Improved control of black rot of broccoli caused by Xanthomonas campestris pv. campestris using a bacteriophage and a nonpathogenic Xanthomonas sp. strain

We examined the potential for biological control of black rot of broccoli, caused by Xanthomonas campestris pv. campestris ( Xcc ), using nonpathogenic Xanthomonas sp. strain 11-100-01 (np X ) mixed with bacteriophage XcpSFC211 (pXS). Inoculation of intact broccoli plants in greenhouse trials with either np X or pXS did not control black rot. After injured plant inoculation, however, np X alone or np X with pXS significantly controlled black rot. When a mixed suspension of np X with pXS was placed on a membrane filter, then washed with distilled water and air-dried, a substantial amount of pXS adsorbed to the surface of np X . In a field trial, broccoli plants were sprayed with a suspension of np X with pXS, then inoculated with Xcc . A meta-analysis of the results from five field trials showed an integrated risk ratio (IRR, the ratio of disease incidence in inoculated broccoli plants to the incidence in control plants) of 0.69 after treatment with only np X and 0.59 with np X with pXS, indicating that black rot incidence was significantly reduced by each treatment. The difference between these two treatments was also significant. IRR was 1.24 when comparing suppression by np X with pXS and that by basic copper sulfate wettable powder; thus, their control was comparable. The combination of np X with pXS improved the preventive effect against black rot. This is the first report describing that a nonpathogenic Xanthomonas sp. strain mixed with a bacteriophage effectively controlled black rot of broccoli in field trials.