First Report of Pseudomonas syringae pv. tomato Race 1 on Tomato in California

Symptoms of bacterial speck have occurred sporadically in California since 1993 on tomato (Lycopersicon esculentum) cultivars that possess the PTO gene for resistance to Pseudomonas syringae pv. tomato race 0. In 1998, the disease was found on numerous cultivars resistant to race 0 in fields throughout the Sacramento Valley. In several fields, it caused severe defoliation of tomato seedlings. Symptoms included dark brown to black leaf and stem lesions surrounded by yellow halos. Seven isolates were identified as P. syringae pv. tomato based on analysis of whole cell fatty acids (MIDI, Newark, DE; similarity indices of 0.746 to 0.710), carbon utilization profile (Biolog, Hayward, CA; similarity indices of 0.556 to 0.840), and LOPAT results: levan production (+), oxidase reaction (-), potato soft rot (-), arginine dihydrolase production (-), and tobacco hypersensitivity (+) (2). The isolates were identified as race 1 by inoculating greenhouse-grown tomato plants of cv. Ontario 7710, which possesses the PTO gene, and cv. H1916, which is susceptible to both known races of P. syringae pv. tomato. At least eight plants of each cultivar were inoculated with each isolate. Plants were inoculated by spraying leaves with a bacterial suspension mixed with Carborundum or puncturing leaves with needles dipped in the bacterial suspension (10⁷ cells per ml). Controls were inoculated with water. All plants inoculated with the bacteria developed speck symptoms in 6 days. The bacteria were reisolated from all inoculated plants and confirmed as strains of P. syringae pv. tomato. This is the first documentation of P. syringae pv. tomato race 1 in California. This race was previously reported in Canada in 1986 (1). References: (1) M. B. Lawton and B. H. MacNeill. Can. J. Plant Pathol. 8:85, 1986. (2) R. A. Lelliott et al. J. Appl. Bacteriol. 29:470, 1966.