Factors contributing to spread and population build-up of potato cyst nematodes in smallholder farms in Nyandarua County, Kenya

The rate of spread and buildup of potato cyst nematodes (PCN) poses a significant threat to potato production in Kenya. A study was conducted to determine factors that could be contributing to the rapid spread and increase in population density of the nematodes in smallholder farms in three agro-ecological zones, namely Upper Highland-3 (UH3), Upper Highland-2 (UH2), and Lower Highland (LH4) of Nyandarua County. A stratified random sampling procedure was used to draw a sample of 65 farms for the study. A total of 62 (95.1%) samples out of 65 were infested with PCN. A significantly (P≤0.05) lower PCN count of 63.1 cysts was recorded in LH4 compared to 203.7 and 289.2 cysts/300cc of soil in UH2 and UH3, respectively. Sandy clay loam soil had a significantly (P≤0.05) higher number of cysts in LH4-229.6, UH2-460.1, and UH3-458.5 cysts/300cc compared to sandy clay soils, which had 2.0 cysts/300cc, and clay loam soils, which had 4.3 cysts/300cc soils in LH4. Nematode numbers were higher, up to 302 cysts, in farms growing more than one potato crop cycle in a year, compared to 197 cysts/300cc in farms where one potato crop cycle was practiced per year. The number of cysts recovered from soil adhering to farm tools, footwear, and potato tubers were 2.6-13.8, 1.8-8.8, and 2.8-12.9 cysts/50cc soil, respectively. Only 15% of the farmers used certified seed, while the rest used farm-saved tubers. A commonly grown variety, Shangi, had significantly (P≤0.05) the highest cyst counts of 463.1 cysts/300cc soil. Very few farmers showed any elements of IPM strategies, probably due to their limited knowledge of the pest and the available strategies. These findings reaffirm the need for an integrated approach incorporating crop rotation, planting of disease-free tubers, resistant varieties, and field sanitation, as well as enhancing farmers' knowledge on management of the pest, in an effort to reduce the spread and buildup of PCN. Key words: Globodera rostochiensis, prevalence, Solanum tuberosum, site-specific factors.