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A Bacterial Leaf Disease of Lavender Caused by Xanthomonas campestris. . S. T. Koike, University of California Cooperative Extension, Watsonville 95076. S. A. Tjosvold, University of California Cooperative Extension, Watsonville 95076 and D. A. Cooksey and H. R. Azad, Department of Plant Pathology, University of California, Riverside 92521. Plant Dis. 79:859. Accepted for publication 12 June 1995. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/PD-79-0859B.

In Santa Cruz County, California, commercial nursery growers observed a leaf disease on container-grown French and English lavender (Lavandula dentala L. and L. officinalis Chaix, respectively). On both hosts, symptoms were observed only on older, fully expanded leaves and consisted of dark brown necrotic leaf lesions that were vein delimited and irregular in shape. Yellow mucoid bacterial colonies were consistently recovered from diseased tissues streaked onto sucrose peptone agar. All strains hydrolyzed starch weakly on MXP (1), but were strongly hydrolytic on Tween medium (2), indicating they were Xanthomonas spp. Biolog analysis identified strains from both hosts as Xanlhomonas campestris, with the greatest similarity (41.2 to 77.8%) to pathovar vesicatoria in the Biolog library. French (cv. Gray) and English (cv. Twickle Purple) lavender test plants were each inoculated with strains isolated from the two nursery-grown lavender species. Inocula consisted of nutrient broth shake cultures grown at 24C to 107 cfu per ml. Tween 20 (1 ml per liter) was added to each preparation, which was sprayed onto plants until runoff with a hand-held atomizer. Inoculated plants were incubated in a greenhouse (28 plus or minus 2C day/17 plus or minus 2C night natural light). After 14 days, dark brown lesions similar to those on nursery plants developed on both types of test plants inoculated with strains from either host. Symptoms did not develop on nutrient broth-inoculated controls. Putative strains of X. campestris were isolated from leaf lesions on inoculated plants and used to inoculate a second set of plants. Again, symptoms developed and strains of X. campestris were isolated from lesions. Recovered strains were phenotypically identical to the initial isolates. This is the first known report of a bacterial disease of lavender caused by X. campestris.

References: (I) L. E. Claflin el al. Phytopathology 77:730, 1987. (2) R. G. McGuire et al. Plant Dis. 70:887, 1986.