Today I removed a lot of leaves from my remaining turnips. They have turned white and moldy on the top side, so I suspect Erysiphe betae (I haven't found the common name). This irks me a bit; those leaves are good eating. Today I just tossed them. I have no idea on what to do with Erysiphe betae, neither do I know if it's possible to eat it. I need to do some more research!
