It isn't just pak choi, but I have seen it in chard and other things.
Quite a lot develop long brown streaks in the stems, which look like
some form of rot, mostly on older leaves. What I don't know is what
triggers it.

I have flea beetle, but I don't think that's it, as I can see no
association with physical damage.

Any ideas?


Regards,
Nick Maclaren.