Abstract
Phosphite is increasingly being used to control for dieback caused by Phytophthora cinnamomi. We wished to study the likelihood of P cinnamomi evolving resistance to phosphite, given the clonal populations of the fungus present in Western Australia. Isolates of P cinnamomi were collected from areas where phosphite has been used intensively for up to 15 years (avocado orchards) as well as areas of less frequent and no use of phosphite. Our testing involved stem inoculating a clonally propagated host (Leucadendmn sp.) that was treated with one of three levels of phosphite (0%, 0.25% and 0.5%). The extent of colonisation by each P. cinnamomi isolate was detem1ined after 8 days of incubation in a controlled temperature plant growth cabinet. Initial trials showed that less aggressive isolates are not present in populations obtained from areas where phosphite has been used. Also, the few isolates that colonise the phosphite treated host to a large extent all come from phosphite treated areas. However, the interaction of aggressiveness of isolates with the length of their previous exposure to phosphite was not consistent. These results may reflect the repeated sampling of the same, more aggressive isolate from a single avocado orchard rather than the development of aggressiveness in response to phosphite treatment. A replicated trial produced similar though less significant results, but required much longer incubation for disease .to develop, probably due to the slightly more mah1re clonal plant material used.