Reaction of Indicator Tobacco Cultivars to Races of Pseudomonas syringaepv. tabaciTox+

Abstract Three races of Pseudomonas syringaepv. tabaciTox+ (wildfire) (races 0, 1 and 2) and two races of Tox- (angular leaf spot) (races 1 and 2) have been confirmed on tobacco in Zimbabwe (Zim). Very few cultivars with no resistance to Ps. syringaepv. tabaci are grown commercially and race 0 has not been isolated since 1996. Because we no longer have a viable culture of race 0, we obtained an isolate of race 0 from Kentucky (0 KY), USA in January 2000. We included this isolate in race tests on standard indicator cultivars K E1 (susceptible to all races), KM 10 (resistance to race 0 derived from Nicotianalongiflora), WZ (resistance to races 0 and 1 derived from N. rustica) and a hybrid, K 35 (resistance to races 0 and 0 and 1 derived from N. longiflora and N. rustica respectively). Two leaves on 10-week-old seedlings were inoculated with a bacterial suspension (106 colony forming units [cfu] per mL) by spraying selected areas until just watersoaked and incubating the plants at 28 C and 70% RH for 10 d. The reaction to race 0, measured as lesion diameter, was different from that previously obtained with race 0 (Zim). Races 0 and 1 (Zim) are avirulent on WZ but race 0 (KY) was virulent. Further isolates of race 0 were received from Maryland (MD) and Tennessee (TN). The TN isolates overcame resistance derived from N. longiflora and N. rustica, except where both sets of genes were present in the same cultivar. Reactions have been variable with the race 0 (MD) isolate suggesting it is a mixed culture. We conclude that there are at least four races of Ps. syringaepv. tabaciTox+ worldwide and race 0 (KY) should be designated race 3. On all cultivars, race 2 consistently caused the largest lesions.