VARIATION FOR BLACK ROT RESISTANCE IN TRONCHUDA REGENERATED FROM EPICOTYL SEGMENTS

Tronchuda (Brassica oleracea var. tronchuda Bailey syn. costata) regen erants with resistance to Xanthomonas campestris pathovar campestris P ammel (Dawson) were produced by culturing epicotyl segments of 3-week- old seedlings on Murashige and Skoog medium supplemented with 2 mg BAP and NAA at 0.1 mg . liter(-1). Cultures were placed in darkness (1, 2 , or 3 weeks) or in a 16-hour . day(-1) light regime. Seedlings of all four cultivars were more susceptible (mean disease severity rating of 3.8 to 4.0; where 1 = trace and 4 = more than 2 cm(2) diseased leaf t issue) than the respective regenerants. There were significant differe nces in disease severity of regenerants among and within the four cult ivars. Dark incubation of cultured explants generated plants with high er disease resistance than evident with control plants. One week of da rk incubation resulted in increased resistance in regenerants of 'Penc a de Chaves', 'Portuguesa', and 'Vilinda' (2.1, 2.3, and 2.7 mean dise ase severity, respectively), whereas 2 weeks of dark incubation increa sed resistance in regenerants of 'Ana Maria' and 'Vilinda' (2.1 and 2. 7 mean disease severity, respectively). The genotype x plant treatment interaction was significant. Chemical names used: 6-benzylaminopurine (BAP); a-naphthaleneacetic acid (NAA).