How to Control Onion White Rot

, written by Ann Marie Hendry gb flag

Onion (allium) white rot on garlic

White rot is the number-one threat to onion family crops worldwide. It’s so severe that, in some areas, it has destroyed the allium growing industry. Alarming? Certainly. Treatable? No – not currently, anyway.

However, there are steps you can take to prevent it from entering your garden, and to limit the damage and spread of the disease if it’s already present.

Onion White Rot Symptoms

Onion white rot can affect all alliums, so onions, garlic, leeks and shallots, as well as ornamental alliums, are at risk. I’ve found garlic to be the most susceptible, closely followed by onions. My shallots haven’t yet been affected, and I’ve only ever had one leek show signs of the disease, but I may have just been lucky – if any aspect of having a white rot-infested garden can be considered to be lucky!

One of the worst things about onion white rot is that appears symptomless until it’s too late to do anything about it. The first sign that anything is wrong is usually yellowing foliage, often just before harvest time when you’d expect the leaves to start dying back anyway. This means it’s possible to overlook the disease entirely.

“Garlic

When you dig up an infected plant it will pull free from the soil easily; no tenacious tugging required! Depending on how far advanced the disease is, you may see a mat of fluffy white mould on the basal plate (where the roots sprout from), which will probably have tiny black growths like poppy seeds strewn across it. In severe cases, the bulb will have turned black and be totally rotten.

The little poppy seed growths on the base of infected allium bulbs are called ‘sclerotia’ and these are the means by which the disease spreads. They will drop off and hang around in the soil until another allium crop is planted in that spot, at which point chemicals in the plant roots trigger the sclerotia to germinate and colonise the bulb with white mould.

“Macro
Close-up of the black sclerotia that are distinctive of onion white rot

Onion White Rot Control

Unfortunately there is currently not a single white rot-resistant variety of allium available, so when buying in young plants make sure they’re from certified disease-free stock. Additionally, inspect the basal plate carefully and discard any that show signs of mould. Or start your plants from seed instead. If your plants do contract white rot, dig up the crop as soon as you’re aware of it, and dispose of every scrap of plant tissue by burning or in your household waste. Do not compost it.

If the infection is not too severe, you may be able to use part of the infected crop. It won’t store dry, so you’ll need to use it fresh or freeze it.

“Garlic
Garlic bulbs that are only lightly infected with onion white rot can be used fresh or frozen

It’s vital to avoid spreading the disease around the garden, so make sure you wash any soil from infected parts of the garden or allotment off your boots and tools before working on other garden beds.

Crop rotation can help limit the prevalence of the disease, but unfortunately the sclerotia can lurk in the soil for many years – far longer than most gardeners can afford to wait between allium crops. You’ll need to avoid growing alliums in the infected soil for at least eight and possibly as many as 20 years. Growing in containers of bought compost may turn out to be a more practical solution.

Garlic Extract Cure for Onion White Rot?

There is a glimmer of faint hope, and that’s adding garlic extract to the soil. The idea is that this causes the sclerotia to sense allicin, the chemical that gives onion family plants their scent. This tricks the sclerotia into germinating and, finding no host plant to infect, they will starve and die. This may help reduce, if not completely eliminate, the disease.

Onion white rot on garlic

To make garlic extract, take a bulb of clean, disease-free garlic and discard the papery wrappers and the basal plate. Crush the whole bulb into 10 litres (two gallons) of water. Water it onto areas of your garden that you’re not currently using for growing allium family crops. Don’t be shy – use a lot!

The whole 10 litres (two gallons) should be applied to two square metres (21 square feet). Do this when temperatures are between 15 and 18°C (59-64°F) as this is the optimum germination temperature for the sclerotia. An easier option is to rake or water in garlic powder, which you can buy quite cheaply in large containers from equestrian suppliers.

I’m trialling this technique this year – so fingers crossed! Have you ever tried these techniques, or had success with other methods of white rot control? If so, share your experiences with other gardeners – and me – by leaving a comment below.

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Comments

 
"I've never seen nor heard of this disease in Australia - has anyone else? "
Gypsy on Saturday 16 July 2016
"I've heard you need to water onion or garlic extract on regularly, like once a month, to keep triggering any spores of the sclerotia, for a number of years. I'm really pleased you're trialling this, looking forward to hearing how you get on."
Elspeth Campbell on Sunday 17 July 2016
"Gypsy, if you click the link in the first paragraph, this shows a distribution map which suggests that white rot is present in Australia - as well as pretty much everywhere else in the world, unfortunately! Elspeth, if you're able to water on the garlic solution regularly this could help to trigger more sclerotia to germinate, though I can't say what affect this could have on beneficial soil organisms. It's also important to do it within the temperature range shown above, as this is the typical range that the sclerotia will germinate within. "
Ann Marie Hendry on Tuesday 19 July 2016
"Thanks heaps Ann Marie - unfortunately I keep getting an error when trying the link but I'll keep trying in case it's temporary"
Gypsy on Tuesday 19 July 2016
"Hi Gypsy. It seems to be working again now."
Ann Marie Hendry on Tuesday 26 July 2016
"G'day Ann Marie Thanks heaps. An interesting read but thankfully, nowhere near us. I'll go plant more winter leeks now :) Hooroo Gypsy"
Gypsy on Thursday 28 July 2016
"is it possible to apply garlic powder in the fall and plant garlic in that field in the spring. i planted garlic this past spring and had very good results, good size and a healthy crop. this was my first year with garlic and i had not made up my mind to plant garlic until late feb. i planted march 10 4,000 seeds and allows good . i did pull two or three yellow leaf plants and garbaged them. not knowing anything about white rot . I'm not sure now if that was what i pulled or not. i would sooner be safe than sorry so thats why I'm considering the garlic powder application . could you advise please."
gord hansen on Tuesday 6 September 2016
"Hi, I used this method with great results! I watered 1 crushed garlic per square metre, with lots of water. I then dug the patch over a couple of days later and repeated the process. This was in late May when the ground had warmed up a bit. I then planted squash on the patch and onion sets the following spring. It is important to not put onions in for at least 6 months. The results were remarkable. . As I had done this in a new part of the allotment I then decided that it probably was not thegarlic but simply that this part was clean. I therefore did not bother treating the adjacent patch that I was going to grow onions in next year...there was almost complete crop failure! Needless to say I have treated the next patch last spring ready for next year. I am also going to try just digging in garlic powder whenever I am going to dig in the warm months in the hope that it may just keep knocking the sclerotica back... cheers Norma"
Norma on Friday 9 September 2016
"Hi Gord. You need to apply the garlic powder within the temperature ranges shown in the article above, so unless you live in a very warm climate it's unlikely to be effective in the fall. If you haven't seen any white rot and you're growing from seed then adding garlic extract is not necessary - the disease can only be introduced on infected plant tissue from allium family plants, or in infected soil. Norma, it's great to hear you've had success with this method! Growing on fresh soil this year has worked well for me, so just to be safe, I'm going to dig a new bed for alliums next year and will continue applying garlic extract for another year on other parts of the garden to try to reduce the disease to a minimum. "
Ann Marie Hendry on Tuesday 13 September 2016
"Hi I use garlic powder on the ground with the crop before my onions, sweet corn is the norm. I dress the area in mid June then again in mid July Then last of all mid August. in 2014 I lost 95% of my crop of onions to white rot last year 2015 I lost 20% but that was with only one dressing this year 2016 with 3 dressings I lost 1 onion out of the 500 i grew Yes Garlic Powder Does Work"
John Rendell on Sunday 9 October 2016
"Thanks for the back-up advice John, nice to know. Never seen nor heard of it where we live but all bugs/viruses seem to travel these days, so no doubt it will happen at some time. The majority of us grow our alliums from seed, so I'm hoping that gives us some protection but you never know. Using garlic powder does sound less work than the other method but of course, you'd try all of them if you had a problem! Cheers Gypsy"
Gypsy on Sunday 9 October 2016
"Hi John, Great to hear your post. Do you dig your garlic powder in or just use use it as a top dressing? Presumably your corn is already growing when you apply your last 2 dressings.... also do you water it in? cheers Norma"
Norma on Sunday 9 October 2016
"Hi Again the garlic powder was sprinkled around then hoed in , It was washed into the ground by rain or watering. the garlic had no effect on the sweet corn"
John Rendell on Sunday 9 October 2016
"Thanks for that. I forgot to ask if you know what quantity per sq. yard you used. I have only used fresh garlic.... Norma"
Norma on Sunday 9 October 2016
"Hi about 1 oz per sq yard apx"
John Rendell on Monday 10 October 2016
"Hi John, it's very encouraging to hear that you've found garlic powder to be effective. 1 infected bulb out of 500 is a definite win! Well done."
Ann Marie Hendry on Tuesday 11 October 2016
"White rot of Allium species (caused by Sclerotium cepivorum) has been confirmed in garlic from a Perth backyard and a property in the Swan Valley."
Mik on Saturday 18 February 2017
"G'day Mik Ouch, that's closer to home. I'd never heard of this problem prior to this article but I think it's time I took preventative measures rather than wait for it to hit here and then try to cure it Many thanks for that update Cheers "
Gypsy on Sunday 19 February 2017
"No probs Gypsy"
Mik on Sunday 19 February 2017
"i have had a white rot problem on my allotment for many years and have redwith iterest of all the comments, and i will try the remedys that have been posted. thanks to all who have contributed."
les on Sunday 26 March 2017
"Hello! How long a time do you have to leave after using garlic watering before you can plant out onions/shallots? I got a bit of white rot last year here in SW France. Thanks in advance for your help."
Alan Rippington on Thursday 13 April 2017
"At least 6 months, preferably treat the ground and then grow a crop of something else and plant the onions the following spring. So lots of forward planning needed!"
Norma on Friday 14 April 2017
"Thanks Norma..Well...I guess I'll have to keep fingers crossed for this year. However, I've grown some Golden Bear onions from seed that are supposed to be resistant to white rot. I guess I'm going to find out if this true! (Going to plant out tomorrow)"
Alan FR on Friday 14 April 2017
"Keep us posted on the Golden Bean onions Alan - I'm sure we'll all be very interested to know how you get on!"
Ann Marie Hendry on Friday 21 April 2017
"I've had white rot in the soil for several years, sufficient to ruin 65% of the crop in cool years. The rot favours cooler termperatures, but for the last two years I have dosed at 1oz per sq.yd in the autumn with the temp at 15 degrees Celsius, then rotavated it in just prior to a heavy fall of rain. The exercise is then repeated in the spring using the same parameters. Last summer I did not lose one onion and it wasn't a particularly good one (Sark), so the signs are very positive. I have used the Golden Bear variety for several years and though it fares better than shallots and red onions, is still susceptible in poor summers. It appears to be no longer available in Europe through my suppliers, so maybe that tells us something. In a long hot summer the white rot shouldn't be a problem as it seems anything above 20 degrees C. is outside of its operating range."
Peter Cunneen on Thursday 4 May 2017
"the garlic powder can you buy it from anywhere or do you get it from garden shop .thank you .how much do you use .thank you"
haroon on Saturday 20 May 2017
"i have white rot on my allotment in west yorkshire is it worth trying the garlic powder."
haroon on Saturday 20 May 2017
"Haroon, certainly worth using, but too late for northern hemisphere onions this year. Get the powder from an equine supplier - they use it for treating horses for worms or some such - I order mine through Amazon. Use on the soil now if the temperature is no more than 15 degrees and then again in the autumn, then try onions again in the spring of 2018."
Peter Cunneen on Saturday 20 May 2017
"Hi Haroon, garlic powder can be bought from equestrian suppliers (local stores or online). The last three paragraphs in the article above shows application rates and recommended temperatures for applying it. "
Ann Marie Hendry on Tuesday 23 May 2017
"its warm now here in yorkshire uk .then when should i apply garlic powder to soil.thank you"
haroon on Tuesday 23 May 2017
"how come this is not public knowledge everyone i have asked have told me you cannot treat white rot.but i am going to try this i will buy a soil therometer as not sure what temperature the soil it.how long do i have to do this .will this be a nearly job.thank you i am new at my allotment."
haroon on Tuesday 23 May 2017
"Some people put garlic cloves in a food proser then dilute and water on the ground THIS IS A COMPLETE WASTE OF TIME as the bits of garlic act as a host for the white rot and then wait for onions. BY USING DRIED POWDER THERE IS NOTHING FOR THE WHITE ROT TO FEED FROM hence it withers and dies. I also dress 3 times june july and august about 1 oz p s y just hoe in with normal hoeing then water well. Hope this helps"
John Rendell on Tuesday 23 May 2017
"You can not treat white rot but with good planning you can help prevent it"
John Rendell on Tuesday 23 May 2017
"Hi Haroon, if the temperature is above 18 degrees now and stays that way then it's likely to be too warm for now - but if a cooler spell is forecast it would be worth trying the garlic powder. As John Rendell said above, there is no cure for white rot but it's possible to prevent it using crop rotation practices and avoiding buying diseased plants, and there is anecdotal evidence (much supported by some of the comments above I'm pleased to see!) that using garlic powder may help reduce problems."
Ann Marie Hendry on Tuesday 23 May 2017
"thank you ann and john ,peter i will give it ago."
haroon on Tuesday 23 May 2017
"Crushed garlic does work (see my earlier post) as long as it is a paste..."
Norma on Tuesday 23 May 2017

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