Methods to reduce the spread of the black rot pathogen, Xanthomonas campestris pv. campestris, in brassica transplants

Abstract

Black rot of Brassica spp. caused by the bacterial pathogen Xanthomonas campestris pv. campestris (XCC), is a seedborne disease. Because the pathogen can easily be spread by splashing water, there is a high risk of spreading it by overhead irrigation systems during the raising of cabbage transplants. In glasshouse experiments, the efficacy of different methods to reduce XCC dispersal and subsequent black rot development during the raising of transplants were examined. The three methods tested were 1) chlorine dioxide dosing of overhead irrigation water, 2) spray treatments of transplants with commercially available products that contain disinfectant or have bactericidal properties (benzoic acid and copper hydroxide) and 3) an ebb and flood irrigation system. Trays with inoculated cauliflower transplants that had been raised from modules were placed between trays with healthy transplants. XCC transmission from the spray inoculated infector to non-inoculated transplants was detected using PCR after four days. The mean efficiency of 1–2 sprayings with benzoic acid and copper hydroxide in reducing black rot development was 11.6% and 32.2%, respectively. Overhead irrigation with chlorine dioxide enriched water resulted in a mean efficiency of 93.5%. No symptoms of black rot were observed in the ebb and flow irrigation system. The proportion of symptomless, XCC-DNA positive transplants at the end of the incubation period followed the same pattern as the visual black rot development. To obtain a minimum concentration of 0.21 mg l⁻¹ chlorine dioxide effective against XCC, a feed-in concentration of 3 mg l⁻¹ was used, resulting in a final concentration of 0.5 to 1.0 mg l⁻¹ wetting the leaf surface. The positive effects of both “sub-” and “chlorine dioxide dosed” irrigation to reduce the spread of XCC during transplant raising were still apparent in the field and resulted in a healthier crop.