Gardeners worldwide grow beans because they are adaptable, fast-growing and are easily stored for long periods of time. Considered to be one of the oldest cultivated foods on earth, bean plants have been providing healthy, nutritious food to humans for thousands of years. Beans are relatively healthy plants but are susceptible to a few diseases that can damage the bean plant and lessen its crop yield.
Caused by the fungus Colletotrichum lindermuthianum, Anthracnose is destructive and can cause the loss of bean crops. The fungus attacks the seed, causing dark brown or black spots to appear on any part of the bean plant including the leaves, stems and pods. Spots are 1/2 inch in diameter and are coated in a salmon-colored liquid during wet conditions.
The disease is seed-borne and is spread by wind and rain as well as animals, humans and planting tools. Control anthracnose by not planting infected seeds, cleaning gardening tools, rotating crops and applying fungicide according to label instructions.
The fungus Erysiphe polygoni causes powdery mildew. You will notice a light discolored spot on leaves at first, and then, a grayish-white talcum-like powdery growth spreads to all parts of the plant including pods, stems and leaves. Symptoms usually appear on mature plants late in the growing season after long exposure to moist conditions. The plant leaves turn yellow, curl and drop. Pods are stunted and are disfigured.
Plants begin to lose nutrients during the infection, which can result in the decline of growth and vigor in the plant. Prevent the spread of powdery mildew by removing and destroying infected leaves and spray the plants with the proper solution of fungicide.
Angular Leaf Spot
Symptoms of angular leaf spot appear on the surface of leaves as dark brown or gray patches that are sometimes covered in gray mold. Badly spotted leaves will drop from the plant. Angular leaf spot is caused by the fungus Isariopsis griseola and can survive for 2 years in the soil. The spores of the fungus spread by wind, splashing water, tools and insects. Poor drainage and frequent rainfall provide perfect conditions for the fungus to grow.
Reduce the risk of spreading angular leaf spot by cleaning tools and avoid over head watering. Prevent the disease by spraying plants with a proper mix of fungicidal soap.
Bean common mosaic virus and bean common mosaic necrosis virus are both seed-borne viruses. The signs of both viruses are light yellow or green oddly-shaped patterns on leaves, followed by puckering, curling and rolling of the leaves. Plants infected early or grown from an infected seed will be stunted and will produce fewer crops. Death will result if the root system becomes infected by mosaic viruses. These viruses are spread by aphids, sap-sucking insects, that infect the leaves of the bean plant. Controlling mosaic is best done by planting certified seeds that are resistant to the mosaic virus.