Bucket of onions

Owen’s comment on my shallots post about the possibility of introducing white rot into the garden on shop-bought alliums sent me off on a research mission. I know that it’s possible, but is it likely?

There’s a lot of information on the internet about white rot, and a lot of it repeats the same basic information. There is also some on-going scientific research into ways to control white rot, most of which is confined to rather impenetrable scientific journals. So what I’m going to do here is round-up the basic information about white rot and some of the new information I have found. I have no desire to simply repeat conventional wisdom, so I will annotate the text with the websites I found – you can follow the links for more information if you want, and come to your own conclusions.

White rot – the basics
White rot is a fungal disease that is capable of infecting all members of the onion (allium) family (Ref 5). It is caused by a fungus called Sclerotium cepivorum, which is soil-borne and attacks the plant’s roots. An infected plant may simply collapse and die, and if you dig it up the roots may be covered with a distinctive white mould (Ref 5). Unless the infection is severe it’s perfectly possible that only some of your crop will be affected, and that other plants will appear healthy – although they could still be infected. Infected bulbs will not store well (Ref 5), as the rot continues once they have been harvested, even if the bulbs are properly dried.

This disease has a temperature-dependent lifecycle. When the soil temperature is between 10-20°C (50-68°F, March/ April in the UK) (Ref 5), it actively seeks out allium roots in the soil. Once it finds them it hooks into the plant and reproduces via sclerotia, black balls that look a bit like poppy seeds and are sometimes visible at the base of the bulbs. When the bulb rots (Ref 6), or is harvested, there sclerotia are left behind in the soil to begin the cycle again (Ref 2).

Infection rates are worse in cool, damp summers (here in the UK) and can be made worse by over-watering (something alliums really don’t like) (Ref 1).

Sclerotia can remain dormant in the soil for at least 15 years (Ref 5), waiting for their next opportunity to infect, and white rot is a difficult disease to eradicate as there are no chemical controls available to UK gardeners.

The sclerotia can be transferred on infected plants and in soil. The best way to prevent it is to be strict with your hygiene – clean your boots and tools if you’ve been in someone else’s garden. Suspect plant material should be destroyed and not composted. Be careful where you source your plants from.

It’s also important to practice a crop rotation, so your alliums are not grown in the same soil every year. The longer the rotation you can manage, the less likely you are to have problems, but at least 3 years is recommended.

If you suspect that a crop has been infected with white rot, and it’s a new infection, your best defence against it spreading is to remove the crop (it’s perfectly safe to eat if it’s mature enough) and dispose of any remains by burning or in landfill (Ref 5). Only a genuinely hot composter would be enough to kill the disease, so composting is best avoided. Remove the soil in the vicinity of the plants, and dispose of that as well. Disinfect all your tools, wash the soil off your boots, and grow your alliums somewhere else for the next few years, even if that means using pots and grow bags (Ref 1).

If you visit the various gardening forums then other control methods are mentioned. You could try having a bonfire on the contaminated area (Ref 7), as the heat will help if it’s high enough and sustained. Armillatox (Ref 3) is recommended by some as a soil drench.

There is ongoing research into the use of composted onion waste (Refs 2&4) (and other products made from alliums) – it seems to trick the sclerotia from their dormancy, without providing a host in which they can live, thereby reducing their numbers. It could be done on a garden scale, with composted onion waste, garlic powder (sold in pet stores) or a slurry made from garlic gloves and water – once you treat an area you need to remember not to plant any alliums there for the year.

It should also be possible to use an allium trap crop – grow an allium (garlic is reported to be their favourite victim) to the point where it has been infected, then remove and destroy it (or, again, eat it) before the disease has a chance to complete it’s lifecycle. It could work with something like salad onions, that aren’t in the ground for long – but I can’t find any information on how long it’s likely to take for the sclerotia to infect the plant, and how long you then get before more sclerotia are produced. Use of a trap crop in this way is likely to take several seasons to clear the infection, if you get it to work.

There are also suggestions that adding homemade compost (Ref 5) to the soil offers some protection, presumably because it keeps the soil ecosystem healthier and allows the natural soil organisms to keep the white rot in check. You can also try using compost tea (Ref 1).

If you want to get a crop of alliums on ground that you know is infected (an allotment, for example) then it’s useful to know that onion sets and garlic cloves are usually worse hit than alliums grown from seed – it’s all about the size of the root system (Ref 5) that has developed at the time the fungus is seeking out roots. For the same reason, leeks are often least affected.

My research led me to ask several more questions, not all of which I could find an answer to (particularly the timing of the lifecycle, as mentioned above). I wondered whether perennial onions were less susceptible – there’s not much information around, but I did find a scientific research paper that suggested that Welsh onions (which are widely grown in China) are susceptible enough for people to be looking for control measures.

Are there heritage varieties with some resistance? Several forums mentioned the onion variety Up-To-Date, which was dropped from the national seed list as it was deemed to be very similar to another variety. Up-To-Date is supposed to have some resistance to white rot; it’s commercial replacement does not.

The Open Plant Breeding Foundation encourages amateur plant-breeding with the aim of increasing pest- and disease-resistance (since most commercial varieties are drenched in pesticides they are not developing any resistance), and one of the projects on their books is to find onion varieties that are resistance to white rot. If you have the space you can sign up to give them a hand.

There is a new variety now on sale called Golden Bear (an F1 hybrid) that has been bred to have some white rot resistance, but I couldn’t find any reports about how well it had done in gardens last year.

It seems unlikely (but not impossible) that supermarket bulbs could bring white rot onto your plot. I checked over my shallot bulbs (which had been in storage in a plastic bag for a few weeks) for any signs of rot before I planted them, and there were none. They’re planted in the corner of a raised bed filled entirely with homemade compost – in the event that there’s a problem I can remove the infected compost and refill that corner. There are currently no other alliums in that bed, as it’s being planted up with perennials.

References (accessed on 17th January 2011)

1 – Gardeners Corner
2 – UC IMP online
3 – Armillatrox
4 – Farmers Guardian: http://www.farmersguardian.com/fighting-allium-white-rot-and-the-uk%E2%80%99s-waste-problems-with-composted-onions/23430.article (no longer available)
5 – Garden Organic factsheet (available only to members)
6 – NC State University
7 – http://www.theediblegardenshow.co.uk/page.cfm/link=55, accessed on 17th January 2011, but no longer available.