Weed management effects on insects and diseases of cabbage and snapbean

Field studies were conducted to determine insect and plant pathogen management effects on weed competitiveness and crop yield and to evaluate weed management impacts on insect pests, diseases, and crop yield. At similar densities, redroot pigweed (Amaranthus retroflexus L.) reduced snapbean (Phaseolus vulgaris L.) and cabbage (Brassica oleracea L. var capitata) yield more than that of common purslane (Portulaca oleracea L.), a low growing weed. In 1995, diamondback moth [Plutella xylostella (L.)] was greater on cabbage growing in plots with purslane than in plots of cabbage growing without weeds. Imported cabbageworm [Pieris rapae (L.)] was greater on cabbage growing in plots with either purslane or pigweed than when growing alone. However, the amount of feeding damage to cabbage was similar across treatments. Disease incidence was low, but fungicide treatments made redroot pigweed more competitive with snapbean, reducing yield in 1995.