Development of a Diagnostic Test Based on the Polymerase Chain Reaction (PCR) to Identify Strains of R. solanacearum Exhibiting the Biovar 2 Genotype

Abstract

R. solanacearum has been classified into three races based on host range (Buddenhagen et al. 1962) and five biovars based on oxidation of sugars and sugar alcohols (Hayward 1964) the only agreement between the two schemes is that R. solanacearum biovar 2 is equivalent to R. solanacearum race 3. R. solanacearum biovar 2/race 3 isolates have a very narrow host range, being limited almost entirely to potatoes. Bacterial wilt (Brown rot) caused by R. solanacearum isolates of biovar 2/race 3 is a significant disease of potatoes world wide. Infection of potato tubers with R. solanacearum biovar 2/race 3 may become latent under conducive environmental conditions. Other races of R. solanacearum can infect potatoes but it is the biovar 2/race 3 phenotype that is the most persistent and potentially the most destructive phenotype for potatoes. Consequently, there is a need for rapid, sensitive diagnostic tests for identification of plant material infected with the biovar 2/race 3 phenotype. Tests for the detection of R. solanacearum in potatoes have traditionally been directed against the entire species and have not concentrated on the biovar 2/race 3 phenotype. Skoglund et al. (1993) report on molecular and immunological techniques applied to the identification of infected tubers in Burundi, all the tests were directed toward the species R. solanacearum not the biovar 2/ race 3 phenotype. The only test described to identify the race 3/biovar 2 phenotype is the DNA probe based test of Cook and Sequeira (1991). We describe here the first test for the biovar 2/race 3 phenotype of R. solanacearum employing the polymerase chain reaction.