Occurrence of cassava diseases in relation to environmental, agronomic and plant characteristics

Among the seven cassava diseases observed in a survey in the cassava-growing regions of Benin and Ghana, cassava mosaic virus disease (CMD) occurred most frequently in all ecozones, with a higher incidence in Ghana (82.2%) than in Benin (48.1%). Stepwise regression and canonical correlation analysis revealed that CMD was positively correlated with increasing numbers of weeds and with cultivar mixtures. Cassava bacterial blight (CBB) was particularly severe in all ecozones of Benin, but rare in Ghana. Cassava in the savanna zones of Benin was heavily diseased with an incidence of 34.5–84.1% and a high percentage of plants showing systemic symptoms, while CBB was not observed in the rainforest (RF) zone and only sporadically in the coastal savanna zone of Ghana. High incidence and severity of CBB was associated with increasing plant age and clay soils, whereas the disease was less when fields were intercropped or planted to cultivar mixtures. Anthracnose and leaf diseases caused by Cercospora spp. were of minor importance. The severity of diseases caused by Cercospora spp. was positively correlated with profusely branching varieties. Incidence of Cercospora caribaea was associated with increasing rainfall and seemed to be suppressed in cultivar mixtures. Integrated control methods are proposed for the diseases, especially CBB.