Purple blotches on leaves

Folks,

This is a new one to me, my neighbour's rhubarb and my mint mint have both developed intense beetroot red/purple stains on their leaves. His rhubarb appears to be going backwards and my mint doesn't look too great either.

My rhubarb is ok.

I read that this might signify a magnesium issue, if there's too much potash. I've fed using the former but never applied the latter.

So far (10 days) there's been no real signs of recovery - can anyone suggest other potential causes?

Thanks.

Make do