Once you have the fungus (Sclerotica cepivorum) that causes white rot in your soil it’s not likely you’ll get rid of it. The good news is organic home gardeners especially can deal with it and still get a healthy crop of onions and/or garlic (or other alliums).
Your success will come with incessant attention while your alliums are growing. That way only a small percentage of the crop will be lost to the fungus.
In community gardens with many gardeners, success will depend on how diligent each of those gardeners are in doing what’s necessary to keep white rot to a minimum AND to keep it from spreading to other plots.
With market gardeners who grow large areas in alliums, it’ll be a lot more difficult. From what I understand, there are currently no chemical or cultural controls available.
Allium plants like onions, garlic, leeks, and shallots are staples in many gardens. However, these plants can be susceptible to rot, which can ruin an entire crop. In this article, we’ll explore the common causes of rot in allium plants and proven solutions to prevent it.
What Causes Rot in Allium Plants?
There are a few key culprits that lead to rot in alliums
Improper Watering
Too much moisture causes the bulbs and roots to rot. Overwatering is especially problematic in heavy clay soils that don’t drain well. On the other hand, uneven watering that leads to periods of very dry soil can also stress the plants and make them prone to rotting.
Compacted Soil
Alliums thrive in loose, well-aerated soil Compacted soils prevent proper drainage and air circulation around the bulbs, creating an environment perfect for rot organisms to thrive
Disease Pathogens
Certain fungal diseases like white rot are common sources of rot in allium plants. Once disease pathogens are in the soil, they can persist for many years and infect future crops.
Pest Damage
Onion maggots and other pests that feed on allium roots and bulbs provide entry points for rot diseases. Their damage also compromises the plant’s defenses.
Cold Weather
Chilly, wet conditions keep soil cold and favor the spread of rot diseases. This is especially problematic during dormancy over winter.
Effective Solutions to Prevent Rot
While rot can be discouraging, the good news is there are many effective preventative measures you can take:
Ensure Good Drainage
Plant alliums in well-draining soil, raised beds, or mounds. In heavy clay soil, add compost or other organic matter to improve drainage. Avoid soggy sites prone to waterlogging.
Provide Even Watering
Water at the base of the plants, avoiding wetting the leaves and neck of the bulb. Allow the soil to partially dry out between waterings. Mulch around the plants to maintain even soil moisture.
Rotate Crops
Don’t plant alliums in the same spot each year. Rotate to a different area of the garden to prevent disease pathogens from building up in the soil.
Loosen Soil
Before planting, loosen compacted soil and mix in compost or other organic matter to improve drainage and aeration.
Control Pests
Remove weeds that can harbor pests. Use row covers as a barrier. Apply beneficial nematodes or organic pesticides to control maggots and other pests.
Select Resistant Varieties
When available, choose allium varieties bred to resist common rot diseases like white rot. Ask your local nursery for recommendations.
Use Preventative Fungicides
Apply registered biofungicides like Serenade Garden during the growing season to protect against rot diseases. Always follow label directions.
Allow Proper Curing
After harvest, allow the bulbs to cure properly with good air circulation to harden off the necks and prevent rotting in storage.
Improve Overwinter Conditions
Add extra mulch around overwintering alliums. Consider covering with a cold frame or low tunnel to protect from extreme cold and moisture.
By taking a multifaceted approach and implementing a range of preventative measures, you can help ward off rot issues and enjoy a successful allium crop. Be vigilant about regularly checking your plants, and take prompt action at the first sign of problems. Pay close attention to proper watering, drainage, and crop rotation as your primary defenses. With smart prevention strategies, you can outsmart allium rot in your garden.
Frequently Asked Questions about Preventing Allium Rot
What causes onion rot?
Onion rot is most often caused by overwatering, poor drainage, damage from onion maggots or other pests, or fungal diseases like white rot, botrytis, or downy mildew. Cold, wet conditions also promote rot.
How do I stop garlic from rotting?
To prevent garlic rot, plant in well-draining soil, provide even watering, control pests like onion maggots, allow adequate drying and curing of bulbs after harvest, and employ crop rotation. Avoid overwatering.
Why are the bottom leaves of my leeks yellowing?
Yellowing lower leaves on leeks is often a sign of rotting roots or bulb. This can be caused by soggy soil, diseases like white rot, or pest damage. Improve drainage and avoid overwatering.
What causes shallots to rot?
The main causes of shallot rot are excessive irrigation, poor drainage, compacted soils that lack oxygen, damage from onion maggots allowing infection, and diseases like botrytis and white rot.
Should you remove rotting onion leaves?
Yes, any rotting leaves or sections on onion plants should be promptly removed to prevent the decay from spreading. Dispose of diseased debris offsite, not in compost. Be sure to sterilize pruners between cuts with isopropyl alcohol.
How do I treat white rot in garlic?
There is no treatment for plants already infected with white rot fungus. Infected plants should be pulled up and destroyed. For prevention in future crops, employ long crop rotations of 8 years or more and drench soil with garlic tea or powder to reduce inoculum.
The Bottom Line
Rot diseases present a serious threat to allium crops. The keys to prevention are providing good drainage, proper watering, pest control, long crop rotations, preventative fungicides, and selecting resistant varieties when available. With vigilance and integrated preventative measures, you can have great success growing onions, garlic, leeks, shallots, and other alliums in your garden.
How the White Rot Fungus Gets into Your Soil
White rot is usually introduced into your soil by plant material (contaminated seedlings or seed) or soil contaminated by the fungus. Once in your soil it can spread to healthy plants.
White rot on onions or garlic in my first 33 years of growing was NOT a concern.
Soil is never imported into my garden. And since I only purchase seed or seedlings from reputable growers I never thought I’d have this problem. It showed up about 7 years ago.
As far as I can tell it had to come from onion seedlings purchased from a reputable and well known grower.
When Do Symptoms of White Rot Show Up on Onions and Garlic?
Symptoms only show up above ground after the fungus has attack the root.
- stunted grow and/or
- yellowing and wilting of the foliage
- plant leans – indicating it may be loose in the soil
When you see stunted grow – more than likely the plant has been attacked by the fungus. A gentle tug on the plant will tell you. If it’s loose in the soil – pull it up and bag it.
Either discard to the trash or burn the plant(s) after you remove it (them) from the garden.
Sometimes a plant has a slightly yellowed leaf and it’s not the white rot. This can be confusing. But again, a gentle tug should tell you if white rot is the cause.