Occurrence and Distribution of Major Cassava Pests and Diseases in Western Kenya

Field surveys were conducted in October 2022 and March 2023 (60 fields each year) to determine the prevalence, incidence and severity of cassava mosaic disease (CMD) and cassava brown streak disease (CBSD), whitefly numbers and incidence of cassava green mite (CGM) in six counties of western Kenya. A total of 29 varieties were recorded out of which 13 (45%) were improved. The improved varieties were present in 62% of the 120 fields sampled with the most predominant MM96/4271 found in 36% of the fields. There was more CMD infection in 2022 with a prevalence of 68%, incidence of 26.4% and severity score of 2.7, while in 2023 these parameters were 60%, 10.1% and 3.0 respectively. CMD incidence attributed to planting infected cuttings was 20.1% in 2022 but dropped to 4.3% in 2023 while for whitefly-borne infection incidence remained 4.4% for both years. CMD incidence was significantly lower (5.9%) for improved varieties than it was for local varieties (35.9%) in contrast to CBSD incidence which did not differ significantly between improved (2.3%) and local varieties (9.7%). However, the improved variety MM96/4271 was among the varieties most affected by CBSD, with an incidence of 6.3%. Overall, CBSD in 2022 had a prevalence of 20%, incidence of 6.4% and severity of 2.7, while in 2023 these parameters were 12%, 4.1% and 2.7 re-spectively. Cassava brown streak virus (CBSV) and Ugandan cassava brown streak virus (UCBSV) were both detected. Most infections were single CBSV infections (82.9%), followed by single UCBSV (34.3%) and coinfection with both viruses (16.7%). Whiteflies were more abundant in 2023, in which 28% of the fields had super-abundant populations of 100/plant, compared to 5% in 2022. Molecular characterization based on mtCOI sequencing identified the whiteflies as Bemisia tabaci. KASP SNP genotyping designated 92.8% of the specimens as SSA-ECA for 2022 while it was 94.4% for 2023, compared to 72% for an earlier sample collection in western Kenya in 2017. CGM prev-alence was 95% with a severity of 2.7 in both years, but incidence was lower in 2022 (65.4%) compared to 2023 (79.9%). This study demonstrates that the majority of farmers are cultivating improved varieties, although cassava viruses, whiteflies and cassava green mites are still a threat. There is a need to develop and deploy integrated management strategies against these pests which will include components of resistant varieties, clean seed and control of whiteflies and CGM.