When my beans reach appx. 8", right at earth level, the stock turns brown and weak. The wind blows and the stalk bends and plant dies.
There are a number of disease and insects that could cause these symptoms. Best idea for you would be to take some samples of affected plants, especially of the stem area, to the agricultural service or a university agricultural dept. Maybe you could go to a nearby garden store, they might be able to tell you who could analyze this for you.