Examination of some morphologically unusual cultures of Phytophthora species using a mitochondrial DNA miniprep technique and a standardised sporangium caducity assessment.

Using the mitochondrial DNA miniprep technique, the identity of sixteen morphologically unusual cultures allocated to Phytophthora nicotianae, Phytophthora mexicana or Phytophthora porri was determined by comparison with a library of mtDNA band patterns obtained from reference cultures. Seven cultures were identified as Phytophthora nicotianae (including those assigned to Phytophthora mexicana and Phytophthora porri), six as strains of Phytophthora palmivora with small, ovoid, weakly caducous sporangia, and one as Phytophthora citrophthora. Some cultures of P. nicotianae had a low percentage of caducous sporangia. Percentage sporangium caducity, but not sporangium L:B ratio, is considered a useful taxonomic criterion for separating species morphologically similar to Phytophthora nicotianae. One culture from tobacco in New Zealand had a highly unusual morphology and a unique DNA band pattern, but was not identifiable. One culture from Acacia mearnsii in South Africa had a unique DNA band pattern which was identical to that of an isolate from Annona squamosa from Australia previously identified as Phytophthora palmivora, the precise identity of which is still unclear. The identity of most isolates from diseased durian was found to be Phytophthora palmivora, confirming its role as the main pathogen, but P. nicotianae was also identified from this host.