Wouldn’t it be nice if we could just grow plants without worrying about pests and diseases? Once you start your gardening journey you quickly learn that the natural world wants to eat your produce just as much as you do. I find that protection is a necessary evil for almost every crop I grow. The good news is that most pests can be prevented or controlled through the use of netting, fencing and companion planting. Diseases, on the other hand, are often much more tricky to deal with.
Allium white rot
Upon returning from holiday I discovered that many of my onions had been struck down by allium white rot. Often referred to as ‘onion white rot’ or simply just ‘white rot’, this disease is a fungus that can live in your soil for up to 20 years. It is very commonly found on allotments, and due to the nature of allotments it is often spread across most plots on a site over time.
This disease affects anything in the allium family, including onions, garlic and leeks. Garlic seems to be the most susceptible to white rot. The first year I had my allotment I had some garlic that was growing tremendously. You can imagine my excitement to harvest my first garlic – only to discover upon harvest time that my entire crop had been wiped out. The only survivors were most of my elephant garlic. Elephant garlic is actually more closely related to a leek than garlic which is why it is less susceptible. Leeks are the least susceptible and onions fall somewhere in between.
Most people grow their onions from sets. Someone at my allotment who has had a plot much longer than me, has observed over the years how those grown from sets seem to be more susceptible than those grown from seed. He reckons that those who use onion sets frequently lose as much as 50% of their crop to white rot, compared to around 30% from those grown from seed. I suspect this is likely because plants grown from seed are often stronger plants and therefore less susceptible to disease.
Signs of white rot
White rot is a disease that doesn’t present itself until it is nearing harvest time and the damage to your onions is already done. The most visible sign is yellowing and wilting of the leaves, starting from the top of the leaves downwards. Onion leaves can yellow slightly at the top as it nears harvest time but the outer leaves should usually die off first. White rot attacks the roots of the plants, so you will find infected plants will come up very easily. You will then likely see a white fluffy mould-like fungus on the onion. You may find some of the onion is OK to use but it will not store so use it immediately or freeze it.
I have white rot in my soil – what can I do?
Unfortunately unless you grow none of the allium family for 15-20 years in your soil, you’re not going to be able to eradicate it. Crop rotation is the main thing you can do to reduce build up. If possible, try not to grow alliums in the same soil for at least four years. It is important to pull up plants as soon as you notice signs of white rot, to stop the fungus from multiplying further. Do not compost any of the plant material – burn it or put it in your black bin.
The germination and growth of this fungal disease is inhibited above 20C, so it is more prominent in cooler, wetter summers. The dormant fungi cells only germinate once, so if there is no food source for them then they will die. Many sources suggested infusing water with garlic granules or crushed garlic, and watering an area with this prior to planting any alliums. The theory is the scent of the garlic should trick the white rot fungus into germinating, then they will die because they have no food. It is best done when the soil is at optimum temperature for the germination of the fungus – 15C-18C. There seems to be little research or evidence if this actually works, but I think it is worth a try!
How can I prevent white rot from entering my garden?
If you’re lucky enough not to have this disease in your garden, then you should do you best to keep it out. White rot is often transferred via contaminated soil on tools, boots or plants. If you buy plants, make sure they are certified as disease free. Alternatively it is best practice to grow everything from seed if you can. This includes growing onions from seed instead of sets too. This is also a good reason to buy garlic bulbs from a retailer specifically for growing rather than just using a cheap bulb from a supermarket.
A final note on multi-sowing
This year I multi-sowed my onions. I have loved using this method because it is more space efficient and saves time when planting out. However, when you have allium white rot in your soil, unfortunately when one onion in a clump gets infected, the whole clump gets infected.
I feel like so far this year I have lost more onions than I did last year to white rot. There are many factors which could be at play here. It’s been a much cooler and wetter year which means white rot is more prominent, the area I planted my onions may be more severely infected, or it could be because I multi-sowed. Next year I have decided I will grow a mixture of single-sown and multi-sown onions so I can compare them and hopefully understand more.
You could try growing caliente mustard green manure to bio fumigate the soil.You have to dig it in quick after cutting it down for best effect.Worth a try on a small scale before planting onions.
oh this is interesting! My only concerns are that I am trying to stick to no dig, and also there is club root present at our allotments so mustard as a green manure isn’t ideal because it is part of the brassica family! Will definitely have to read up on this though! I’ve also read about watering in a garlic infusion where the theory is the scent will wake up the dormant spores and then they will die because they have nothing to feed on! So I might try that method so watch this space!
When I get white rot in a bed, I grow other non-allium crops in that bed for 2 to 3 years.
At least once a month I water the bed with a garlic wash. The sclerotia in the soil think there is a new load of alliums to devour and they start to grow, only to die when can find no food supply.
Over the course of 2 to 3 years, they all die and it is safe to grow alliums safely again.
I have applied this with success on every bed that I have encountered white rot and I am one of the few that can grow onions on our allotment site
I’ve read about this method, and will definitely have to try it sometime! Unfortunately I think the whole plot is ridden with it, not just a bed!
I have onion white rot. I’m trying two approaches. Firstly the puréed onion/garlic watered onto the soil then followed the next spring by a ‘sacrificial’ crop of onions sown as seed and picked as spring onions. I’ve noticed that the white rot doesn’t really become obvious until the onion or garlic bulbs approach maturity, however any allium presence in the soil should be triggering the germination of the sclerotia, so removing the onions at a juvenile stage should still starve the sclerotia that would have germinated when they detected the presence of live onion material.
I hope, by using both methods, live in the spring, purée in the summer, to seriously reduce the number of sclerotia in the soil.
Yes I’ve read about the first approach! Would love to know how that works for you. I’ve decided not to grow onions this year (except for shallots and spring onions) because between the white rot and allium leaf miner, the space and efforts for the yield I get just doesn’t feel worth it! Maybe I’ll try again another year!