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.

SOMATIC HYBRIDIZATION BETWEEN CITRUS RETICULATA AND CITROPSIS GABUNENSIS THROUGH ELECTROFUSION

Protoplasts were isolated from embryogenic calli of Citrus reticulata cv. Ponkan and Citropsis gabunensis, and fused in electric fields. The maximal fusion efficiency was obtained by application of AC at 75 V/cm (1.0 MHz) and DC square-wave pulse at 1.125 KV/cm for 40 usec. Fusion-treated protoplasts were cultured on MT medium without phytohormone, solidified with 0.6% agar. Colonies from the protoplasts were proliferated on MT medium with zeatin 1 mg/l and 0.9% agar. Selection of somatic hybrid callus was based on chromosome count and isoenzyme analysis. The somatic hybrids were tetraploid (2n=36). C. reticulata and C. gabunensis were both homozygous at Got-1 locus, but distinguishable easily because band of the latter migrated faster than that of the former. In zymogram of somatic hybrid, both parent bands were retained and a new hybrid band was also evident between them. Embryos from somatic-hybrid callus regenerated intact plant. The hybrid plants showed intermediate morphological characteristics of the parents.