Action of the fungicide phosphite on Eucalyptus marginata inoculated with Phytophthora cinnamomi

Although phosphite has been effective in the control of P. cinnamomi in E. marginata (jarrah), the biochemical mechanisms behind phosphite protection are poorly understood. Using an aeroponics system, jarrah clones with moder ate resistance to P. cinnamomi were treated with foliar applications of pho sphite (0 and 5 g L-1). The roots were inoculated with zoospores of P. cinn amomi at 4 days before and 0, 2, 5, 8 and 14 days after phosphite treatment . Root segments were then analysed for activity of selected host defence en zymes (4-coumarate coenzyme A ligase [4-CL], cinnamyl alcohol dehydrogenase [CAD]) and the concentration of soluble phenolics and phosphite. Lesion de velopment was most effectively reduced when phosphite concentrations within the roots were highest (i.e. days 8-14). During this time, the levels of h ost defence enzymes remained relatively unchanged. Lesion development was a lso effectively restricted when phosphite concentrations within the roots w ere lowest (i.e. days 2 and 5); a significant increase in host defence enzy mes was associated with this decrease in lesion development. It was conclud ed from these studies that the effect of phosphite in controlling the patho gen is determined by the phosphite concentration at the host-pathogen inter face. When phosphite concentrations within the roots are low, phosphite int eracts with the pathogen at the site of ingress to stimulate host defence e nzymes. At high phosphite concentrations, phosphite acts directly on the pa thogen to inhibit its growth before it is able to establish an association with the host, and the host defences remain unchanged.