Wilt Pathogens of Solanaceae in Tanzania: Clavibacter michiganensis subsp. michiganensis, Pseudomonas corrugata, and Ralstonia solanacearum
Authors: Black R. , Seal S. , Abubakar Z. , Nono-Womdim R. , Swai I. 
Published: May 2007

Surveys of vegetables in the southern and northern growing regions of mainland Tanzania and Zanzibar during 1997 and 1998 indicated the presence of three wilt pathogens of tomato. Clavibacter michiganensis subsp. michiganensis (cause of bacterial canker) was isolated from wilting plants in the southern highlands and Lushoto District (Tanga Region, north) on selective King's medium B with polymyxin B (1). The identity of the isolates was confirmed by cultural, morphological, and biochemical characteristics and a plate-trapped antigen enzyme-linked immunosorbent assay (PTA-ELISA) kit (Pathoset 113-08, Adgen, Auchincruive, U.K.). Pathogenicity was confirmed by host inoculation. In addition, the bacterium was detected directly by the same methods in several sources of seed, including commercial farmers' saved seed and seed extracted from infected plants. Of 61 tomato seed lots tested, 18 samples were positive. Ralstonia solanacearum (cause of bacterial wilt) was isolated from tomato and potato by semiselective media and detected directly in stem and tuber tissues, respectively, by polymerase chain reaction and ELISA (4) in all vegetable-growing areas surveyed. In 1998, R. solanacearum was detected for the first time in Zanzibar on tomato and eggplant. Pathogenicity was confirmed by inoculation on and reisolation from tomato seedlings of cv. Money Maker. Only biovar 3 (2) occurred in tomato. Biovar 3 also was found in midaltitude potato. Biovar 2 has been found only in potato plants grown above 1,500 m. Pseudomonas corrugata (cause of pith necrosis) was isolated from tomato on semiselective media at only one location. From wilted tomato plants in the southern highlands, 38% of samples tested positive for C. michiganensis subsp. michiganensis and ≈ 10% for R. solanacearum. Of samples collected from the northern highlands, 43% tested positive for R. solanacearum. Wilt incidence of ≈35% was observed in tomato fields where the bacterial wilt pathogen was isolated in the northern highlands compared with gt;90% incidence and almost total crop loss in tomato fields of the southern highlands infected with bacterial canker. Although all three pathogens caused systemic wilt of plants, bacterial canker occasionally caused downward turning of lower leaves, unilateral wilting, and marginal necrosis of leaflets as well as fruit spotting. In general, wilts caused by C. michiganensis subsp. michiganensis, R. solanacearum, and P. corrugata were not readily differentiated in Tanzania prior to this research. Plants with pith browning had often been assumed to be infected by P. corrugata, until R. solanacearum was isolated (3) frequently from such plants.

References: (1) Anonymous. OEPP/EPPO Bull. 22:219, 1992. (2) A. C. Hayward. J. Appl. Bacteriol. 27:265, 1964. (3) J. B. Jones et al., eds. 1991. Compendium of Tomato Diseases. The American Phytopathological Society, St. Paul, MN. (4) S. Seal and J. G. Elphinstone. Pages 35–57 in: Bacterial Wilt. The Disease and Its Causative Agent, Pseudomonas solanacearum. A. C. Hayward and G. L. Hartman, eds. CAB International, Wallingford, U.K. 1994.