Relationships among soilborne bean seedling diseases, Lablab purpureus L. and maize stover residue management, bean insect pests, and soil characteristics in Trans Nzoia district, Kenya

Smallholder farmers who practice continuous maize (Zea mays L.) and bean (Phaseolus vulgaris L.) cultivation in the highlands of eastern African have been introduced to new leguminous crops for soil fertility enhancement. However, little is known about the impact these crops may have on farmers’ pre-existing crop pest problems. We investigated the cumulative effects of 7 years of differential management of Lablab purpureus L. and maize residues on soilborne bean seedling diseases caused by Pythium spp. and Fusarium solani f. sp. phaseoli. Residue management effects on soil fertility and seedling damage by bean stem maggots (Ophiomyia spp.) and root-feeding chafer grubs (Schizonycha spp.) were also evaluated. Differences in disease dynamics were investigated in the laboratory (seed and stem assays). Three consecutive bean seedling crops were grown in the field plots (seedling assays) to assess disease severity and the impact of bean insect pests. The experimental design included three residue management factors: (1) lablab residues removed (L0) or retained (L1); (2) maize stover removed (S0) or retained (S1); and (3) residues left on the soil surface (I0) or incorporated (I1).Residue management practices altered several fundamental soil properties and indirectly affected disease dynamics. Residue incorporation and maize stover removal decreased soil water holding capacity and enhanced Pythium infection of incubated bean seeds. Crop residue removal acidified the soil and reduced nutrient availability, which was linked to a reduction in the number of Fusarium lesions formed on incubated bean stems. In the field, seedling damage by bean fly maggots and chafer grubs increased soilborne disease severity. Lablab residue incorporation plus maize stover removal increased chafer grub abundance. Bean stems incubated in soils from this treatment had significantly more Fusarium lesions, suggesting that differences in relative grub abundance may be related to differential build-up of Fusarium inoculum. Crown and root rot severity of field grown seedlings increased as levels of nutrient availability increased. However, enhanced nutrient availability and maize stover retention reduced disease-induced mortality of the seedlings. Our results point to a complex web of interrelationships among residue management practices, soil characteristics, and soilborne bean seedling pests in Trans Nzoia district Kenya.