Susceptibility of native plant species to Phytophthora cinnamomi and the spread of Phytophthora dieback in South Australia.

Abstract

Phytophthora dieback, caused by Phytophthora cinnamomi Rands, affects a wide range of Australian native plants. In South Australia, the pathogen has affected large areas of native vegetation to threaten plant biodiversity. Lack of information on the disease in the local environment hampers management. The main objectives of this project were to: a) determine the rate of pathogen and disease spread in naturally infested native vegetation, b) assess the susceptibility of plant species native to South Australia to the disease and c) assess ability of antagonistic soil actinomycetes to protect susceptible species from Phytophthora dieback. A confirmed P. cinnamomi-infested site, with gentle slope, at Mount Bold Reservoir Catchment Reserve in the Mount Lofty Ranges, was selected to assess pathogen and disease spread in native vegetation. The soil was loamy sand. The vegetation was open woodland dominated by Eucalyptus obliqua L’Hérit with an understorey dominated by Xanthorrhoea semiplana F. Muell, a highly susceptible species which was used as an indicator to assess disease spread. An area of 70 m x 70 m, extending from two disease fronts into the adjoining healthy vegetation, was marked into 10 m x 10 m quadrats. The number of dead and dying X. semiplana was counted and soil samples from each quadrat, collected every spring and autumn from 2008 to 2010, were baited for P. cinnamomi using cotyledons of E. sieberi L.A.S. Johnson. P. cinnamomi was regularly detected along the disease front. However, the pathogen did not spread across the slope into the adjoining healthy vegetation despite annual rainfall of 626 to 900 mm for three consecutive years (2008 to 2010). The slow spread of the pathogen was reflected in the small numbers of dead and dying X. semiplana observed in each quadrat at each assessment time. The limited spread of the pathogen may be due to unfavourable weather conditions. In winter (June to August), when the precipitation was high (ca. 50% of the annual rainfall), soil temperature was generally too low (average temperature 9.3°C) for formation of sporangia. On the contrary when the temperature was warm (≥ 15°C) during spring (September to November) and autumn (March to May), the average soil water potential, ≤ -200 kPa, may have been too low for movement of zoospores. Further, sporadic distribution of P. cinnamomi and the patchiness of disease spread might have reflected the efficiency of the baiting technique. Thirty-seven South Australian native plant species, including 15 threatened or locally endangered species, were assessed for susceptibility to Phytophthora dieback in a greenhouse from October 2009 to July 2010. Seedlings or cuttings were raised in potting mix for native species then transplanted to 15 cm-diameter pots filled with limed University of California mix or Bio Gro® (Bio Gro, South Australia). Plants were inoculated with P. cinnamomi via pine wood-inoculum plugs when up to 6 months old, maintained in moist conditions and monitored for disease symptoms for 3 to 6 months. Twenty-four of the 37 species studied, including 8 threatened species, were susceptible to the disease. Nine of these 24 species were ranked as highly susceptible. Another nine species were assessed as resistant. All species classed as susceptible were trees or shrubs while herbs were unaffected. In South Australia, where native vegetation has been extensively cleared or degraded, Phytophthora dieback represents an additional threat to the remnant native flora that might cause the extinction of native plant species, particularly the rare and endangered species, if not brought under control. Actinomycetes were isolated from soil collected from roots of Acacia pycnantha Benth and young, healthy X. semiplana growing close to dead X. semiplana at the field site. Of 127 actinomycetes isolates selected, 78% inhibited P. cinnamomi in dual culture. Eight Streptomyces spp. which exhibited strong to weak antagonism, were compared in the greenhouse for ability to protect 2-month old E. sieberi. One isolate delayed infection of E. sieberi by P. cinnamomi, although none prevented disease. The high soil moisture (≥ -10 kPa) required to induce disease was probably not conducive for the growth of the actinomycetes. Knowledge generated in this project can be used in Phytophthora management to help prioritise threatened plant species in South Australia for protection, inform revegetation programs and to provide the basis for further research in the state.