Phosphite is used to protect plants from the soil borne pathogen, Phytophthora cinnamomi. Although phosphite stimulates resistance to P. cinnamomi, this is the first histological study of its effect on Eucalyptus marginata, an economically important forest tree in Western Australia. Clonal lines of E. marginata, considered resistant and susceptible to P. cinnamomi, were underbark inoculated with P. cinnamomi. 4 days later, they were treated with 0, 2.5, 5 or 10 g L -1 phosphite. Transverse hand sections were stained for suberin and lignin, and histological responses to infection were examined. Defence responses were stimulated at all phosphite concentrations in both clonal lines, and the genotypic difference in lesion length was eliminated within 8 days of treatment. In the resistant line, suberin production was stimulated while in the susceptible line both lignin and suberin were stimulated. By 2 days after treatment, phosphite stimulated a faster rate of suberin production in the resistant line than the susceptible line, but by 4 days after treatment, there was no difference in the increase between the lines. Damage caused by P. cinnamomi was found to extend furthest in the cortex and outer phloem in transverse sections in both genotypes. In the presence of P. cinnamomi, phosphite stimulated mitosis as part of the defence response, with meristematic activity involved in the compartmentalisation of damaged tissue (formation of periderm) and closure of healthy tissue (callus). Phytotoxicity had a detrimental effect in healthy tissues and this was more apparent in the resistant line, where it did not provide the best protection from lesion extension and plant mortality, suggesting phytotoxicity could disrupt defence responses. Phosphite increases the capacity of susceptible and resistant E. marginata clonal lines to wall-off and contain P. cinnamomi colonisation through lignin and suberin deposition, and increased meristematic activity.
A detailed site investigation of a eucalypt nursery suffering disease losses revealed the causal agent to be Phytophthora boodjera. The pathogen was detected in vegetation surrounding the nursery production area, including the lawn, under the production benches during the growing season, and, most importantly, from plant debris in used trays. However, it was not found in the container substrate, water supplies, or production equipment or on the workers themselves. The sterilization methods used by the nursery were shown to be ineffective, indicating that a more rigorous method was required. Boiling trays for 15 min or steaming at 65°C for 60 min eradicated P. boodjera. This pathogen was more pathogenic to the eucalypts tested in their early seedling stage than P. cinnamomi. Tracing of out-planting to revegetation sites showed that P. boodjera was able to spread into the environment. Dispersal via out-planting to native vegetation may affect seedling recruitment and drive long-term shifts in native plant species. Inadequate nursery hygiene increases the risk of an outbreak and can limit the success of biosecurity efforts as well as damage conservation efforts.
Although Phytophthora species cause serious diseases worldwide, until recently the main focus on disease in natural ecosystems in southern Australia has been on the distribution and impact of P. cinnamomi. However, new Phytophthora pathogens have emerged from natural ecosystems, and there is a need to better understand the diversity and distribution of these species in our natural forests, woodlands and heathlands. From a survey along a 70 km pipeline easement in Victoria, Phytophthora species were isolated from 249 rhizosphere samples and 25 bait bags deployed in 21 stream, river, or wetland locations. Of the 186 Phytophthora isolates recovered, 130 were identified to species based on ITS sequence data. Ninety-five isolates corresponded to 13 described Phytophthora species while additionally 35 isolates were identified as Clade 6 hybrids. Phytophthora cinnamomi was the most common species isolated (31 %), followed by P. elongata (6 %), both species were only recovered from soil. Samples from sites with the highest soil moisture at the time of sampling had the highest yield of isolates. Consistent with other studies throughout the world, Clade 6 species and their hybrids dominated water samples, although many of these species were also recovered less frequently from soil samples. Many of the species recovered in this study have not previously been reported from eastern Australia, reinforcing that Phytophthora species are widespread, abundant and diverse in natural ecosystems. We have probably been underestimating Phytophthora diversity in Australia.
SummaryThe ability of Phytophthora cinnamomi to survive long dry Mediterranean summers is the key to its persistence in the southwest of Western Australia. It has been proposed that dead Banksia grandis are a key long term reservoir for P. cinnamomi inoculum. To test this, 36 healthy B. grandis trees were inoculated in April 1999. The presence of viable inoculum in the trees' tissues was determined in two experiments from tissue 10 and 30 cm above and below ground: Experiment 1 from core samples 3-34 months after tree death; and Experiment 2, separating bark and wood in cross sections of trees 2-33 months after tree death. In the first summer (10 months after inoculation) 75% of the trees had died, with the remaining trees dead by the following summer. P. cinnamomi was more commonly recovered from bark than wood, except from those trees that died in the drier second summer where it was recovered more often from the wood than the bark. Recovery of P. cinnamomi from B. grandis cores declined over time from 60% at 3 months after death to 33% at 10 months, 5.5% at 12 months and 0.1% at 34 months after death. Two months after tree death, the recovery rate of P. cinnamomi was 87% from trunks and roots; this decreased to 0.5% by 33 months. P. cinnamomi was more commonly recovered from above-than below-ground tissue until 8 months after plant death, after which it was more frequent in below ground tissues. This study suggests that the pathogen does not behave as a competitive saprophyte in these plant tissues and B. grandis is unlikely to be a long term reservoir for P. cinnamomi. However, the manipulation of the frequency of B. grandis mid-storey of the jarrah forest areas and the use of fire to facilitate the breakdown of dead Banksia stems may reduce the effects of P. cinnamomi. Collins et al 2011 Pc surviving in Banksia grandisForest Pathology 2
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