Evaluation of the role of Phytophthora cinnamomi in Victorian ecosystems

摘要

Phytophthora de Bary species are among the most notorious plant pathogens capable of causing large-scale damage to plant communities as well as enormous economic loss in agriculture, horticulture, forestry and natural environments worldwide. Disease caused by P. cinnamomi in Victoria apparently declined during the long drought in Victoria and P. cinnamomi became undetectable in several previously infested areas. This research project focused on studying the effect of drought (from 1996 to 2009) and bushfires on the presence and survival of P. cinnamomi at such sites. The project focused on identifying any possible misidentification or unidentified species among previously collected isolates thought to be P. cinnamomi, studying their pathogenicity and the implications of more than Phytophthora species on the same host. Sampling conducted at 32 sites in 4 Victorian National Parks showed that Kinglake National Park had the greatest rate of infection at 100%, followed by Wilsons Promontory National Park (88%), Brisbane Ranges National Park (60%) and lastly the Grampians National Park (46%). It was concluded that the absence of susceptible host plants was a major factor responsible for the decline in disease in Victoria and not just drought as previously thought. Even high-intensity fires did not eliminate P. cinnamomi from the soil. Fire restored disease symptoms and detection of P. cinnamomi in previously infested areas but not until at least a year after the fire, when susceptible host plants had re-grown. Smoke water inhibited growth of P. cinnamomi and could possibly be used as an efficient and economical control measure. Seventeen of the ninety-one isolates (18%) previously collected by various researchers were misidentified as P. cinnamomi. Phytophthora species such as P. cryptogea P. gonapodyides and P. niederhauserii, and Pythium species such as P. undulatum, P. sterilum, Pythium sp. UZ 612 and P. helicandrum were identified among the culture collections using DNA sequencing and PCR-RFLP. A modified and efficient method for isolation and detection of P. cinnamomi from field-collected samples was designed. The procedure included collection of whole sample with roots, litter and soil, pre-moistening and incubation of soil samples at 28°C for 2 days prior to baiting, use of Pimelea ferruginea leaves as baits and using V8 agar as the medium for studying growth and sporulation of P. cinnamomi. This method significantly increased the rate of P. cinnamomi isolation, from 40% to 70% of samples. A combination of different Phytophthora species, including P. cinnamomi, P. cryptogea and P. niederhauserii, when inoculated on germinated lupins showed that larger lesions and significantly greater root restriction was observed when two different Phytophthora species were present in the same host as compared to the presence of any single Phytophthora species. This suggested a possible increase in disease incidence and pathogenicity if more than one Phytophthora species is found in the same location and host.