To help keep plants healthy, grow beans in the same spot only once every three years. Destroy infected plants (don't save seeds from them), and do a good fall clean-up. To prevent fungal diseases, weed or harvest beans when plants are dry; control weeds and avoid overcrowding to provide good air circulation. Following are specific problems to watch for.
Mexican bean beetle
Spiny yellow larvae and copper beetles that look similar to ladybugs, with 16 spots, feed on the undersides of leaves, skeletonizing them. Hand-pick adults and larvae; squish yellow-orange egg clusters. Interplant bean rows with potatoes or a mixture of aromatic herbs and flowers.
Tiny green and black aphids infest new growth, spreading viral diseases. Control by washing off with a jet of water from the hose; use insecticidal soap as a last resort.
Fat, one-inch (2.5-centimetre) long grey to brown grubs hide in the soil and chew off tender seedling stems at ground level. Search in the soil around the base of cut-off seedlings and destroy. Protect stems in heavily infested areas with foil collars.
Leaves develop light and dark green patterns, and may become curled and stunted.
Plants develop dark brown, sunken spots on leaves, pods and seeds; spots may later develop pink centres. Carried by seed; if a problem, plant resistant varieties.
Reddish-brown circular lesions form on leaves and pods; leaves turn yellow and drop off. Plant resistant varieties.
Dark green lesions on leaves turn to velvety grey, hair-like mould; they form in pods, too.
Dark green lesions on leaves increase in size, and affected tissues turn brown.