Inoculum sources and management of bean scab caused by Elsinoë phaseoli

Bean scab caused by Elsinoë phaseoli is a major challenge to common bean cultivation in Kenya. However, knowledge about its epidemiology and management is limited. This study conducted three experiments in Kakamega, Kenya, evaluating scab inoculum sources and control options. The first experiment evaluated three different inoculum sources over two seasons; bean crop residue caused the highest scab incidence in both the 2021 (62.1%) and 2022 (81.1%) seasons. The second experiment assessed the effect of rotation history on scab over three seasons. Scab incidence consistently surpassed 90% within the first 30 days after planting in half-yearly rotated fields across all seasons, while the disease was absent after planting in fields with no recent history of legumes. Fields rotated out of legumes consistently had the highest grain yield, averaging 1.5 t ha⁻¹ over the seasons. Additionally, the rotation land treatment displayed significantly more pods per plant during the 2021 (9.7), 2022a (9.8), and 2022b (12.5) seasons. In the third experiment, the impact of five fungicides and four cropping systems on bean scab was investigated over two seasons. Neither fungicide treatments nor cropping systems had an effect on bean scab incidence even when only severe symptoms were considered. Carbendazim had the highest yield (1.9 t ha⁻¹) outperforming the unsprayed control only in the 2022 season while none of the other fungicides outperformed the control treatment in yield parameters in either season. This study emphasizes the challenge of managing bean scab without proper crop rotation and underscores the role of crop residue as a critical inoculum source.