First Report of Bacterial Speck of Tomato Caused by Pseudomonas syringae pv. tomato in Tanzania

Abstract

In April 2004, there was a serious outbreak of a tomato (Lypersicon esculentum Mill.) leaf spot disease in Mgeta, Mvomero District of Tanzania. The disease was characterized by lesions on green tomato fruits that were small, sunken, and black and were surrounded by darker green haloes. Lesions on ripe tomato fruits were dark brown to black, superficial, and measured approximately 1 to 2 mm in diameter. On the leaves, lesions were small, black, and surrounded by chlorotic (yellow) haloes. In some cases, the specks coalesced to form large lesions on older leaves. Black lesions were also observed on stems and petioles. A disease survey of selected tomato-producing areas in Arusha, Dodoma, Iringa, and Morogoro regions of Tanzania during 2004 and 2005 revealed that the disease was widespread in farmers' fields in all areas surveyed. Disease incidence was approximately 80%, while severity, rated on the scale of Chambers and Merriman (1), ranged from moderate (11 to 40 lesions per plant) to severe (>40 lesions per plant). A bacterium that produced a greenish, diffusible pigment on King's medium B was consistently isolated from lesions on tomato fruits collected from the fields in all the surveyed areas. All 56 isolates obtained were gram negative, oxidase negative, and fluoresced on King's medium B under UV light. None utilized phenylethylamine as the sole carbon source, while three isolates utilized i-erythritol and lactulose. Biolog analysis of the isolates, along with two reference strains of P. syringae pv. tomato (Pst CEP-3 from Sokoine University of Agriculture, Tanzania and Pst BB6 [Race 1] from Göttinger Sammlung Phytopathogener Bakterien, Göttingen, Germany) identified them as P. syringae pv. tomato, with similarity indices of 0.518 to 0.933. They also were positively identified as P. syringae pv. tomato by repetitive sequence-based-PCR (2,3) and fragment length polymorphism analysis. Pathogenicity of the strains was confirmed by spraying 35-day-old tomato seedlings (cv. Tanya) with suspensions of the isolates at a concentration of 10⁸ CFU ml^-1 of sterile water. After approximately 72 h, small, water-soaked, dark brown lesions similar to those observed on the field plants were observed on leaves of all the inoculated tomato seedlings. There were no symptoms on control plants. The bacterium was reisolated from the infected plants and identified as P. syringae pv. tomato, in accordance with Koch's postulates. To our knowledge, this is the first report of the occurrence of tomato bacterial speck in Tanzania. References: (1). S. C. Chambers and P. R. Merriman. Aust. J. Agric. Res. 26:657, 1975. (2). F. J. Louws et al. Appl. Environ. Microbiol. 60:2286, 1994. (3). M. Zaccardelli et al. Eur. J. Plant Pathol. 111:85, 2005.