No abstract

No abstract

Changes in vegetation, pathogen population and distribution were monitored periodically in both defined infested quadrats and similar pathogen-free quadrats at six sites representing major types of forest and woodland. Assessments were recorded in May 1977, 1979, 1981, 1983–1984, 1995 and 2000. The susceptible eucalypts in the overstorey of infested sites, such as Eucalyptus obliqua, E. baxteri, E. willisii and E. macroryncha, showed severe dieback, loss of crown or deaths. All the trees died on some sites, others presented dead leaders with epicormic growth on lower branches. Dieback followed by death occurred in 54% of the understorey species, including the dominant Xanthorrhoea australis, thereby changing community structure and species composition. At the time of its greatest prevalence, the pathogen’s activity resulted in a decline in species richness in infested quadrats to a mean of 25.6 species compared with a mean of 39.2 for pathogen-free quadrats. Percentage cover and percentage contribution to the community by susceptible species were negligible. On steep sites, 65% of the ground remained bare, but on other sites the susceptible flora was replaced by field-resistant species of sedges and rushes, such as Lepidosperma semiteres and Hypolaena fastigiata, and by partly resistant tea-trees Leptospermum myrsinoides, L. continentis and L. scoparium. The dense, field-resistant understorey consisted of the ground cover of H. fastigiata, scattered clumps of various sedges and above this a mass of tea-tree scrub, approximately 1 m in height, with moderately severe dieback of the branches. From 1976 to 1984, the pathogen was isolated from 100% of the 345 root and soil samples and from all of the infested quadrats, but then gradually declined. In 2000, Phytophthora cinnamomi Rands was rare at four sites and was not isolated from two sites. Regeneration of 30 susceptible species, previously eliminated, was recorded from infested sites and 21 of these species were growing in more than one quadrat. Vigorous regeneration of the previously dominant but highly susceptible X. australis occurred at two sites and was similar to that recorded from some recovering infested sites in the Brisbane Ranges, Victoria. The decline of the pathogen and the regeneration of susceptible species may be associated with low spring rainfall from 1995 to 2000 and the consequent reduction in zoospore production, enabling a partial recovery from dieback. The disease cycle from invasion and destruction of a susceptible indigenous flora by this virulent pathogen to the decline of the pathogen and the regeneration of that same susceptible indigenous flora was almost complete on sections of two of the six sites studied. In other areas, the post-infection colonising flora of field-resistant species remained dominant, except at one steep site where the ground remained uncolonised and subsequently eroded following the death of susceptible flora. Extinction following infection by P. cinnamomi, however, remains a grave threat to endangered, endemic species if susceptible.

ABSTRACIThe concentrations of zeatin-type and isopentenyladenine-type cytokinins were reduced in the xylem extrudate collected from seedlings of Eucalyptus species following infection by Phytophthora cinnamomi Rands. The use of an enzyme-linked immunosorbent assay (ELISA) allowed the detection of these cytokinins over the range of 03 to 7 picomoles for the isopentenyladenine-type and 1 to 1000 picomoles for the zeatin-type. Isopentenyladenine-type cytokinins occurred in concentrations less than 10% of the zeatin-type, but they could be readily detected and measured. This is the first report of their presence in xylem. The sensitivity of the assay allowed a short collection period (30 minutes) reducing any confusion with trauma-induced changes. Infection of the susceptible species Eucalyptus marginata Donn. ex Sm. resulted in significant reduction of zeatin-type cytokinins within 3 days of infection, and at 14 days postinfection the concentration of both cytokinin types was reduced to 26% of uninoculated controls. No reduction in cytokinins occurred with the field resistant Eucalyptus calophylla R. Br. It is suggested that failure of cytokinin transport from the root system may be responsible for the failure in water transport and symptoms of P. cinnamomi infection observed in infected susceptible eucalypts.The fungus Phytophthora cinnamomi Rands is a pathogen with a broad host range, and the diseases it causes are ofeconomic importance throughout the temperate and tropical zone (36). While the primary symptom of infection by P. cinnamomi in a susceptible species is root rot, the organism also induces secondary symptoms in the shoots of many perennial host plants which resemble droughting. These include wilting, chlorosis, microphylly, and shoot and bud death (dieback) (36).Studies of seedling eucalypts grown under controlled conditions showed that susceptible species died when as little as 8% of the root system was infected (9), indicating that loss ofroot tissue alone is unlikely to be the cause ofeither the secondary symptoms or of host death. Moreover, the same study demonstrated a significant reduction in hydraulic conductivity ofthe root system of a susceptible species 2 to 4 d after inoculation. This decline in root conductivity preceded reductions in leaf xylem water poten- tial, leaf transpiration rate, and wilting in these plants. These changes were not observed in seedlings of field resistant species.Histological examination failed to show xylem blockage or extensive damage to the conducting system under these conditions (9) and there is no evidence of toxin production sufficient to account for the secondary symptoms (7).However, reductions in the concentration of cytokinin-like compounds in the tracheal fluid of crop plants infected with wilt by other fungal root pathogens, Verticillium spp., have been reported (16,20,24) with symptoms which also resemble drought. In this paper, we report that infection of susceptible eucalypts by P. cinnamomi also induces a major reduction in the concentration of cyt...

Chlamydospore survival was investigated for six soil types, collected from disease-free areas of native forest in Victoria, in 50-g packs of non-sterile, unamended soils and gravels at five different matric soil water potentials ( ψ ). No chlamydospores survived in gravel free from OM, and only one chlamydospore survived at ψ -3000 kPa. In other packs the numbers of chlamydospores declined for 2 months then increased markedly at 4-6 months. Many chlamydospores remained viable for 8 months and some for 10 months despite the use of non-sterile soil and the absence of hosts. Maximum numbers survived in gravel from the Brisbane Ranges 6 and 8 months after inoculation at ψ -500 kPa. Decreasing soil moisture appeared to stimulate chlamydospore formation while a low rganic matter content and small numbers of microorganisms increased survival.