Variation in population levels of Phytophthora cinnamomi in Eucalyptus forest soils in eastern Victoria.  G.C. Marks, F.Y. Kassaby and P.C. Fagg.  April 1975.  31 pp. (unpubl.)

SUMMARY

The behaviour of Phytophthora cinnamomi was examined over a two-year period on newly cleared and replanted, eucalypt sites in the coastal forests of east Gippsland, rated as low, moderate and high die-back hazard, in relation to drainage and disease characteristics of the original forest cover and surrounding trees.

The fungus behaved like a saprophytic survivor (sensu Garrett). Maximum population levels, assessed as a “Population Density Index” (PDI), occurred within the soil influenced directly by the root mass. The PDI in adjacent bare soil was very low. The PDI depended, inter alia, on fungal pathogenesis, soil temperature and soil moisture. The ratio of the mean maximum PDI recorded for the high, moderate and low-hazard sites was about 8:4:1 respectively. This was directly related to the depth of the clay pan beneath the surface.

There was a marked seasonal variation in PDI, that was greatest on the high-hazard site and least on the low-hazard site. The minimum and maximum PDI’s were recorded in June (winter) and December-March (summer) respectively. Moderately heavy rainfall had little effect on the PDI in well-drained soils even during mid-summer when soil temperatures were optimal for the PDI to increase. Fire affected the PDI only temporarily.

The PDI in the surface soil was great than that at a depth of 75 cm, and the distribution of the fungus in the soil profile was influenced by soil texture. The PDI was greater under root-rot tolerant than sensitive eucalypt species before canopy closure occurred.

Canopy closure reduced the seasonal fluctuation in soil temperature and, on sites where tree growth was vigorous, the reduction in PDI was striking. Addition of fertilisers to the soil had no direct effect on PDI, the reduction observed being an indirect effect produced possibly by accelerated growth, increased transpiration and rapid canopy closure.

These results suggest that root-rot sensitive eucalypt species growing on coastal sites with impeded drainage will be vulnerable to die-back, and that any form of logging activity that reduces the amount of green cover on infected coastal sites will aggravate root-rot in these species if the fungus is present. Consequently it is unlikely that these stands can be managed successfully using conventional methods.

On steeper, will drained sites the disease should not prove to be a serious hazard except in exceptionally wet summer and autumn periods.

Also published:

Marks, G.C., Kassaby, F.Y. and Fagg, P.C. (1975)  Variation in population levels of Phytophthora cinnamomi in eucalypt forest soils of eastern Victoria.  Aust. J. Bot. 23 (3): 435-49.