Possibly cabbage root fly grubs? Have the roots died back or could they have been eaten?

Thanks for the response, I will look into the problem.

It could be the answer as not all the plants are affected.

Like you I am committed to organic growing. Living in area carved out of woodland it is constant battle to keep the nature that I love from invading the crops I like to eat.

With good wishes, Paul.

'For a while' is how long? I would suspect Bulbaholic's diagnosis as this shouldn't affect every plant. The plants should pull out easily if affected and there will only be a stump with almost no root hairs.
I garden organically and find that mulching immediately around around the newly planted plants with a deep layer of grass cuttings or even the stuff out of a shredder will help the problem. Or you could always use the paper or card 'collars' which are supposed to help... but I find the mulch just as good.
Don't blame 'nature' and the forest, as these should be helping with the natural balance and introducing predators which eat the root flies! Encourage even more pollinating insects by planting things like phacelia and you should find the problem diminshes.

The plants seem fine for a week or so before the problem starts showing although some survive.
A badly affected one has just a bare stump as you say.
I will try the mulch idea which will also help with watering as we are hot and dry here.
As for natural world we do try to encourage the good bugs but I seem to be losing this particular battle just at the moment.
One year a badger found its way through the fencing and systematically dug up the brassicas, presumably for the worms underneath. At least I could replant them the next morning in that case.
Thanks for the information.

I'm still struggling with dying brassica plants and have been doing some investigation.

Pulling out a badly effected plant just showed the main taproot devoid of side root growth.

I then dug up a badly effected plant with surrounding soil to search for evidence such as grubs that might be eating the roots but found nothing.

This morning I dug up a Calabrese plant just as it started to wilt and this time found the rootball swarming with small black ants with reddish brown heads. I could also clearly see that the side roots were still there but appeared to be severed off from the taproot starting from the top.

There is no visible evidence of other insects that might be eating the roots and encouraged by the ants as they do in my annual battle for the broad bean plants.

So, I am wondering if the ants are cutting through the roots to get at the sap and once a plant is denuded moving on to another.

I am also wondering what on earth I can do about it without resorting to some sort of insecticide.

We are committed to growing organically and brassicas are the mainstay of our winter production.

Any ideas on how to deal with this would be welcome. I have long experience of how persistent and industrious ants are so am not feeling very optimistic at the moment.

An interesting observation, Pellegrino, and one that is new to me. Is the soil on the dry side? Ants, if they are actually ants, don't like wet soil, but then neither do cabbages. Sorry, I have no suggestions but hope that someone can help.

Brassica update.....
Yes, we have sun-baked clay interspersed with stone here in our little corner of northern Tuscany, so lots of ant nests.
(When we first arrived we put the potatoes in with a pickaxe but fortunately conditions have improved since then).
Still, finding one of my plants in a nest of moist compost must be like finding an oasis in the desert to a hungry bug- and it seems that the bug in question is probably a mole cricket.
I found one coming up for air and reluctantly decided that our 'live and let live' policy towards the rich and varied wildlife we share our lives with only goes so far.
The ants were probably not the culprits after all, just opportunistic for whatever reason.
The good news is that there hasn't been further significant loss and some of the plants I whisked off into intensive care as they started to wilt have made a good recovery and are back in the ground.
Thanks for taking an interest, happy gardening!