GROWTH OF LESIONS OF PHYTOPHTHORA-CINNAMOMI IN STEMS AND ROOTS OF JARRAH (EUCALYPTUS-MARGINATA) IN RELATION TO RAINFALL AND STAND DENSITY IN MEDITERRANEAN FOREST OF WESTERN-AUSTRALIA

The effects of reduced stand density on (i) water status of jarrah (Eu calyptus marginata Donn ex Sm.) and (ii) growth rate of artificially i nduced lesions of Phytophthora cinnamomi Rands in phloem of E. margina ta were examined. Plots in the high (>1100 mm . year(-1)), intermediat e (900-1100 mm . year(-1)), and low (<900 mm . year(-1)) rainfall zone s of the jarrah forest were thinned to controlled fractions of origina l stand density. Phytophthora cinnamomi lesions in the phloem of stems and roots were established by wound inoculation. Lesions of P. cinnam omi were longer in stems of trees with small water deficits than in tr ees with larger water deficits. For example, in 1991 at the intermedia te rainfall site, water potentials and lesion lengths of trees on unth inned and thinned plots were -1.1 and -0.9 MPa and 23 and 45 cm, respe ctively. Lesions grew more slowly in roots than in stems (average 10.0 and 26.2 cm, respectively, after 55 days at unthinned high rainfall s ite in 1991); however, the relative difference between lesions in tree s with the highest and lowest water potentials was greater in roots (u p to 2.7 times) than in stems. Lesion extension was also affected by s ummer rainfall, with longer lesions occurring in summers of highest ra infall. Viability of P. cinnamomi in 10-week-old and 1-year-old lesion s decreased as tree water deficits increased. Differences in lesion ex tension between jarrah in different amounts of summer rainfall were la rgely explained by differences in dawn water potential.