Effect of phosphite on in planta zoospore production of Phytophthora cinnamomi

The efficacy of phosphite to control the production of zoospores of Phytoph thora cinnamomi on infected trees grown in a glasshouse and in a revegetate d mined area was examined. Banksia grandis and Eucalyptus marginata seedlin gs in the glasshouse and E. marginata seedlings in the minepit were sprayed with 0, 5 and 10 g phosphite L-1. In both trials, zoospores were produced from infected tissue of plants treated with all concentrations of phosphite . In the glasshouse, spray application of 5 and 10 g phosphite L-1 signific antly reduced the production of zoospores from both B. grandis and E. margi nata seedlings. In the mined area there was a similar, though nonsignifican t, reduction in the number of zoospores produced from phosphite-treated and nontreated E. marginata seedlings. However, the average number of zoospore s produced was greater in plants not treated with phosphite (1.75 zoospores mL(-1)) than from plants treated with 5 or 10 g phosphite L-1 (0.04 and 0. 09 zoospores mL(-1), respectively). Pimelea ferruginea leaves were used to bait the water surrounding the plants in the mined area to determine if zoo spores produced from phosphite-treated plants were able to infect plant mat erial. Significantly more baits were infected by zoospores from plants not treated with phosphite compared with plants treated with 5 or 10 g phosphit e L-1. These results suggest that phosphite reduces, but does not prevent, the production of viable zoospores on infected trees. Thus phosphite applic ation may not remove the risk of P. cinnamomi spreading from infested, spra yed areas.