Occurrence and distribution of cassava pests and diseases in Rwanda

A survey was conducted in 2007 to obtain information on the distribution, incidence and severity of cassava pests and diseases in Rwanda, and determine how these parameters relate with cassava varieties and intercropping. Local (unimproved) cultivars predominated in most farmers’ fields (over 83%) and 78% of the fields were intercropped. Cassava green mite, Mononychellus tanajoa and cassava whitefly, Bemisia tabaci, were the most abundant pests. Within-field incidence of green mite averaged 42% but damage was mild (average score of 2.3 on a scale of 5). Typhlodromalus aripo, the mite predator of green mite, was found in 28% of the fields surveyed, with a mean incidence of 5.7% within fields. The mean number of B. tabaci whitefly adults on the apical five leaves was 0.92 whereas the mean number of whitefly nymphs on a middle leaf was 5.2. Incidence of cassava mosaic disease within sites averaged 33.2%. Cutting infection accounted for 66% of infected plants but the relative contribution of cutting and whitefly infection varied among the major varieties. Mite damage as well as mosaic disease incidence and severity were higher on local varieties. However, whitefly populations were higher on improved varieties. Intercropping was associated with lower pest populations and disease incidence and severity.

Research highlights
► Populations of whiteflies (Bemisia tabaci) on cassava have increased while the incidence and severity of cassava mosaic disease (CMD) has remained mostly the same in Rwanda over the past few years.
► Improved varieties reduce CMD.
► Using clean cassava planting material is a major component of integrated management of CMD.
► Intercropping reduces CMD.