As a new vegetable grower on a small farm in South Africa this is my first attempt at Asian vegetables. The pak choi and tat soi plants were doing well up until 2-3 days ago when the leaves suddenly started going brown. I thought it might be downy mildew so I treated with a Burgundy mixture spray but this has not stopped the "rot". Does any one have any idea of what could be causing the problem. There are other brassicas planted nearby and they are all fine.

(The colours in the picture are not exacly the same as the true colours)