[fatcat1955]

Having suffered this on my plot i want to try growing in a raised bed. I have a number of those wooden folding pallet sides which are 48x40 inches and 6 inches deep. Does anybody know how many i would need height wise to eliminate the white rot problem. I was thinking 1 filled with 6 inches of gravel then 2 more on top with planting medium in them.

[simonwatson]

I would think that would be enough, as onion roots don't go that deep. However, I'd be a bit worried that such a well drained bed would dry out very quickly.

[Annen]

I grew mine in a raised bed this year hoping to avoid the white rot, but it still affected quite a few, but not all, of my maincrop onions. It was a new bed in a new area and the bed was filled with council soil improver (supposedly sterilised and certainly it was steaming when it arrived), so I don't know how you can avoid it. I wonder it it is not just soil conditions but atmospheric conditions are important too.
I'm just wondering if I should stick to winter onions as they don't seem to be affected.

[beesrus]

Rather than the whole raised bed thing, why not just hollow out under each onion and fill that with compost, in the way one would dib, waggle and fill for parsnips on a stoney soil. It's something I was prepared to do until I found some beds on my plot that have so far been rot free, fingers crossed.

[moose]

Rather than the whole raised bed thing, why not just hollow out under each onion and fill that with compost, in the way one would dib, waggle and fill for parsnips on a stoney soil. It's something I was prepared to do until I found some beds on my plot that have so far been rot free, fingers crossed.

On a recent Beechgrove Garden episode it showed  that George Anderson tried this on his allotment and it did not work.

[Kristen]

Having suffered this on my plot i want to try growing in a raised bed

I've not encountered White Rot, so no first hand advice, but my understanding is that it is very infectious so likely to contaminate the [raised bed] soil easily.

I have read of people "watering" the soil with a Garlic solution, the idea being that this gets the Onion Rot spores to "germinate" thinking that there are onions present, none are found and the germinated spores die.  I suppose it needs repeating a few times, but that then leaves the remaining spores few / non existent / exhausted.

[JayG]

I've not encountered White Rot, so no first hand advice, but my understanding is that it is very infectious so likely to contaminate the [raised bed] soil easily.

Me neither fortunately, but I have read that it is important to sterilise tools used for onions in a 'clean' bed to avoid transferring the spores from infected areas, bearing in mind they can live for up to 15 years even in the absence of their host plants. Even with a no-dig approach you are likely to use at least a hoe and some sort of hand tool on your onion bed, and they must be sterilised before use unless used exclusively for that bed.

a 9:1 water:bleach solution for 30 minutes, followed by rinsing off, should do the trick, not forgetting that bleach does not last forever hidden away at the back of the cleaning cupboard.

[moose]

I have read of people "watering" the soil with a Garlic solution, the idea being that this gets the Onion Rot spores to "germinate" thinking that there are onions present, none are found and the germinated spores die.  I suppose it needs repeating a few times, but that then leaves the remaining spores few / non existent / exhausted.

I have read that this has been tried on a field scale and works but the timing is important. (Google may be your friend here)

[simonwatson]

With garlic watering, I believe it's a good idea to do it a few times during the year, whilst actually growing something else in the bed, such as your spuds. I don't think you can get rid of it, but you can seriously reduce it's effect.

[Kristen]

Amongst the articles I found, mostly vague on the subject, was this one:

"Root Exudate Solution (before planting)
A solution to stimulate the sclerotia to germinate in the soil can be made using onion or garlic juice using culls from your crop. Once the sclerotia [Onion White Rot fungus] are stimulated and germinate they then die due to lack of nutrient reserves because the garlic is not actually present. The optimum conditions for germination of sclerotia occur when soil temperatures are between 15-18C. Garlic works better because it's stronger than onions. An effective dilution for garlic is one to a thousand parts garlic and water. The idea is to somehow distribute it over the area that you're going to crop in the future getting this juice stimulant concoction across the area and down into the soil. It needs to get it as deep as you've tilled it. It usually takes 6 months before the sclerotia are receptive. Garlic powder [available in equestrian stores or on-line] that's tilled in at 250lbs/acre has also been used."
http://bcseeds.org/white-rot.php

I wonder for how much of the summer a soil temperature of 15-18C is available?

and:

"It is best [for the garlic solution]to be watered onto the ground when rain is expected, so as to take the garlic water deep into the soil to contact the maximum number of “sclerotia”."

[realfood]

More information here with quantities. I have had no problems with white rot since adopting this system or organic control.
The control method is to trick the dormant “sclerotia” into thinking that there is an allium growing beside it, by watering on a ground-up garlic bulb solution of 1 part garlic solution  to 1000 parts water, say one ground-up garlic bulb to a watering can of water. This should be watered onto 1 sq m of damp soil. It is best to be watered onto the ground when rain is expected, so as to take the garlic water deep into the soil to contact the maximum number of “sclerotia”.
After carrying out the first treatment using the ground-up garlic water, turn over the soil with a fork. Then apply the ground-up garlic water again. This will enable the soil to be treated to a fork depth, hopefully killing most of the white rot sclerotia.

Strip the garlic cloves of their papery wrapping and cut off the basal plates before you grind the cloves up, to cut down the danger of introducing any disease. You should be able to taste and smell the garlic in the solution. It should be applied when the ground temperature is between 10c and 20 c with an optimum temperature of 15c.
It should be watered onto the ground that you intend to use for alliums, during the preceding year while the ground is moist and warm.

It is also possible to use garlic powder which you can find in equestrian stores or on-line, as it is used for the treatment of horses. This should be applied at a rate of 250 lb to the acre. Roughly 125 Kg per 4000 sq m, or roughly 30 gm per sq m. This has the advantage of having been sterilised and unable to pass on any allium infections but is not so effective as using fresh garlic bulbs.