Distribution and Races of Xanthomonas axonopodis pv. malvacearum on Cotton (Gossypium hirsutum) in Uganda

Surveys in 1995 and 1996 showed that bacterial blight caused by Xanthomonas axonopodis pv. malvacearum occurs throughout the main cotton growing areas of Uganda, causing seedling blight, angular leaf spot and bacterial boll rot. During the vegetative and early fruiting stages of crop growth, severe symptoms of `blackarmʹ spread from leaves to the stem, causing loss of fruiting branches. A set of Upland cotton cultivars (Gossypium hirsutum) were then used to determine the races of the blight bacterium present in Uganda.Many of the isolates induced moderate to severe symptoms on all the test hosts except 101±102B, indicating infection with race 10 or 18. The next most common isolate was race 7. Races 16 and 6 were also identi®ed and 23%of isolates caused symptoms on all the di€erential cultivars including 101±102B, results indicating the presence of a race of the pathogen which may be the same as that identi®ed in countries neighbouring Uganda and designated as race 20.