Species-specific PCR identification and detection of Phytophthora drechsleri, P. cryptogea and P. erythroseptica

Phytophthora drechsleri, P. cryptogeaand P. erythroseptica are phylogenetically closely related Oomyceteous plant pathogens which are morphologically similar. In order to discriminate these taxa from each other and from species with convergent morphological characteristics a simple as well as a nested-PCR based method was developed. A collection of isolates of each species from different hosts representing world-wide diversity of species were examined for unique regions of nuclear as well as mitochondrial genes. Six candidate PCR primers were designed and calibrated for species-specific amplification of P. drechsleri based on the DNA sequences of rDNA internal transcribed spacer regions and the cytochrome c oxidase subunit I, and also three candidate PCR primers specific for P. cryptogeaand P. erythroseptica were designed and calibrated based on cytochrome c oxidase subunit I. Studies showed that the best primer set for identification of P. drechsleri was the combination of ITS-DF2 and ITS-DR2, which amplified a 567 bp band. The combination of COX-CF1 and COX-CR2 was the best set for discrimination of P. cryptogea/P. erythroseptica from other species which amplified a 415 bp product from both species. A restriction map analysis of P. cryptogea/P. erythroseptica indicated that the non-palindromic Mnl I enzyme restriction site was unique to amplicons of P. erythroseptica isolates and could be employed to distinguish this species from P. cryptogea. Based on this study, nested-PCR was at least 100 times more sensitive than conventional PCR for detection of these species.