Pythium spp. from cavity spot and other root diseases of Australian carrots

A survey of Pythium spp. from carrot-growing regions of Australia showed that P. sulcatum was the most common pathogenic species and occurred in all Australian states. It was isolated from carrots with a range of symptoms including cavity spot, and brown, discoloured, forked or hairy roots. P. violae was also isolated from tissue with cavity spot symptoms, but only from South Australia and from properties along the Murray River in Victoria. All P. violae isolates tested were pathogenic. This is the first record of P. violae from Australian carrots. Some isolates of P. irregulare, P. ultimum, Pythium HS-group and Pythium 'D' were also pathogenic in in vitro tests. RAPD analysis of a subset of Australian P. sulcatum isolates showed that they clustered into two clades. The type and isotype formed a third clade. Isolates from Tasmania were in clade 1, isolates from Queensland and southern Victoria were in clade 2; isolates from New South Wales, Western Australia and the Murray River basin were in both clades. Comparisons between RAPD clades showed that isolates differed in growth rate and oospore diameter but not in oogonium diameter. Australian isolates of P. sulcatum are plerotic, whereas the type is aplerotic. The restricted host range of P. sulcatum, its genetic diversity, together with the small number of sites where carrots are grown, indicate that it may be a pathogen of native Apiaceae that has become a pathogen of carrots.