Resistance to bacterial soft rot (Erwinia carotovora subspecies carotovora) in Brassica rapa vegetables

Optimized inoculation procedures were established to identify accessions of Brassica rapa and related Cruciferae for resistance to bacterial soft rot caused by Erwinia carotovora subsp. carotovora (Ecc). Mist-chamber inoculation provided the best correlation of mean disease severity ratings with results obtained in field inoculation (r = 0.673**). The optimum mist-chamber incubation conditions for distinguishing genetic resistance were 23The continuous distribution of mean disease severity ratings in the populations included in this study indicated that resistance to bacterial soft rot in B. rapa is a quantitative trait. The broad-sense heritability varied from 39% to 72%, and the narrow-sense heritability varied from 13% to 60%. Additive genetic effects were most significant. The resistant sources involved in this study appeared to have different genes or alleles, which were additive and partially dominant. XRecurrent phenotypic selection was used to increase the level of resistance to bacterial soft rot in B. rapa by combining different resistance genes from the most promising sources. After three cycles of recurrent selection, the levels of resistance increased markedly. When the cycle 3 population was compared with the cycle 0 population, the mean disease severity rating was reduced 38% from 6.3 to 3.9 and the percentage of plant survival increased from 65% to 97% based on average of two year experiments using the mist-chamber inoculation method. The dramatic improvement in resistance was also demonstrated in the field experiments. Four cycle-3 lines with the lowest disease ratings (about 3.3) were released to seed companies for use in commercial breeding programs.Another strategy, protoplast fusion, was used to combine different sources of resistance genes. Twelve somatic hybrids were produced between selected genotypes of B. rapa and B. oleracea, and were confirmed by morphology, nuclear DNA content, and RAPD analysis. The somatic hybrids were fertile, and set seeds following self-pollination and in backcrosses to B. rapa. Most somatic hybrids had lower disease severity ratings than the B. rapa parents did, and some progenies from selfing and backcrossing of somatic hybrids showed much higher level of resistance (with disease severity ratings of 1 to 2) than either parent. The data confirmed the possibility of obtaining highly resistant material of B. rapa by further backcrossing and selection.