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Isolation of Pepper Races 4 and 5 of Xanthomonas campestris pv. vesicatoria from Diseased Peppers in Southeastern U.S. Fields. C. S. Kousik, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616 . D. F. Ritchie, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616 . Plant Dis. 79:540. Accepted for publication 5 April 1995. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/PD-79-0540C.

Five host-differentiated races (P0-P4) of Xanlhomonas campestris pv. vesicatoria (Doidge) Dye (Xcv) that cause bacterial spot on pepper (Capsicum annuum L.) have been reported (1). Three single dominant genes in pepper confer resistance to these races. Gene BsI provides resistance to races P0, P2, and P5 (described below), gene Bs2 to races P0-P3, and gene Bs3 to races P0, PI, and P4 (1). Strains of Xcv virulent on pepper near-isogenic line ECW-20R (contains resistance gene Bs2) recently were isolated from tomato in Barbados (3). Previously, no field strains of Xcv were reported to overcome the Bs2 gene. In summer 1994, pepper cultivars Camelot (susceptible to all races), X3R Camelot (resistant to races P0-P3), and King Arthur (resistant to races P0, P2, and presumably P5) were grown in a 0.25-ha research plot not planted previously with peppers and located 20 km from any commercial pepper fields. Inoculum on diseased pepper plants was introduced into the plot 1 week after transplanting in late April and consisted of two strains each of race PI (copper resistant and streptomycin sensitive [CuR,SmS]) and race P2 (CuR.SmR). Bacterial spot developed rapidly on Camelot and King Arthur with the predominant race being PI. In late July, several Xcv strains (CuR,SmS) isolated from each of the three cultivars elicited a hypersensitive reaction (HR) on ECW-30R (contains resistance gene Bs3) but not on ECW-20R or ECW-10R (contains resistance gene BsI) thus, they were race P4. In mid-January 1995, pepper plants (cv. Camelot) expressing bacterial spot symptoms from a commercial planting in southwestern Florida and submitted by a consultant based in North Carolina, were received in the Plant Diagnostic Laboratory at N. C. State University. Pepper race P2 strains and a strain that elicited an HR in ECW-10R and a compatible reaction in ECW-20R and ECW-30R were detected. Based on the current system for race designation (1), strains eliciting this host response would be designated race P5. Currently, it is unknown how widespread and how fit strains of P4 and P5 are under field conditions. Strains that overcome resistance gene Bs2 have been selected in the laboratory and were found to have reduced growth rates (2). Races capable of individually overcoming the three resistance genes currently used in pepper now have been isolated from pepper fields in the southeastern U.S. This finding has significant implications for the successful use of current resistance to bacterial spot of pepper.

References: (1) H. Bouzar et al. Phytopathology 84:663, 1994. (2) B. Kearney and B. J. Staskawicz. Nature 346:385, 1990. (3) L. W. O’Garro and S. Tudor. Plant Dis. 78:88, 1994.