Screening and identification of resistance to bacterial soft rot in Brassica rapa

Mist-chamber, field, and detached leaf inoculation procedures identified plants resistant to bacterial soft rot [ Erwinia carotovora subsp. carotovora, ( Ecc)] in Brassica rapa and related species. The mist-chamber seedling inoculation provided the best correlation of mean disease severity ratings with the field plant inoculation (r = 0.67**) and was used to identify resistant materials. The optimum mist-chamber incubation conditions to distinguish the resistance of accessions were 23 degreesC and 100% RH and were used for primary screening. A total of 752 accessions of B. rapa and related Cruciferae were screened. In general, accessions of B. oleracea were more resistant than accessions of B. rapa. Within B. rapa, subspecies pekinensis and chinensis were more susceptible than other subspecies. No completely resistant material was found. In species of B. rapa, only 7% of accessions showed some degree of resistance with plant-to-plant variation within the accessions. G30444, G30449, and AVRDC2837 were identified as the most resistant materials in B. rapa by both mist-chamber and field inoculations. The resistance was correlated between an USA isolate (Geneva-1) and two Chinese isolates (RL4-1 and RL-19) of Ecc.