Colonization of Cauliflower Blossom (Brassica oleracea) by Xanthomonas campestris pv. campestris, via Flies (Calliphora vomitoria) Can Result in Seed Infestation

Inoculation of cauliflower blossom with Xanthomonas campestris pv. campestris (Xcc), by brush or pollination with blue bottle flies (Calliphora vomitoria) as a vector, can result in seed infestation. Two years of poly-tunnel experiments with fly inoculation of cauliflower has shown that in approximately 30% of seed batches of a breeding line, high densities of Xcc (>10 000 CFU/g) were found both before and after seed treatment with warm water. The presence of Xcc in seed derived from fly-inoculated plants was confirmed in a grow-out test. After brush-inoculation, on average 45% of seed batches were contaminated with high densities of Xcc before disinfection and 40% after seed disinfection. A grow-out test, however, did not confirm the presence of internally infected seed. In a 1-year trial, no seed infections were found in warm water-treated seed derived from cv. Opaal, an open pollinated cultivar. No indication of internal seed infection was found after blossom inoculation with X. c. pv. armoraciae (Xca). To obtain contaminated flies as a vector for Xcc, flies were fed on agar-grown bacteria. Initial populations of Xcc per fly were 105 CFU, but population densities decreased rapidly and survived for 5 days maximum. During the same period, populations of other bacteria associated with flies increased slightly. The implications of blossom infection and the role of pollinating insects in dissemination and transmission of Xcc are discussed.