Influence of On-farm Pesticide Practices and Processing Methods on Pesticide Residue Levels in Potato Tubers (Solanum tuberosum L.) in Nyandarua County, Kenya

In Kenya, the extensive use of agrochemicals in potato farming raises concerns about pesticide residues in potato tubers and products. This study aimed to document the pesticide application practices of potato farmers in Nyandarua County, Kenya and evaluate the effect of the practices on pesticide residue levels in raw potato tubers. Also, the study evaluated the effect of various heat processing methods on the pesticide residue levels in potatoes. A cross-sectional survey using semi-structured questionnaires was conducted on 275 randomly selected farmers. Alongside, raw Shangi potato variety samples (n = 16) from respective farmers were analyzed for some of the most commonly used pesticide residues using LC-MS/MS and GC–MS/MS. The study found that 98.8% of farmers use synthetic pesticides, with 96.4% using fungicides, 68.2% insecticides, and 28.7% herbicides. Common fungicides contained mancozeb, metalaxyl, and cymoxanil as the main active ingredients. Insecticides contained α-cypermethrin and imidacloprid, while herbicides had glyphosate and 2,4-D as the main active ingredients. These chemicals were used either alone or in mixtures. Only 11.85% of farmers adhered to the recommended manufacturer’s application rates, with the majority relying on advice from agrochemical retailers (74.63%) or other farmers (13.32%). The frequent mixing of pesticides and weekly applications were also common practices. Residue analysis revealed that adherence to the manufacturer’s instructions resulted in lower residue levels. Mixing pesticides and frequent applications led to higher residues, particularly for fungicides containing azoxystrobin. Longer preharvest intervals generally reduced residue levels. Most of the pesticide residues were below the EU and Codex Maximum Residue Limits (MRLs) in potatoes, however, the banned insecticides containing chlorpyrifos and fenitrothion were found at levels exceeding the EU and Codex MRLs of 0.01 mg/kg. Processing methods such as frying, baking, boiling, and steaming significantly reduced pesticide residues, below the Codex MRLs. Frying and boiling were particularly effective for most pesticides. However, baking, roasting, and frying were not effective in reducing chlorpyrifos and fenitrothion below EU and Codex MRLs. The findings highlight the need for farmer education on proper pesticide use and adherence to recommended practices to minimize residue levels and ensure food safety.