Effect of cassava mosaic disease, soil fertility, plant spacing and their interactions on cassava yields in Zanzibar

Cassava Mosaic Disease (CMD) and low soil fertility are limiting factors for the production of the preferred cassava variety Kibiriti mwekundu on the northern part of the island of Pemba in Zanzibar. Because CMD severity, assessed 7 months after planting in shifting cultivation fields at wide spacing of cassava (2.5 x 2.5 m), was always low, the hypothesis was tested whether damage due to CMD could be decreased by applying green manure. A total of 10 tonnes of fresh Gliricidia sepium leaves applied at 2 1/2, 4 and 5 1/2 months after planting increased yield by 40% to 7 tonnes per ha. The effect of green manure on yield was greatest on plants with highest CMD scores. The effect of plant density on CMD severity was also studied. Cassava yields at 1600, 2667, 6667 and 10 000 plants per ha and with soil of high fertility were 15, 17, 19 and 30 tonnes compared with 3, 6, 10 and 8 tonnes per ha respectively at a low fertility site. Under high soil fertility conditions, the CMD score was highest at close plant spacing, while under low soil fertility conditions there was no effect of spacing. Yield compensation occurred by plants neighbouring CMD affected plants, but only at the close plant spacing of 10 000 plants per ha and under high soil fertility conditions. Our results indicate that the impact of CMD on cassava can be reduced by applying green manure in fields with low soil fertility, that increasing organic matter content in the soil lowers CMD severity, and that CMD scores are reduced by increasing both soil fertility and plant spacing.