It is hard to say for sure whether the plant has a leaf disease or something else without actually seeing the plant and perhaps even send the affected parts to theplanta clinic for culturing to confirm disease. Saying that though if the top portions of the plant are fine and it is procing peas and only the bottom portions are brown, peas can often show brown lower foliage when there is some type of stress to the plant such as very warm temperatures, or lack of sufficient moisture to keep the plant growing. It could be that the plant is showing signs of this type of problem. If it did in fact have a leaf disease fungicides are best put on before the problem to protect the leaves. Once the leaves have the problem fungicides are not usually as effective. So not much can be done at this point in time if it is a disease. My suggestion make sure the plants are well watered and when warm weather starts to be an issue pea season tends to be over. Fungides labeled for vegetable crops could be used at the start of the season for best effect.