GENETICALLY BASED RESISTANCE OF EUCALYPTUS-MARGINATA TO PHYTOPHTHORA-CINNAMOMI

Sixteen half-sib families of jarrah (Eucalyptus marginata) seedlings w ere screened for resistance to Phytophthora cinnamomi using soil inocu lation and stem inoculation in pot experiments, and soil inoculation i n a P. cinnamomi-infested field site. Low mortality following soil ino culation and short lesion lengths following stem inoculation were used as indicators of P. cinnamomi resistance. Resistance levels varied co ntinuously across families from high to low values in all experiments, but family rankings were consistent among experiments. The narrow-sen se heritability of the resistance character was high at both family (0 .74-0.85) and individual-tree (0.43) levels. The resistance of jarrah to P. cinnamomi is under strong genetic control. Selection of lines wi th high levels of resistance is feasible, and such lines can be used i n rehabilitation plantings of jarrah forest sites. Selection of resist ant parent trees in the forest based on a single assessment of crown h ealth met with little success. Seedlings of five healthy parent trees in diseased forest exhibited a wide range of resistance levels and wer e only marginally more resistant than seedlings of trees with symptoms of root rot. Stem-inoculation of jarrah seedlings at least 9 mo old i s recommended as the standard screening test to be used in selecting f amilies and individuals resistant to P. cinnamomi, based on lesion siz e.