Prevalence, Identification and Management of Bean Scab (Elsinoë Phaseoli) of Common Bean (Phaseolus Vulgaris) in Kenya

Abstract

In Kenya, common bean (Phaseolus vulgaris L.) production has increased slightly due to new varieties and expanded cultivation areas but still does not meet demand due to biophysical stresses like climate variability, soil fertility loss, and disease. Bean scab, caused by Elsinoë phaseoli, significantly reduces yields but remains poorly understood. This study investigated the identity of the causal agent of bean scab in Kenya, explored sources of its inoculum and possible management options. This study further determined the prevalence of bean scab in Kenya , as well as agronomic practices and challenges that may affect its management. Common bean plants showing characteristic bean scab symptoms were observed and photographed in Kakamega, Kenya. The infected bean materials were examined under stereo- and light microscopes. Two methods were used to isolate the causal agent: the tissue plating method, which involved disinfecting and plating infected bean parts on full strength potato dextrose agar (PDA), and the streaking method, which utilized a suspension of scab lesions streaked onto full strength unamended PDA (39 g/L) plates. The cultures were incubated, and their characteristics were documented and compared with known features of the pathogen. Koch’s postulates were satisfied using purified isolates to confirm disease causation. For genotypic characterization, DNA was extracted from fungal mycelia followed by PCR amplification of the internal transcribed spacer region of ribosomal DNA. The amplified products were sequenced, and the sequences were compared with those in the GenBank database to identify isolates. Phylogenetic analyses were conducted using BioEdit and MEGA11 software to compare and identify Elsinoë species accurately. To explore bean scab management options, a series of experiments were conducted at the Kenya Agricultural and Livestock Research Organization (KALRO) - Kakamega station. Assessment of inoculum sources for bean scab was conducted over two consecutive bean planting seasons, employing a completely randomized design with nine plots and three treatments of genotype GLP 2; in sterile soil, farm-saved seed, and soil mixed with scab-infested debris. A second field trial evaluated the impact of cropping history on bean scab occurrence over three seasons, comparing fields with recent bean history to those cropped with non-legume for three years. A third field trial assessed the effects of fungicide applications and cropping systems on bean scab, using a split-plot design over three seasons. Data collected in these experiments included emergence, disease incidence, stand count, and grain yield components. Data analysis was performed using R software, incorporating linear..........................................