Bacterial spot ( <i>Xanthomonas campestris</i> pv. <i>vesicatoria</i> ) of tomato and sweet pepper in Tanzania

Abstract

Bacterial spot of tomato (Lycopersicon esculentum) and sweet pepper (Capsicum annuum) is recorded from several countries of eastern and southern Africa, including Ethiopia, Kenya, Malawi, Mozambique and South Africa. With the growing importance of solanaceous cash crops in Tanzania, particularly tomato, it was important to confirm the findings of preliminary reports of the possible presence of the pathogen and to record it officially. It was also desirable to assess the degree of contamination of seed as potentially the most significant source of inoculum. Field surveys during the rainy seasons of 1997 and 1998 showed that bacterial spot could be found in tomato and sweet pepper fields in all the mainland vegetable regions of the northern and southern highlands, but not in Zanzibar. In fields of tomato surveyed (59 in 1997, 50 in 1998) in which bacterial spot could be seen, incidence (percentage of plants with symptoms) varied greatly between years and fields (from <5% to >90%). In pepper, incidence was never greater than 5% (10 fields surveyed in 1997, three in 1998). Xanthomonas campestris pv. vesicatoria (Xcv) suspected from symptoms on tomato and sweet pepper fruit was confirmed by isolation on semi-selective media including Tween B (McGuire et al., 1986), followed by biochemical tests for Xanthomonas and pathogenicity tests on tomato. Although the name X. c. pv. vesicatoria remains a valid taxonomic name, it has recently been proposed for reclassification into three separate species (Jones et al., 2000). However, available data for the Tanzanian isolates are not sufficient for them to be allocated to the newly proposed taxa. In some cases, bacterial spots masked symptoms of Clavibacter michiganensis ssp. michiganensis which was also present (Black et al., 1999). For detection of Xcv in seed, extraction methods recommended for quarantine purposes were used (Anon., 1992). Using dilution plating, Xcv was found in five out of 26 samples of farmers' saved seed, was absent from 12 commercial tomato seed lots and present in two out of three commercial pepper seed lots sampled.