Comparative differences of whitefly-transmitted diseases between local and hybrid bean varieties in Kitui County, Kenya

The typical dry bean, Phaseolus vulgaris, is the essential food legume for direct human consumption. They play a significant role in food security and nutrition. Despite their nutritional importance, its production growth rates have declined in Kenya due to diseases, insect pests, plant nutritional deficiencies, and drought. Therefore, this study's main objective was to determine whether there are differences in bean varieties' tolerance to whitefly transmitted viral diseases. The study had a survey and laboratory phases. The survey covered wetter midlands of Central Kitui County, where fifty-two farms were repeatedly covered in two bean production sites in two years (2017-2018), in Kyangwinya West Ward and Kitui East Ward. A questionnaire was administered to collect data on bean production acreage, type of cropping system, pest and diseases, pesticide, and fertilizer use by farmers on bean production. Each farms’ location was marked using the global positioning system (GPS) device. Rainfall data of the production period was secured from Kenya Agricultural and Livestock Sub-Centre of Ithookwe, centrally placed at 2 Kilometres west of Kitui Town. The Whitefly specimens were collected from the named regions and the specimen preserved in 70% ethanol. Bean disease lesions were noted following the procedure of observation of the bean leaves for disease symptoms. Viral symptoms were recorded based on the data from observation. Fungal identification was done using pathogens on Potato Dextrose Agar (PDA) combined with a drug (streptomycin and neomycin) while bacteria were differentiated through selective media YDC (yeast extract-dextrose-CaCo3). The fungi were grouped using spore characters on PDA, while bacteria were grouped depending on their color on YDC. Bean production acreage and yield levels were subjected to analysis of variance. The highest density of B. tabaci was observed in Kyangwithya-West ward at close to 2 whiteflies per field plot of 1m2. After laboratory analysis, disease incidence among the varieties showed that improved varieties had a higher incidence of fungal, bacterial, and viral diseases than the local cultivars. Mwezi-Moja had the least disease incidence of 1%, while Rosecoco, B1, and Nyayo, with ≥ 5% of fungal diseases, identified as Altenaria and Phoma species as common ones on the dead lesions. The bacterial diseases found on the leaves were Pseudomonas spp, grouped in the fungal lesions. The local cultivar with the least incidence was Wairimu indicating 1% fungal disease occurrence, mainly being Altenaria species. The local variety bearing the highest viral disease was Mwitemia at 3%, of Bean Golden Yellow Mosaic Virus (BGYMV). However, the varieties had a less than 10% incidence of both fungal and viral diseases. There is a need to develop bean genotypes through seed breeding, which are ecologically adaptive to the waterstressed environment to increase food security.