Survival of Phytophthora cinnamomi Rands in, and colonisation by P. cinnamomi and Pythium spp. of, mountain ash forest soil in eastern Victoria.  F. Y. Kassaby, P. C. Fagg and G. C. Marks.  April 1977.  14 pp. (unpubl.)

SUMMARY

Surveys of the distribution of Phytophthora cinnamomi in eastern Victoria indicate that a large portion of the high altitude forests above 800m are uninfected. The main objective of this study was to ascertain whether land hygiene measures are needed to protect the mountain forest soils in the Errinundra, Nunnet and Nunniong Plateau forests in eastern Victoria, which are stocked almost exclusively with valuable root-rot sensitive eucalypt species. A naturally infected soil inoculum was introduced into mountain forest soil, at altitudes between 800 and 1240m above sea level, at three sites: virgin forest; 3 year-old eucalypt sapling regeneration; and freshly burnt bare soil. The population density index (PDI) of Phytophthora cinnamomi in the inoculum was monitored for 3 years and the colonisation of the mountain soils by Phytophthora cinnamomi and Pythium spp. was examined. Before inoculation the sites were sampled and no isolations of either pathogen were made.

Slow soil colonisation by Phytophthora cinnamomi occurred after a lag period on the bare soil site, but the virgin forest and regeneration sites were not colonised even though the fungus was still detectable in the inoculum at the end of 3 years. Pythium spp. was detected much in advance of Phytophthora cinnamomi at the end of the second year and its rate of movement was greatest on the regeneration and bare soil sites. The population density index of Phytophthora cinnamomi declined in the inoculum on all sites, the rate being greatest in virgin forest at least on the bare soil site.

No shoot damage was observed on the virgin forest or regeneration sites and only an insignificant number of seedlings died on the bare soil site.

It was concluded that the soil temperatures in these forests, under a closed canopy, do not favour either Phytophthora cinnamomi establishment or disease establishment over a 3 year period, even when soil moisture is favourable. It was also concluded that Phytophthora cinnamomi is not indigenous to the area, and as the long-term effects of its introduction with other pythiaceous fungi are unknown, land hygiene measures remain a justifiable insurance against disease in the region.