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Onion Smut!
Green Magpie
Posts: 806
in Fruit & veg
No, it's not a rude joke about an onion, it's this year's new disease. I've been gardening this plot for ten years now, and yet there seems to be a new pest or disease every year. Last year it was leek moth - oh, and codling moth - and this year it's a new one: onion smut (I think). Both my red onions and my shallots seem to have it. The tops go soft and have a black sooty deposit on them, and then the base of the bulbs develops a white soggy mould.
If I've made the correct diagnosis, this means that I can't grow onions or shallots on that ground for 5 years (or 8 years?). Has anyone else suffered with this? What did I do wrong?
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I am pretty sure that I've diagnosed it correctly, using the web. What I'm wondering is whether there's a lot of ot about this year, or whether I've just been unlucky or done something wrong.
The RHS gives this piece of information which may well be useful ...
"Onion smut only infects plants through the first leaf produced. Plants raised in a disease-free seedbed can therefore be transplanted into contaminated soil without risk of infection " https://www.rhs.org.uk/advice/profile?PID=734
So presumably you can start your onion seeds off in a greenhouse/coldframe etc, then plant them out with no risk of smut! Presumably it'll also be fine to grow from onion sets.
Hope that helps
Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.
I did see that RHS stuff, thanks, but it's not a lot of help. I grow onions and shallots from sets, and this can't be safe, as sets produce their first leaf when in the soil. The affected plants were all grown from sets, in a bed that has never had onions in it before, so I don't know where the infection came from.
Luckily I have enough space to be able to comply with this rule, but it's a bit dispiriting, as it may happen again if I don't know how to prevent it. I've already given up on leeks, which used to do very well, because of leek moth that completely ruined the whole crop last year.
But it's swings and roundabouts, I suppose; I have great carrots and beans, and masses of mangetout (which failed last year). We are just eating up the sound parts of the onions as fast as we can before they rot.
I now always start my onion sets off in modules in the greenhouse. This gives them an early start and also prevents the birds from pulling them up as they have decent roots when transplanted into the onion beds. In your case this should prevent smut, too. However, if you also have white rot in your soil, it won't help:
https://www.rhs.org.uk/advice/profile?pid=226
My thought was that sets have grown the previous season, so they had their first leaf then, in the first year - if they were all grown from sets, in a bed that has never had onions in it before, perhaps the sets were infected when you bought them ........?
Or
are we barking up the wrong allium so to speak
Could this be Onion Downy Mildew? https://www.rhs.org.uk/advice/profile?PID=743
Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.
I don't think this is Onion Smut - according to the info I've found it only affects onion seedlings, not plants grown from sets http://learningstore.uwex.edu/assets/pdfs/A3796.pdf
Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.
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?
Think I'd buy in new shallots next time - just to be on the safe side.
Sorry not to be more helpful
Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.