Evaluation of the effect of seed treatments, bactericides, and cultivars on bacterial leaf spot of lettuce caused by Xanthomonas campestris pv. vitians

As a part of a broader program for management of bacterial leaf spot, the e ffects of lettuce-seed treatments, greenhouse application of bactericides, and cultivars were evaluated. Seed artificially inoculated with Xanthomonas campestris pv. vitians was treated with bactericides or heat treated and e valuated for the incidence of contaminated seed and seed germination. Seed soaked in 1% sodium hypochlorite for 5 and 20 min had an incidence of conta minated seed of less than 10%. Dry-heat (1 h), hot-water (50 degrees C, 2 h ), and organic-acid treatments significantly reduced seed germination. Cons idering both the effects on incidence of contaminated seed and seed germina tion, the best treatments were soaking the seeds in 1% sodium hypochlorite for 5 or 20 min. Copper sulfate, alone or mixed with Zineb or Dithane, fail ed to control the disease and caused phytotoxicity. All of the other bacter icides significantly reduced the severity of bacterial leaf spot. However, the differences among bactericide efficacy were too small to allow comparis on between the different forms of copper used, as well as the effect of add ing manganese and zinc (Dithane) or zinc alone (Zineb) to the copper produc t. Nevertheless, copper hydroxide alone, mixed with Zineb or mixed with Dit hane, and basic copper sulfate reduced disease severity by 86.89, 78.67, 80 .42, and 81.82%, respectively, without causing phytotoxicity. For the two y ears of cultivar evaluation, no significant difference in mean disease seve rity was observed among the cultivars. Based on disease incidence, the most susceptible cultivar was Bellagreen. Cvs. Ideal cos, Grand Teton, Great La kes, Paris Island, Ithaca, and Optima showed intermediate susceptibility, a nd the least-susceptible cultivars were Waldmann's and Grand Rapids, both g reen-leaf type. There was no significant difference between the three romai ne (cos) cultivars and between the two crisphead cultivars, but a significa nt difference was observed between the two butterhead types, Bellagreen and Optima, which had 80.04 and 48.01%, respectively, of their leaves diseased at the time of harvest.