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I had this happen to a batch of brassicas but gave them comfrey tea and they managed to pull through and I only lost two plants they are outside now and growing away fine. I am blaming the compost - Homebase. It was suggested to me there was not enough nitrogen in it. So I quickly gave them a high nitorgen fix. For me one variety was worse than the others. Are you sure they are totally dead not not just yellow leaves.