The Use of Subtractive Hybridization to Obtain a DNA Probe Specific for Pseudomonas Solanacearum Race 3

Pseudomonas solanacearum, the causal agent of bacterial wilt, has been classified into three races based on host range and into five biovars based on physiological properties. Strains of race 3 belong exclusively to biovar 2 and primarily affect potatoes. Although this race is thought to have originated in the Andean highlands, it has unusual physiological properties that make it a potential threat to potatoes grown at the cooler latitudes worldwide. Consequently, there is need for a rapid and sensitive method for detection of race 3 strains. We have used subtractive hybridization to enrich for race 3-specific DNA sequences in total race 3 genomic DNA, and thereby obtained a 2 kb clone homologous to DNA from all 28 race 3 strains tested, but with only five of 90 non-race 3 strains. In addition, two larger regions of the genome, containing a minimum of 23 kb of DNA, are also specific for race 3. Deletion of this DNA did not affect virulence. This race 3-specific DNA is a potentially useful diagnostic tool for the detection of race 3 strains.