Purple sprouting Broccoli

I am completely new to growing Brassicas and my purple sprouting Broccolli was growing well until last week. I noticed some small holes appearing in the leaves on some of the plants and thougt at first it was slugs. However I have now noticed that black spots are forming on the underside of the leaves, which are eating right through to the surface side and forming holes. I think this is the wrong colour for the Blister Fungus. Any ideas on what it is, whether it will effect my crop and if so how to treat it?

Nathalie

After extensive research on the internet and finding images to match my purple sprouting broccolli leaves, I have diagnosed the problem; Ring spot.  A disease favoured by wet, mild conditions. Sound familiar?!!  I can only hope in removing the infected material that the promised frost will kill off any further spread.