Site and Seasonal Effects on Susceptibility of Eucalyptus marginata to Phytophthora cinnamomi

Abstract

Susceptibility of Eucalyptus marginata stems and roots to invasion by Phytophthora cinnamomi was compared at four sites in the northern jarrah forest and reasons for differences in tree susceptibility were sought. The sites were located in both the low (750 mm year -I ) and high (1 100 mm year -1) rainfall zones and differed in understorey composition and soil nutrient status. Stems were inoculated at monthly intervals between October 1983 and April 1984. Measurement of stem lesions induced by inoculation showed that rate of fungal growth in trees at all sites generally increased during the October-December (1983) period. During February and March (1984) there was a large difference in the susceptibility of stems and roots in low-rainfall zone sites compared with those in high-rainfall zone sites. Roots inoculated during February at three of the four sites showed the same relative susceptibilities as stems inoculated one week later. Mean relative water contents (RWC) of the phloem (inner bark) were used to compare the water status of the saplings at the four sites and the observed inhibition of the fungus in the sapling stems and roots at the driest sites, coincided with the months when phloem RWC values were at their lowest. Phloem was also sampled from pole-sized trees at the four sites and seasonal changes in RWC values, soluble carbohydrate concentrations and phenols were monitored for 12 months. Soluble carbohydrate concentrations in the phloem of some of the saplings inoculated at each site were also determined. Concentrations of carbohydrates and phenols did change seasonally and differed between sites but no evidence was found to suggest that they had a direct effect on fungal growth.