[shewolf]

Hi there, wondered if someone could give me some advice on my dismal onion crop please? I've been growing my onions from seed for several years now very successfully, using a 4 year crop rotation method, but this year my Bedfordshire Champion crop has failed dismally! Within a week my healthy, promising looking onions developed a black powdery fungus(?) which dissolved the outer leaves, leaving the onions looking as though they had been involved in a fire!
I would like to learn from this please, so 1) can you tell me what this disease is? 2) do i destroy the onions or is it safe to eat them? 3) presuming the soil is carrying the fungal(?) spores, how long to leave the ground before growing on it again? 4) how do I prevent this from happening again (or similar diseases)?

[Salmo]

That is downy mildew, a disease of wet Summers. Most of my onions got it this year. Once they had got it they stopped growing. The small bulbs look OK but there is some doubt whether they will keep. Make sure you clear all tops and skins from the plot and do not put anything from the onions on the compost heap. Notably one variety, Santero, did not get it and the onions are beautiful. It is advertised as resistant.

As well as infection from the soil, spores can blow in on the wind, so it is not your fault that you got it. Just a wet Summer.

https://www.rhs.org.uk/advice/profile?PID=743

[shewolf]

Thankyou Salmo for your thoughtful reply. However, this doesn't look like downy mildew as the outside leaves of mine are dry, blackened and have crumbled away.
Has anyone else got any thoughts on this?
And how to treat the soils/how long to leave this site until I can grow onions on it again?
Thankyou

[Salmo]

The outside leaves finally die and dry up. That is what yours have done.

Look at the RHS site I quoted. As long as your rotation has a reasonable gap and your plot hygiene is good you may not see it again, unless we have another wet Summer.

I shall grow varieties with some resistance to downy mildew. I may also look at whether I can sensibly separate my onions into two or three lots rather than grow them all together.

[shewolf]

Ah, I see!
I've been away on holiday and must have missed the first stages of the disease where the outer leaves wither, as pictured in the RHS link you sent me. Returned to see the onions in the final throes, so to speak!

This is going to help my allotment neighbours (and me in the long run) who also have the same onion malady, and if we follow good preventative measures, hopefully it won't happen so easily.

Funnily, I grew some Santero, too. They were hard to germinate, so I only got three, but they have grown into beauties too!
Maybe next year I will try to source some sets and try them.
With a crop rotation scheme, it will be hard for me to separate them.

Thanks again