The behaviour of Phytophthora cinnamomi Rands was examined over a 2 year period on newly cleared and replanted eucalypt sites in the coastal forests of eastern Gippsland, rated as of low, moderate and high die-back hazard in relation to the drainage and disease characteristics of the original forest cover and surrounding trees. The fungus behaved like a saprophytic survivor. 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. Mean maximum PDI values recorded respectively for the high, moderate and low hazard sites were in the ratio of about 8/4/1. This ratio was directly related to the depth of the clay pan beneath the surface. There was a marked seasonal variation in PDI, most pronounced on the high hazard site and least on the low hazard site. The minimum and maximum values 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 midsummer when soil temperatures were optimum for fungal populations to increase. Fire affected PDI values only temporarily. PDI values in the surface soil were greater than at a depth of 75 cm, and the distribution of the fungus through the soil profile was influenced by soil texture. Before canopy closure occurred, PDI values were greater under eucalypt species tolerant to root rot than under sensitive species. 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 fertilizers to the soil had no direct effect on the 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 prove vulnerable to die-back, and that any form of logging activity that reduces the amount of green cover on infected sites will aggravate root rot if the fungus is present. Consequently it will not be possible to manage these stands by conventional methods. On steeper, well-drained sites the disease should not prove to be a serious hazard except in exceptionally wet summer and autumn periods.

The die-back tolerance of 16 fertilized (17/9/7 nitrogen-phosphorus-potassium) and unfertilized eucalypt and two conifer species was tested in the coastal forests of east Gippsland on sites rated as of "high", "moderate", and "low" hazard on the basis of previous damage, internal soil drainage, and infection by P. cinnamomi. Measurements were made of the population density index (PDI) of P. cinnamomi, of soil moisture, and of soil temperature. Supporting greenhouse and laboratory experiments are also reported. The high hazard site showed the most uniform infection and the greatest PDI, the low hazard site the least uniformity in the infection pattern. During the first year's growth, five renantherous eucalypt species showed considerable sensitivity to root rot and die-back; the intensity of the disease and the number of deaths were directly proportional to the hazard rating of the sites. The 11 species of Macrantherae tested were very tolerant to die-back. The disease was aggravated by temporary waterlogging during a 7 day period, but waterlogging did not cause die-back. The disease first appeared in the plots when soil temperatures rose above 15°C. Greenhouse tests showed that P. cinnamomi was most virulent at 22°, and visible injury became evident between 15 and 18°. Fertilizers produced striking growth responses during the first year in both subgenera of eucalypts on the low hazard site, with only minor differences between the two groups. Similar responses were seen only on the Macrantherae on the moderate and high hazard sites. The growth of the surviving renantherous eucalypts was uneven, and fertilizers greatly increased their sensitivity to die-back disease. The response to fertilization in both subgenera was directly related to the disease hazard of the site and the intensity of infection by P. cinnamomi. Differences in response to fertilization between the Macrantherae and the Renantherae were directly proportional to the population density and distribution of P. cinnamomi in the soils. The initial tolerance to die-back of the two conifers, Pinus radiata and P. Elliotii, was similar to that of the most resistant macrantherous eucalypts tested.

Soil surveys of the distribution of Phytopthora cinnamomi in eastern Victorian eucalypt forests showed it to be widely distributed in flat, poorly drained coastal forests extending from Wilson's Promontory to the New South Wales border and from 15 to 25 km from the coast. P. cinnamomi was scattered in the foothill forests up to an altitude of about 800 m. It was sometimes found in high altitude forests, at sites where recent logging, clearing and road con- struction had occurred. Its frequency of occurrence was related to the intensity of forestry activity, to internal soil drainage and to the occurrence of warm soil temperatures. Data are provided on a number of factors affecting soil populations of P. cinnamomi, and its possible origin is discussed.

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