Diseases are hindrance to tomato production in Kirinyaga County, Kenya. However, information on farmer’s knowledge about tomato diseases to warrant pesticide usage is scanty. Further, there is information gap on disease predisposing factor such as varietal choice and seed source. This study assessed the tomato farmers’ socio characteristic, varieties grown, seed source and knowledge of selected foliar fungal disease among tomato farmers in agro-ecological zones (AEZs) of Kirinyaga County. A cross sectional survey design that in cooperated purposive sampling and snowballing approaches was adopted in the study. Data were collected from 120 tomato farmers using structured questionnaires. A chi square (Ӽ2) test was used to examine the association between different variables at α= 0.05 using SAS version 9.4. No significant association (p > 0.05) was observed between gender of farmers and AEZ. Nonetheless, there were more men (83.33%) than women (16.67%). Terminator F1 variety was popular among farmers (25%). No significance (p > 0.05) association was observed between source of tomato planting material and AEZs. However, Agrovet was a popular seed source among farmers (40%). The reasons for choosing a particular tomato variety was significantly (p < 0.05) associated with the AEZ with 40.83% of farmers preferring tomato varieties with good marketability traits such as fruit size. Farmers’ knowledge of causative agent of early blight, late blight and Septoria leaf spot was significantly (p < 0.05) associated with AEZs. The source of farmer’s knowledge on tomato foliar fungal diseases was not significantly (p > 0.05) associated with AEZ. However, farming experiences was a popular source of knowledge (51.67%) among farmers. Inability of some farmers to identify tomato diseases negates the efforts on disease management in tomato production in Kirinyaga County. Therefore, measures such as coordinated education on tomato diseases is necessary to empower farmers on disease causes and identification to enhance disease management and improve tomato yields in Kirinyaga County in Kenya.