Competence of Xanthomonas campestris from Cruciferous Weeds and Wallflower (Erysimum cheiri) to Induce Black Rot in Cabbage

Abstract

In order to verify the role of cruciferous weeds and ornamentals to serve as a source of primary inoculum of Xanthomonas campestris pv. campestris (Xcc), the causal agent of black rot of Brassicas, Xanthomonas isolates from shepherd’s purse (Capsella bursa-pastoris), fanweed (Thlaspi arvense) and wallflower (Erysimum cheiri) were characterized by molecular data and by their pathogenicity on kohlrabi and shepherd’s purse. These characteristics were compared to those of xanthomonads isolated from infested Brassica crops and contaminated transplants. Only one of 30 Xanthomonas isolates from shepherd’s purse in boundary ridges and field paths adjacent to Brassica fields, and from plants outside the Brassica growing area, was able to induce black rot (BR) on kohlrabi; nearly all of them (except one) were able to cause black rot like symptoms with black vascular veins (BRL) on shepherd’s purse. Six of ten tested isolates from different asymptomatic weeds growing in BR infested cauliflower fields were able to induce BR on kohlrabi, but all isolates caused BRL on shepherd’s purse. All Xanthomonas isolates from symptomatic wallflower induced BR on kohlrabi and BRL on shepherd’s purse. Inoculations with isolates from BR infested cauliflower, representing the pathovar campestris (Xcc), always led to BR on kohlrabi and BRL on shepherd’s purse. The isolates from rucola and radish field crops and from Brassica transplants showed typical symptoms of the pathovars campestris (Xcc) and raphani (Xcr), respectively. In a PCR with the newly designed primer set SP-1, only the isolates which were able to induce BR on kohlrabi could be detected. The sequence analysis of the gyrB-gene showed that all Xanthomonas isolates which induced BR on kohlrabi clustered in one major clade, while the remaining isolates clustered into several different clades. Based on these results, the authors conclude that Xanthomonas isolates from shepherd’s purse may not constitute a serious risk to serve as primary inoculum for BR epidemics in Brassicas. By contrast, Xanthomonas isolates from wallflower have this potential. Most of the isolates from cruciferous weeds and the wallflower isolates do not fit into the actual classification of Xanthomonas campestris pathovars.