A comparison of the occurrence, sporulation and survival of Phytophthora cinnamoni rands in soils supporting native forest in south-eastern New South Wales and south-western Western Australia

Abstract

Four aspects of the behaviour of Phytophthora cinnamomi in soil collected from healthy and diseased sites at Durras in south-eastern New South Wales and Jarrahdale in south-western Western Australia were examined. The occurrence and distribution of P.cinnamomi in these two areas was compared using two different sampling and isolation techniques over a 16 month period. Samples of soil and fine roots collected from along a transect from the New South Wales site yielded P.cinnamomi when baited using the lupin seedling technique while similar baiting of comparable samples from Western Australia failed to yield the organism. Direct plating of samples of upper roots and root collars of recently dead Banksia grandis from Western Australian sites yielded P.cinnamomi while the organism was not isolated from comparable samples of chloratic Macrozamia communis collected at the New South vJales site. These results suggested that the form of occurrence of P.cinnamomi and its association with disease in Australia varies in different situations. A comparison was made of the microbial populations in soil from healthy and diseased forest at both sites using the soil dilution plate technique. There was a general association between low microbial populations and the occurrence of disease. However, it could not be ascertained whether disease occurred in areas where lower microbial populations existed or whether microbial populations dropped as a result of disease incidence. An inverse association was demonstrated between the frequency of isolation of P.cinnamomi and the population of the Aspergillus+ Penicillium group of microorganisms. The survival of chlamydospores and mycelium of P.cinnamomi in the field and in soil subjected to three moisture regimes was investigated using soils from diseased and healthy forest at both sites. The moisture regime appeared to have a greater effect on the survival of the fungus than the particular soil employed. P.cinnamomi survived for three to four months in the field when the soil was moist and for a comparable period in the laboratory in soil maintained at -5 kPa. P.cinnamomi may have survived even longer in waterlogged soils but the fungus could not persist for three weeks in soils which were allowed to air dry. These results provided an explanation for the observed pattern of ready isolation of P.cinnamomi from soil samples collected from Durras in New South Wales in contrast with those from Jarrahdale in Western Australia. The sporangial inducing ability of soils from healthy and diseased forest at Durras and Jarrahdale were examined. It was concluded that there was probably no major difference in the sporangial inducing ability of these soils because soils as distinct as washed river sand and garden loam induced comparable numbers of sporangia on mycelial mats of P.cinnamomi inserted directly in soil. It was suggested that the sporangial inducing principle per se had little potential epidemiological significance in P.ainnamomi induced disease except in exceptional circumstances. Using three different techniques it was demonstrated that the results of experiments studying sporangial induction performed on separate occasions were not comparable because of variation in the sporangial producing capacity of different cultures of a single P.ainnamomi isolate. Hence, experiments were not reproducible. The ecological and management implications of the differences in the behaviour of P.ainnamomi in the two areas studied were discussed and some future research priorities were briefly outlined.