Xanthomonas manihotis (Arthaud-Berthet) Starr, Causal Agent of Bacterial Wilt, Blight and Leaf Spots of Cassava in Zaire

Abstract

Bacterial wilt of cassava has been causing a severe reduction of tuber yield in the savanna of the southern part of Zaire since 1970. Infection of young shoots often takes place through insect punctures made mainly by Pseudotheraptus devastans Distant. Systemic colonisation of the stem leads to vascular browning, formation of bacterial pockets in the bark which extrude a yellow-white exudate and sudden drying of the leaves and necrosis of the stem tips. Angular leaf spots, accompanied by small droplets of exudate on the lower surface of the lesion, rapid blight of large areas of the lamina and defoliation, are endemic in Zaire. Epidemics develop under conditions of heavy rainfall. The cultural appearance, the morphological and biochemical characters as well as the cross-inoculation tests demonstrate that the bacteria causing wilt, leaf spotting and blight are identical. They are identified as Xanthomonas manihotis (Arthaud-Berthet) Starr and can be distinguished from a Brazilian isolate of X. manihotis only by a lower amylolytic activity. A critical analysis of the literature suggests a worldwide distribution of the disease. The recent epidemic development of bacterial wilt in Zaire and the control by resistant cultivars are discussed.