Sprouting Broccoli Trouble

[Suru]

Hello!
It's my first attempt at growing Sprouting Broccoli.I started them off in modules in the greenhouse and planted them out in May.They've been growing well,but suddenly all the lower leaves have yellowed.Is this normal?I have removed the yellowing leaves and apart from this the plants seem healthy.Any advice appreciated-excuse my ignorance!
           Thanks Suru :)

[Suru]

[john_miller]

Like this?
http://www.allotments4all.co.uk/yabbse/index.php?board=7;action=display;threadid=3485
or different?

[Suru]

 Yes John,it does look very similar to the one in the picture , although only one or two plants are affected.It seems to have come on very suddenly.We had a lot of heavy rain yesterday after a very dry spell and I wondered if it could be linked to that.
         Suru