Mark Dobrowolski

Genetic studies of Phytophthora cinnamomi using isozymes have revealed low levels of diversity suggesting, though not proving, clonality in a large proportion of worldwide populations (1). In Australia, only three isozyme types (representing both mating types) are found with no evidence for sexual recombination (2). Using microsatellite markers, we have shown that these isozyme types are clonal lineages of P. cinnamomi and that these same lineages are found elsewhere in the world (3). Our study used 647 isolates from three intensively and hierarchically sampled P. cinnamomi disease fronts located in south-west Australia. In addition 133 isolates from an Australia-wide culture collection and 27 isolates from elsewhere in the world were analysed with four microsatellite markers. One disease front contained all three clonal lineages within close proximity in soil and plant tissue but no sexual recombinant isolates were found, even with very intensive sampling. However, within these clonal lineages we frequently found evidence for mitotic recombination (mitotic crossing over). This mechanism for producing genetic variation may explain phenotypic variation known to occur within the identified clonal lineages.

Phosphite is increasingly being used as a means of control for dieback caused by Phytophthora cinnamomi. We wish to study the likelihood of P. cinnamomi evolving resistance to phosphite, given the clonal populations of the fungus present in Western Australia. We have collected isolates of P. cinnamomi from areas where phosphite has been used intensively for up to 15 years (avocado orchards) as well as areas of less frequent use add no use of phosphite. Our testing involved stem inoculating a clonally propagated host (Leucadendron sp.) that was treated with one of three levels of phosphite (0%, 0.25% and 0.5%). We measured the extent of colonisation by each P. cinnamomi isolate after eight days of incubation in a controlled temperature plant growth cabinet. Preliminary results suggest 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 areas of phosphite use. Research is continuing to replicate these results and investigate their significance to the control of P. cinnamomi using phosphite.