Field crop residue and other potential inoculum sources for the bacterial spot pathogen in Ontario

Bacterial spot is a major disease of Ontario field tomatoes. The primary causal agent is the Group D form of Xanthomonas campestris pv. vesicatoria, also known as Xanthomonas gardneri. Seeds are thought to be the major source of inoculum but it is not known if this pathogen can survive and spread through an association with weeds, volunteer tomato plants and infected crop residue. To determine overwinter survival on crop residue, we buried debris from field plants that had been infected with Group A, B, C or D strains in nylon bags at a depth of 10 cm in mid-November. The bags were recovered the following April and sampled for the pathogens using DNA probes and a semi-selective medium. The ability of Group B and D strains to survive for two or more winters on buried crop debris was determined using a similar experimental design. Over three summers, samples (foliage and roots) of various weed species, volunteer tomatoes and wheat plants were collected from fields that had been in tomatoes the previous year and had been severely infected with the pathogen. These samples were screened for bacterial spot using DNA probes and the same semi-selective medium. The presence of the pathogen in any probe-positive sample was confirmed by PCR and pathogenicity tests. Groups B, C and D survived an Ontario winter well on crop residue but could not be found after a second winter. Small Group D populations were confirmed as present on two weed species but the association appeared random. Although the sampling was relatively small (153 samples including 32 weed species), it appears that Group C and D forms of the pathogen may not survive from one season to the next on volunteer tomatoes, wheat plants or the weeds commonly found near Ontario tomato fields.