Black bean aphid
These large black aphids overwinter on shrubs and migrate to beans in spring. To prevent damage, treat colonies as soon as they're seen. Insecticides based on derris, fatty acids, plant and fish oil or pyrethrum can be effective if carefully applied to the underside of foliage, and all are consistent with organic gardening. If these fail, consider using a synthetic insecticide containing bifenthrin.
Glasshouse red spider mite
In hot, dry summers, mites can turn the foliage of outdoor beans bright yellow. Insecticides based on fatty acids, soap or plant and fish oil will usually have sufficient effect on the mites to save the crop. Use biological control, introducing predators as soon as the damage is seen, where mites are troublesome every year.
Bean rust
The whitish spots, later turning rusty brown, of this fungal disease usually occur on runner and French beans late in the season and do little significant damage. There are no remedies, and if a crop is badly affected it should be removed. Avoid growing beans on that site for several years.
Halo spot
This bacterial disease, carried on runner and French bean seeds, causes water-soaked spots in the leaves with a yellow ‘halo’. There are no remedies, so destroy infected crops and buy fresh seed from a reputable supplier.
Bean anthracnose
This fungal disease of French beans is common in wet weather. There are no remedies, but gardeners in wet districts should avoid growing plants crowded together, practice rotation and use resistant cultivars.
Sclerotinia
Fluffy moulds on rotted stems with large grey and black bodies (sclerotia) within the stems are a sign of this fungal disease. There are no remedies, but gardeners should practice crop rotation and remove and destroy affected plants as the sclerotia will persist in the soil.