Kerstin Wydra, A. Fanou and A.G.O. Dixon
The present studies aimed at elucidating the effect of ecozones on disease development and yield loss due to cassava bacterial blight. In field experiments in five ecozones – the humid forest zone, the coastal and the inland forest savanna transition zone, the dry savanna, and the Sudan savanna – cassava bacteria] blight reduced root yield by 12.5-50% in the susceptible variety and by 24-41 % in the resistant variety in 4 ecozones, when two infection levels were compared. Symptom severity was highest in the humid forest and the forest savanna transition zone, but yield loss was highest in the dry savanna zone. In general, the resistant variety developed less symptoms than the susceptible variety, but the susceptible variety generally yielded higher root weights. Only in the dry savanna zone, relative yield loss was higher in the susceptible variety than in the resistant one. In the stepwise regression of symptoms against root weight, blight was the most important variable determining root yield. A high variability in yield loss over 3 years was observed in trials in 2 ecozones. Since varietal resistance appeared to be variable in different environments, it was concluded that only integrated control methods comprising the use of resistant or tolerant varieties and cultural and sanitary methods can reduce the disease and are recommended especially for the dry savanna zone.