Answer: I believe the problem lies within the roots - root knot nematodes or fusarium wilt both cause the kind of symptoms you describe. Both are very challenging problems to overcome. Fusarium fungus invades the roots, clogging the stems and limiting the flow of water and nutrients to the leaves. As the fungus advances, more and more leaves die. Try planting cabbage or sweet corn into your bean plot for a few years to starve the fungus out. Avoid planting any other beans, peas or limas there - you'll need to find a new spot for them. For severe infestations, soil solarization can be effective. Remove any plant debris, rake the area smooth and moisten the soil. Lay a sheet of four mil thick clear plastic over the area, bury the edges with soil, and let the sun beat down on it for at least a month - five or six weeks is even better.
If the problem is nematodes, you'll find the roots of the beans all swolen and knobby. Solarization works well to control nematodes, too, but only in the first few inches of soil. For long-term control of nematodes, the best approach is to mix organic matter in the soil, because it contains beneficial fungi that kill nematodes. Crab shells, shrimp hulls, and eggshells are especially helpful because they boost the number of microorganisms that eat chitin, a substance contained in nematode eggs.
Also, try to choose lima beans that are resistant to these pests (look for varities with "V" and "N" listed next to their names). Good luck!
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