When you pull up an infected bean plant, it will have a skimpy root system with most small roots missing. A dark area of decay may be present on the main stem near the soil line.
Plant beans in soil that has been thoroughly cultivated, and do not follow potatoes with beans. Thin as needed to grow plants at proper spacing, because crowded conditions can contribute to the development of root rot diseases. You will lose fewer seedlings to root rot diseases if you wait until the soil is warm to plant beans.
Pull up affected plants and compost them. Place a pile of mature compost on the spot where the failed bean plant had been growing.