Yellow leaves can appear on green bean plants due to being oversaturated. If overwatering has been ruled out, a variety of bacterial, viral and fungal pathogens can cause green bean plants to present yellowing leaves. Powdery mildew, bacterial blights, rust diseases and mosaic viruses are all common and probable causes for yellow leaves on green bean plants.
Powdery mildew (Erysiphe polygoin) can cause yellow leaves on green beans if untreated. Symptoms start off as white to grey powdery growth on the leaves. The new leaves can appear contorted, abnormally small and curled. These diseased leaves can turn yellow and drop off. Green bean pods may also appear dwarfed and distorted in plants affected by powdery mildew. Some prevention can be obtained by giving each bean plant plenty of room to grow. Control can be obtained on mature plants with sprays of fungicides containing sulphur.
Halo blight and common blight can attack green bean plants. Plants affected by halo blight present symptoms on the leaves, stems and fruit. Light green to yellow halolike rings appear around brown spots on the leaves. If untreated, the spots will merge and the entire leaf will turn yellow and then die. Stem symptoms appear as long red spots. Common blight causes bean leaves to turn brown. Preventive measures include avoiding overhead watering and crop rotation. A fungicide containing fixed copper can be applied every 10 days until the day before harvest.
Rust diseases can plague green bean plants during periods of humid weather. Symptoms of rust (Uromyces appendiculaters) include small rust coloured spots on the lower leaves of bean plants. Left untreated, the infected leaves will turn yellow, wilt and eventually drop off. Stems and bean pods can be affected as well. Preventive sprays containing sulphur or chlorothalonil can be applied at weekly intervals. Cease applications approximately one week before anticipated harvest dates.
The most common and destructive disease of beans are the mosaic viruses. Both the bean common virus and the bean golden mosaic virus can cause yellowing leaves on bean plants. The disease is worse when young plants contract it. Symptoms can include stunted, curled and yellow-mottled leaves. Affected plants will have poor or no production and have a dense canopy. Prevention can be obtained by obtaining virus-free transplants and by removing infected plants before they can transmit the disease to other beans.