How to Control Onion White Rot

14 July 2016, written by Ann Marie Hendry

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.

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.

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.

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 soil 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.