Incidence, severity and spread of cassava mosaic disease in Western Kenya

Abstract

The impact and spread of cassava mosaic disease was studied through a survey, by harvesting of diseased and healthy plants in farmer's fields to determine current yields and the effects of cassava mosaic disease spread experiments, yield of spread experiments, determination of virus type by polymerase chain reaction and determination of cassava mosaic disease incidence in ratoons of cassava variety SS-4. The survey of incidence and severity of cassava mosaic disease and whitefly populations was carried out in October to November 1999 in Mumias/Butere, Kakamega, Busia and Teso in western province and Siaya and Suba in Nyanza province. Many varieties of cassava were encountered, indicating a rich genetic diversity. Individual districts showed high cassava mosaic disease incidence and the overall cassava mosaic disease severity for all varieties was 3.33 (p0.01) on the 1-5 scale of increasing severity. Abundant whitefly populations associated with cassava mosaic disease were encountered in Suba district only, indicating that the spread of the pandemica is still active. The overall cassava mosaic disease incidence in the region had dropped to 62.3percent and was attributed to an infusion of improved cassava varieties by farmers of Busia and Teso district from Uganda. A survey of yield from farmer's fields revealed that fresh tuberous weight per plant ranged from 0.3 - 4.4 (+ 0.86) kg in local and improved varieties. Cassava mosaic disease accounted for 28% of the total fresh tuber yield variability in the districts, and cassava mosaic disease infections accounted for 53% poor harvests encountered in all the district, thus indicating that cassava mosaic disease reduces overall yield in western Kenya.

Disease spread experiments done in Bungoma, Kakamega and Siaya in 1999 revealed considerable spread between April and October 1999, at all sites. There was little spread at the Siaya site between, August 1999 and March 2000 and no spread at Kakamega site at the same period. Most period occurred at Siaya and it was significantly greater than at other sites. The local variety Serere used as a control showed severe infection towards the end of experiment of April 1999 plantings, whereas the improved varieties TMS 30337, TMS 30572 showed apparent decline in disease incidences towards the end of the experiment. Cassava mosaic disease incidences at Kakamega were higher than at Bungoma in plantings of April 1999. Mean cassava mosaic disease severity was not significantly different between Siaya (mean = 2.1, p = 0.05) and Kakamega (mean = 2.0, p = 0.05) but their severity was significantly different from that at Bungoma (mean = 1.6, p = 0.05). The variety SS-4 had only one plant infected at Kakamega in plantings of April 1999. Numbers of adult whiteflies differed significantly between sites, and between varieties in all experiments.

Evaluation of yield from the four cassava varieties showed that the improved varieties yielded more than the local variety Serere. Combined yield for all varieties in all sites ranged from <0.1 - 5.0 (+1.0) kg per plant. The yield of improved varieties varied from the lowest 11.1 t/ha of SS-4 at Bungoma to 27.7 t/ha of SS-4 at Kakamega, the local variety Serere had a production of 11.24 t/ha at Bungoma to 18.43 t/ha at Kakamega. Adult whiteflies were not encountered on the plants at harvest 10 months after planting. Cassava mosaic disease was caused by the newly discovered Uganda Variant/East African cassava mosaic virus-Uganda Variant that predominated in the region. SS-4 was highly resistant to cassava mosaic disease and its ratoons showed low disease incidence whereas its cuttings showed no disease and can be used for propagation.