Studies on myrtle wilt

Nothofagus cunninghamii, or myrtle, is the dominant tree species in many Tasmanian and Victorian cool temperate rainforest communities. The main cause of myrtle death in undisturbed stands is the disease myrtle wilt, which is caused by the pathogenic hyphomycete Chalara australis. Early literature, aerial surveys and aerial photography indicated that myrtle wilt was endemic in at least part (and possibly most) of the range of myrtle, but that in some areas, disease levels may have increased in the recent past. In Tasmania, measured mortality rates were found to be variable but not escalating, with no apparent overall trend. A new estimate of annual mortality due to myrtle wilt was calculated to be 0.61 % p.a. Logging, thinning and reading of myrtle-dominated rainforest led to increased myrtle wilt incidence. For some disturbed areas, there was evidence that after an average of nine years, elevated myrtle wilt mortality levels declined to background levels. The spread of myrtle wilt into areas adjacent to disturbances was clearly detectable up to 180 m from the disturbance, although not all sites were affected. v Small, experimental stem wounds on myrtle saplings provided suitable infection courts for C. australis spores, with most infections occurring within 14 days. Functional root grafts commonly occurred in young myrtles, and experimental inoculation, root excavation and sectioning strongly indicated underground, tree to tree transfer of C. australis. Re-isolations of C. australis were made from these trees, and characterisation of these isolates verified spread via root grafts. Root grafting probably predisposes stands to epidemics, plays a major role in the spread of myrtle wilt, and causes the clumped pattern of infected trees. Clumping occurred on a scale of 2.5-14 m and gave rise to patches of dead and diseased myrtles, often resulting in large gaps in the forest canopy. Floristic studies showed that there was no distinct set of species which characterised myrtle wilt gaps, but that there were generally more myrtle seedlings than in control forest. Data from one site suggested that the vegetation composition in large, old gaps was reverting to that of the surrounding forest, and that myrtle was self replacing in such gaps. The probable long-term effects of myrtle wilt on Tasmanian myrtles were investigated using a simple population model. In summary, if current levels of myrtle wilt continue, it is unlikely that the disease will lead to any permanent change in forest structure. In undisturbed forest, myrtle wilt acts primarily as a mechanism facilitating stand rejuvenation.