What organism cause lactuca sativa dark brown spot?
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1995, a bacterial leaf spot disease was observed on lettuce plants grown on muck soils in north central Ohio. Characteristic symptoms were water-soaked circular, dark brown-or olive-colored spots, approximately 1 to 2 mm in diameter, along the margin of leaves; V-shaped lesions beginning at the leaf margins and advancing along the veins; and individual black spots scattered on the leaf surface. Five bacterial strains were isolated from infected leaves of two lettuce cultivars, Darkland and Go Go Green. These strains were identified as Xanthomonas campestris pv. vitians based on morphological, physiological, biochemical, and pathogenicity tests, as well as fatty acid methyl ester (FAME) analyses. One of the five strains tested was resistant to streptomycin sulfate (100 mu g/ml), and none were resistant to copper sulfate. Eight commercial lettuce cultivars were evaluated for resistance to X. campestris pv. vitians. Of these cultivars, Redine was highly resistant; Focus and Crisp and Green were partially resistant; Slobolt, Tiara, and Carmona were susceptible; and Darkland and Go Go Green were highly susceptible to the pathogen. Surface disinfection with 0.52% sodium hypochlorite for 5 min nearly eliminated the bacterium from lettuce seed without affecting seed viability.