2.5.4
TEMPERATURE CHANGES RESISTANCE OF CLONAL EUCALYPTUS MARGINATA TO PHYTOPHTHORA CINNAMOMI

D HÜBERLI1, IC TOMMERUP2, GESTJ HARDY1 and IJ COLQUHOUN3

1Murdoch University, School of Biological Sciences, Perth 6150, Western Australia; 2CSIRO Forestry & Forestry Products, Private Bag, PO Wembley 6014, Western Australia; 3Environmental Department, Alcoa of Australia Limited, PO Box 252, Applecross 6153, Western Australia

Background and objectives
Eucalyptus marginata (jarrah) varies in its resistance to colonization by the introduced pathogen, Phytophthora cinnamomi. This trait has been exploited to yield jarrah clones ranging in resistance to P. ;cinnamomi [1]. However, isolates of P. ;cinnamomi vary in their capacity to induce disease in resistant jarrah clones, with no association between pathogenicity levels and either A1 and A2 mating types or isozymes types [2]. We have shown that isolates differ in their capacity to colonize jarrah and marri (E. ;calophylla) tissue. Disease outbreaks in jarrah, other native vegetation and horticultural crops due to P. cinnamomi are more likely to occur in warm moist conditions. These factors raise questions about the interactions between the pathogen, hosts and temperature, and consequent disease development.

Materials and methods
To assess the durability of resistance in jarrah at different temperatures, development of disease caused by P. ;cinnamomi in three jarrah clones was measured at 15, 20, 25 and 30°C. Resistant and susceptible clonal trees were used. A pathogenic isolate of P. ;cinnamomi, selected on the basis of previous experiments using clones, was inoculated under the bark on the stem above the soil surface. Tree survival was monitored. Stems from dead plants (100% leaves permanently wilted) were plated onto a Phytophthora-selective agar medium to confirm the presence of P. cinnamomi.

Results and conclusions
At high temperatures (25 and 30°C), survival of all clones was low compared with that at low temperatures (15 and 20°C). The susceptible clone survived longer than both resistant clones at the high temperatures. At low temperatures there was no difference between the susceptible and one resistant clone; however, the other resistant clone survived longer than the susceptible one at 15 and 20°C. P. ;cinnamomi was recovered from all dead plants.

Temperature appears to have a direct effect on host resistance since susceptible clones survived longer than the resistant ones at high temperatures. Temperature is therefore a major factor for inclusion in the screening stage of programmes breeding and selecting for tree resistance to P. ;cinnamomi.

References
1. Stukely MJC, Crane CE, 1994. Phytopathology 84, 650-6.
2. Dudzinski MJ, Old KM, Gibbs RJ, 1993. Australian Journal of Botany 41, 721-32.