Posted: 18/07/2015 at 17:24
Thank you, Dove, that's really interesting. It does sound as if it would only affect seedlings or very new plants, whereas mine were all doing fine until a couple of weeks ago, and the onions are full sized now.
I think you are right, and it is probably downy mildew. This apparently starts as a whitish mould but then other moulds join the party, which could account of the brown and black colourings on the soggy leaves. Some of the red onions seem sound underneath, but some are rotting, and most of the shallots are rotting too. I suppose moulds are not particular about which party of the plant they attack. There may be White Mould there too, and the prognosis is much the same. Whatever it is, it looks as if I still have to keep onions away from that soil for several years.
The cause might have been that I have been saving and storing my shallots from one year to the other. Perhaps one or two of them had a bit of mould on them and I didn't notice. Or perhaps the onion sets (bought from a large garden centre) were contaminated. Do you think that could be what's caused it, whatever it is?