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A Serious Outbreak of Race 6 of Xanthomonas campestris pv. vesicatoria on Pepper in Southern Florida

In the 1997-1998 winter vegetable season, a widespread and serious outbreak of bacterial spot occurred on pepper in commercial fields throughout southern Florida. This was the first serious epidemic of bacterial spot on pepper in southern Florida since the 1993-1994 season. Cultivars affected included those with resistance to races 1--3 of X. campestris pv. vesicatoria, commonly found in previous Florida surveys (1). Field sampling designs, isolation methods, and pathogen identification were carried out as previously described (1). Twenty-seven fields were sampled in Palm Beach, St. Lucie, Martin, Broward, and Collier counties. The race of each strain was determined by infiltration of 3.0 × 10⁸ CFU per ml of sterile tap water suspension into Early Calwonder and three near-isogenic lines of Early Calwonder with genes for vertical resistance to specific races of the pathogen. Test plants were maintained in the greenhouse and observed for hypersensitive and disease reactions over several days. Of a total of 244 X. campestris pv. vesicatoria strains collected, 73.4% were identified as race 6 and 10.2% as race 4. Irrespective of the presence of resistance genes in cultivars sampled, race 6 (eastern Florida) and race 4 (southwestern Florida) predominated in this study. Copper tolerance was widespread among X. campestris pv. vesicatoria strains of all races, with 91% of the strains showing tolerance to 250 g per ml of CuSO₄ · ₅H₂O incorporated into 0.5% (wt/vol) glucose-nutrient agar. Selection pressure resulting from widespread planting of race 1, 2, 3-resistant cultivars may have contributed to this outbreak. High disease pressure also may be related to the unusually wet winter. Rainfall accumulation from December 1997 through March 1998 was 25.6 cm above normal.

Reference: (1) K. Pohronezny et al. Plant Dis. 76:118, 1992.