Growth of lesions of Phytophthoracinnamomi in stems and roots of jarrah (Eucalyptusmarginata) in relation to rainfall and stand density in mediterranean forest of Western Australia

Abstract

The effects of reduced stand density on (i) water status of jarrah (Eucalyptusmarginata Donn ex Sm.) and (ii) growth rate of artificially induced lesions of Phytophthoracinnamomi Rands in phloem of E. marginata were examined. Plots in the high ( > 1100 mm•year−1), intermediate (900–1100 mm•year−1), and low ( < 900 mm•year−1) rainfall zones of the jarrah forest were thinned to controlled fractions of original stand density. Phytophthoracinnamomi lesions in the phloem of stems and roots were established by wound inoculation. Lesions of P. cinnamomi were longer in stems of trees with small water deficits than in trees with larger water deficits. For example, in 1991 at the intermediate rainfall site, water potentials and lesion lengths of trees on unthinned and thinned plots were −1.1 and −0.9 MPa and 23 and 45 cm, respectively. Lesions grew more slowly in roots than in stems (average 10.0 and 26.2 cm, respectively, after 55 days at unthinned high rainfall site in 1991); however, the relative difference between lesions in trees with the highest and lowest water potentials was greater in roots (up to 2.7 times) than in stems. Lesion extension was also affected by summer rainfall, with longer lesions occurring in summers of highest rainfall. Viability of P. cinnamomi in 10-week-old and 1-year-old lesions decreased as tree water deficits increased. Differences in lesion extension between jarrah in different amounts of summer rainfall were largely explained by differences in dawn water potential.