Potential of weed species to serve as a reservoir for Xanthomonas campestris pv. vitians, the causal agent of bacterial leaf spot of lettuce

The ability of Xanthomonas campestris pv. vitians (Brown) Dye, the causal agent of bacterial leaf spot of lettuce, to colonize plant surfaces as well as its ability to cause symptoms was investigated for several weed species belonging to the Asteraceae, Chenopodiaceae, Malvaceae, Polygonaceae and Portulacaceae families. A bacterial suspension of X. campestris pv. vitians was sprayed onto aerial parts of the weed plants. Bacterial population size was determined after 4 weeks of incubation in a greenhouse at 24 °C and 95% relative humidity. Bacterial population sizes were determined by dilution plating of the leaf washings and expressed in CFU (colony forming units) per gram of plant tissue. X. campestris pv. vitians was recovered from all weed species. The bacterial population size was found to vary significantly among the different weed plant species (P < 0.0001, F = 7.358). Significantly larger populations were recovered on Lactuca species (>105 CFU/g of plant) than on Chenopodiaceae, Polygonaceae and Portulacaceae and Astereae (a tribe of the Asteraceae) (<102 CFU/g of plant). Symptom development was investigated using two inoculation methods: spraying and injection. Symptoms were evaluated 2, 7 and 14 days after inoculation. In each trial, independently, symptoms developed on all Lactuca species, regardless of the method used. Symptoms were observed less consistently for other species of the family Asteraceae with the exception of members of the tribe Astereae, in which no symptoms developed. For species belonging to other families, no symptoms were observed, except on Nicotiana tabacum, which belongs to the family Solanaceae.

► Plant species affect the bacterial population size of Xanthomonas campestris pv. vitians.
► X. campestris pv. vitians maintained higher population densities on Lactuca species.
► X. campestris pv. vitians caused symptoms on several Asteraceae species.
► X. campestris pv. vitians caused symptoms on a Solanaceae species.