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

Mist-chamber, field, and detached leaf inoculation procedures identified pl ants resistant to bacterial soft rot [ Erwinia carotovora subsp. carotovora , ( Ecc)] in Brassica rapa and related species. The mist-chamber seedling i noculation provided the best correlation of mean disease severity ratings w ith the field plant inoculation (r = 0.67**) and was used to identify resis tant materials. The optimum mist-chamber incubation conditions to distingui sh 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 Cru ciferae were screened. In general, accessions of B. oleracea were more resi stant than accessions of B. rapa. Within B. rapa, subspecies pekinensis and chinensis were more susceptible than other subspecies. No completely resis tant material was found. In species of B. rapa, only 7% of accessions showe d some degree of resistance with plant-to-plant variation within the access ions. G30444, G30449, and AVRDC2837 were identified as the most resistant m aterials in B. rapa by both mist-chamber and field inoculations. The resist ance was correlated between an USA isolate (Geneva-1) and two Chinese isola tes (RL4-1 and RL-19) of Ecc.