[Brambles]

Have a HUGE very sad problem with my brussels    I had a beautiful bed of luscious healthy sprouts, they had been attacked a bit by pigeons, (we have masses around here) then suddenly my beautiful plants started to wilt...lower leaves turned yellow almost overnight... today I dug two of the almost dead plants up and there was NO root left at all on either of them.... Please can anyone tell me if this is cabbage root fly and if I plant anything else (as this is my brassica bed this year)  should I do anything to avoid the same in other plants like put collars on?   
In future I will put collars on ALL my brassicas
 

[Brambles]

OK.... found the problem...  first two I pulled up were almost dead and I found no trace of anything in the root.   This afternoon I pulled up two more, the whole plant came out easily and the stub of root that was left was riddled with maggots    I don't hold out any hope of any early brussels this year    But have to say the chickens LOVED the little wriggly things!

[Salmo]

Sorry to hear about your sprouts. At least you know the problem.  The collars must be heavy and stiff enough so that they do not blow about. A circle with a slit to the middle. It helps to cut out a bit in the middle to fit round the stem. I use pond liner but you can use all sorts of material such as roofing felt, underlay or cardboard.

[Brambles]

Thank you Salmo, was wondering what to use as collars.. pond liner sounds just right, will definitely have a go at that. 
So far there are just two plants standing, they have a forrest of annual weed around them, am wondering if the weeds stopped the fly getting to the stem of the plant - time will tell!