extract agar, the growth of A. novae-zelandiae was symmetrical with surface mycelium leathery in texture while growth of A. limonea was asymmetrical with crustose mycelium dominating the surface of colonies. On the other hand for cultures grown in the dark, differences were less clear cut and involved matters of degree only e.g. the more prolific branching of rhizomorphs of A. limonea than in A. novae-zelandiae and the higher proportion of broad aerial and submerged rhizomorphs of A. novae-zelandiae than in A. limonea on malt extract agar; the bifurcation of rhizomorphs of A. limonea emerging from the surface of dextrose-asparagine agar. Further studies on the effect of light on the growth of these species revealed that the inhibition by light of rhizomorphs of A. novae-zelandiae was of a transient nature and was not a result of innate or light-mediated difference in the nutritional status of the media. There was a threshold value (regular photoperiod levels below lh at 1500 lux or continuous light intensity levels below 650 lux) below which rhizomorphs were produced although there was a gradual decrease in number until the threshold value was reached. The relative suitability of 11 indigenous as well as 6 exotic tree species as food bases for the fungi was investigated. It was found that a number of the most common tree species present in New Zealand forests can act as potential food bases for the fungi and that A. novae-zelandiae is a more prolific rhizomorph producer than A. limonea. Among the food bases tested, rhizomorph production by both Armillaxda spp was most prolific on Beilschrrtiedia taua. Sections of woody stem of such species as Dacrydium cupressinum, Toronia toru and Nothofagus truncata, which were colonized slowly, failed to produce rhizomorphs until they were completely colonized. If persistence in these host species is correspondingly prolonged, there is an inference that both short term and long term risks from Armillaria attack may be encountered in exotic plantations on land converted from indigenous forests. Pathogenicity trials on P. radiata and the four Eucalyptus spp revealed that seedlings of P. radiata were more susceptible to infection and died more rapidly than those of the Eucalyptus spp. Variation in susceptibility to infection was observed between the Eucalyptus spp., with E. saligna being most resistant. All food bases used in the inoculation trial provided suitable food reserves for the fungi to infect P. radiata seedlings. On the other hand B. tabla was markedly better than P. radiata as a food base in regard to infection of Eucalyptus seedlings. Overseas work suggests that Armillaria can spread in a forest by means of subterranean rhizomorphs and/or root to root contact for many years to form what are essentially large clones. Investigations in the Mamaku and Omatorua State Forests indicated that in the local situation, numerous small clones are present rather than single large clones. It is suggested that basidiospores may be involved with the spread of the fungus.