Plant Problems – Onion White Rot

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Onion white rot – Stromatinia cepivora (syn. Sclerotium cepivorum) — the unforgiving fungal pestilence of the allium family. There is nothing so annoying as expectantly walking up to the lovely crops you’ve grown, picking your first onion, and looking at it in dismay, wondering what happened to it. This happened to me this week as I picked my very first onion of the season, only to look at the base of it and see that it is covered in onion white rot. As soon as I spotted it, I knew that it is likely that the entire crops of onions, shallots, leeks, and garlic will show signs of infection and be useless. It’s very disheartening after working so hard to keep your crops from bindweed and pests to suddenly find you’ve got white rot in the soil, and there’s not that much you can do. So, this inspired me to write this post to discuss what white onion rot is and what you can do to treat it — naturally, of course.

What is Onion White Rot?

Onion white rot is one of the most serious and common problems with growing allium crops. The fungus appears during the summer and is prevalent until early autumn. It stays in the soil for up to 15 years so if you have a break out of onion white rot, the chances are you won’t be able to grow any more alliums for a long time. If your alliums have been infested by the allium leaf miner, they are even more susceptible to white rot.

Symptoms of Onion White Rot

The problem with onion white rot is that the chances are you won’t know you’ve got it until you harvest the first crop from the allium family. The first sign of onion white rot is the wilting of the foliage on your veg. Unfortunately, this happens at the same time as you would expect the foliage to begin to change color and wilt for healthy plants, so by then it is far too late and a waste of the entire onion growing season. In wet conditions, the onions’ foliage won’t wilt, but the onion gets loose in the soil. Once you remove the onion, you’ll see white fluffy mold on the bottom of the bulb. It is soft to the touch and appears rotten. Severely infected onions have small black small growths on them that look like poppy seeds.

Life Cycle of Onion White Rot

Onion white rot starts as sclerotia, which are tiny organisms. They lay dormant in the soil until they sense a member of the allium family. It’s believed by the scientific community that the sclerotia detect the odors that the onion roots give off. Once they know the onions are there, they germinate and produce smaller fungi that attach themselves to the roots of the onions. As these fungi mature, they produce more sclerotia and the cycle repeats itself. In late autumn the fungus lays dormant until the following season. The only flaw in this life cycle is that if they do not sense a member of the allium family, then the organism dies.

How Does Onion White Rot Spread?

The most common way that onion white rot is spread is on gardening equipment and items of footwear. Simply treading on an affected area and wandering to another is enough for onion white rot to infect a new area.

onion white rot

Treating Onion White Rot

The simple answer to “how to treat onion white rot” is that you can’t. There’s no known cure for this fungus. There may, however, be a glimmer of hope. Research is currently being carried out on garlic extract and whether adding this to your soil may reduce onion white rot. The theory behind this is that the sclerotia sense the garlic and begin to germinate. However with no host to feed on, the sclerotia will starve and die. Water an area where allium crops are not being grown with one crushed garlic bulb mixed in 2 gallons of water if you want to try this trick to clear up your soil.

Preventing Onion White Rot

The best way to prevent onion white rot in your garden is by keeping everything clean. From your gardening boots to spades, forks, and trowels, they need to be clean at all times, especially when moving around different areas. You can learn how to clean and care for your boots here. Beware of any gardening equipment bought at yard sales and always clean them before using them. Plants with onion white rot should be burnt and not placed in the composting bin.

Can You Eat Plants Infected With Onion White Rot?

There seems to be some debate on whether or not vegetables that have onion white rot should be eaten. If the infection is not too severe, then some of the crops can be eaten. They need to be eaten straight away though as they will not store or dry. Wash the vegetables well, and either use fresh or freeze.

As soon as you notice the beginning of onion white rot in your soil, then dig up the affected vegetables as soon as possible. This limits the chance of every member of the allium family being destroyed and all your hard work going up in smoke.

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2 thoughts on “Plant Problems – Onion White Rot”

    1. Hey Robert,
      Yes. Onions and tomatoes are two distinct species, as is white rot, which is a fungus that attacks the onion family – leeks, onions, garlic, chives, and so on. So you should be fine to plant tomatoes there, which are part of the nightshade family, assuming that you haven’t also got any other soil-borne fungus or pathogens that attack tomatoes.

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