I think James' suspicion has it. Its a fungal disease, so try using a fungicide, not an insecticide. It is the copper that gives the fungicide action. Very similar principle and action to Bordeaux mixture which is copper sulphate and lime, i.e. copper sulphate fungicide with carbonate to reduce scorch and retain on leaf surfaces longer. Cheshunt compound is very effective in minimising damping off which is caused by a range of soil borne fungal pathogens.
Commercial products based on copper oxychloride are equally as good.
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