Multilocus Sequence Typing of Strains of Bacterial Spot of Lettuce Collected in the United States.

Studies on genetic diversity and recombination in bacterial pathogens are providing a better understanding of the mechanisms shaping bacterial diversity, which can affect disease control. Xanthomonas campestris pv. vitians, causal agent of bacterial leaf spot of lettuce, is a threat to the worldwide lettuce industry. We examined the genetic variation within a sample of 83 strains from California, Florida, and Ohio using multilocus sequence typing of six housekeeping genes, totaling 2.7 kb. Additionally, polymorphism in two virulence-related genes, hrpB2 and a putative glycosyl hydrolase, were examined. Based on housekeeping genes, we found three genetic groups of strains that were all able to induce the disease. These included strains collected from weeds and irrigation water that had haplotypes identical to strains from diseased lettuce. High linkage disequilibrium across the sequenced loci indicates that the pathogen is predominantly clonal but recombination has contributed to the observed sequence variation. Although there was significant genetic variation in X. campestris pv. vitians within and among sampled states, identical haplotypes were observed across all three states. This finding suggests that seedborne inoculum may contribute to the diversity of X. campestris pv. vitians in the United States. Knowledge of the genetic structure of the pathogen may be used for developing resistant lettuce varieties.

Bacterial Leaf Spot of Lettuce: Relationship of Temperature to Infection and Potential Host Range of Xanthomonas campestris pv. vitians.

Epidemiological aspects, including optimum temperature for infection and host range of Xanthomonas campestris pv. vitians, causal organism of bacterial leaf spot (BLS) of lettuce, were investigated. The optimum temperature for infection was determined to be 22.7°C based on growth chamber studies. Internal populations were monitored over time in lettuce, tomato, pepper, parsley, cilantro, and beet. Each plant species was infiltrated with the bacterium at 105CFU/ml. Highest populations developed in lettuce (108CFU/cm2) followed by pepper with 106CFU/cm2, whereas the other plant species harbored much lower populations (105 to 103CFU/cm2). Infectivity titration endpoints were similar in pepper and lettuce (103 to 104CFU/ml). For other plant species tested, infectivity titration endpoints were 106 to 107 CFU/ml. Electrolyte leakage data and corresponding internal population data support the conclusion that fresh-market tomato is not a host of X. campestris pv. vitians but, instead, interacts in an incompatible response. Electrolyte leakage from cells of tomato plants inoculated with X. campestris pv. vitians or a pepper strain of X. axonopodis pv. vesicatoria peaked at 48 h, suggesting that tomato is not a host for the BLS pathogen. Both electrolyte leakage and population dynamics results point to pepper as a potential host of X. campestris pv. vitians.

Investigation of Seed Treatments for Management of Bacterial Leaf Spot of Lettuce.

Chemical seed treatments were evaluated for efficacy of disinfestation of lettuce seed that had been inoculated with Xanthomonas campestris pv. vitians. Three concentrations of each chemical were evaluated by treating seed lots for 5 or 15 min. In addition, the effects of each seed treatment on seed germination and early plant growth were examined by observing seed germination rates. Bacteria were not detected when seed were treated with 3 or 5% hydrogen peroxide for 5 or 15 min. Treatment of seed with 0.52% sodium hypochlorite was relatively ineffective at 5 and 15 min. When sodium hypochlorite was used at a 1% concentration for 15 min, the level of bacterial infestation was reduced to 2%. Suspensions of copper hydroxide plus mancozeb also reduced seedborne inoculum to ≤2%. Treatment of seed with copper hydroxide alone, benzoyl peroxide, or calcium peroxide did not reduce seed infestation levels significantly. Seed germination rates were 90% or greater for the majority of seed treatments tested in laboratory assays. Hydrogen peroxide treatments at a concentration of 5% reduced seed germination up to 28% compared with controls. However, no significant differences in germination were observed among control treatments (noninoculated, nontreated seed and inoculated, nontreated seed) and any of the chemical seed treatments when seed were sown in a pasteurized soil mix in the greenhouse.

Epiphytic Populations of Xanthomonas campestris pv. vesicatoria on Pepper: Relationships to Host-Plant Resistance and Exposure to Copper Sprays.

Epiphytic populations of Xanthomonas campestris pv. vesicatoria were monitored on leaves and in buds of resistant and susceptible pepper cultivars over five seasons. Epiphytic populations were higher on the susceptible cultivar Jupiter than on the resistant cultivar Boynton Bell in four of the five seasons. In copper-sprayed plots, populations in buds of Jupiter plants were consistently higher than on leaves. In one test, copper sprays reduced X. campestris pv. vesicatoria populations by 99% on leaves, compared with only a 51% reduction in buds. In another test on a commercial pepper farm, no differences were found between populations on resistant and susceptible plants spaced as close as 1.2 m. However, combined data on buds for the three cultivars showed populations were higher than those on leaves on three of four sampling dates. These differences were attributed to the heavy use of copper on this farm.