Limited Genetic Diversity in North American Isolates ofPhytophthora erythrosepticaPathogenic to Potato Based on RAPD Analysis

Pink rot of potato (Solanum tuberosum), caused by Phytophthora erythroseptica, is found wherever potatoes are grown, and in the last decade, it has reemerged as an economically important disease in Canada and the United States. A selection of isolates of P. erythroseptica from major potato-growing regions in North America, namely Prince Edward Island and New Brunswick, Canada, and Maine and Idaho, U.S.A., was assessed for genetic diversity with randomly chosen decanucleotide primers which were used to amplify regions of DNA to reveal polymorphisms among templates (random amplified polymorphic DNA [RAPD]). The isolates varied in their geographic origin as well as in their sensitivity to mefenoxam, as determined by an in vitro assay. In three separate RAPD screens (I, II, and III) with 23 isolates of P. erythroseptica chosen from a larger collection, 1,410, 369, and 316 robust, scorable bands were amplified, respectively. However, among the bands amplified in screens I, II, and III, only 3, 1, and 3 bands, respectively, were polymorphic. When three primers yielding polymorphisms were used to screen 106 isolates from Prince Edward Island and New Brunswick, or a representative collection of 32 isolates from Prince Edward Island, New Brunswick, Maine, and Idaho, no major variation was discovered. RAPD markers were not correlated with geographic origin or mefenoxam sensitivity of the isolates. From an evolutionary standpoint, the absence of genetic diversity among the isolates of P. erythroseptica we examined may be attributable to the relatively recent introduction of a small founding population of the pathogen in North America.