White Rot – Onions and Garlic (& Other Alliums)

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.

Even for conventional gardeners who use chemicals, the effectiveness is not reliable.

How Long Does White Rot Last in the Soil

The answer to this depends on the source you’re reading.

• many years
• decades
• 15 years
• 40 years

From these answers we can assume for sure it’s a long time. At least longer than we’d want to wait to grow onions, garlic, and other alliums we might favor.

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.

Conditions the Fungus Favors

The development of the white rot fungus is favored by cool and moist soil conditions.

Although optimum temperatures for infection are 60° to 65°F it can develop at lower temperatures. Warmer soil temperature (usually above 78°F) inhibit the fungus.

How It Spreads

The fungus produces seed-like structures (called sclerotia) that remain dormant until alliums are grown again.

When compounds that are unique to alliums are released in the soil the sclerotia germinate. Fungal growth is then produced and spreads through the soil and invades the roots of the alliums.

New seed like structures form on rotting bulbs further contaminating the soil.

They’re spread by plant material,  tools and equipment, water movement, soil movement (tilling is soil movement), and even your shoes or boots.

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.

Above Ground Symptoms

• 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.

Roots of Alliums Infected by White Rot

A healthy allium has a large and fairly long root system.  A healthy onion or garlic pulled from my garden has a multitude of roots that are 2 1/2 to 4 inches long.

If the roots have been attacked by the white rot fungus they’ll be fewer in number and shorter.

If you’ve missed the first symptoms and pull a plant that has been infected for a while there may be no roots at all.  A fluffy white growth appears at some point on the base of the bulb. Later the growth covers the bulb (especially in garlic) with a black crusty substance which is the fungus growth.

As unpleasant as that is, you may be glad to know that after removing the garlic plants from your garden you can take off all that fungus (bag it for trash or burning). You’ll find pure white garlic cloves that you can wash off and use and/or freeze for future use.

Onions don’t fare as well unless you catch them almost immediately. If they’re still firm they’re usually ok.

They won’t cure and store, but for immediate use they’re fine.

Best Practices to Follow for White Rot – Onions and Garlic

• It’s best to check your alliums everyday or at least every other day for symptoms.
• Remove all infected plants from the garden. (Either burn or dispose of off-property.)
• Never compost any infected plants.  Compost piles may not get hot enough to kill the fungus.
• A healthy soil, high in organic matter, will help minimize any disease problem.  Continue to improve your soil.

A  Suggestion

As long time readers already know, I don’t water my garden.

From what readers tell me, most water their gardens.

If you have white rot in your soil, I would strongly suggest you refrain from watering your beds containing alliums. It’ll help minimize infection.

Possible Solutions I’ve Read that I’d Consider Doing

#1 – Some organic growers decontaminate their garlic seed stock with peroxide.

Please do your own research on this, but this seems  something you do as you plant.  In other words: Prepare the solution (probably 1 part peroxide to 9 parts water), dip the garlic clove, and plant while it’s wet.

#2 – As mentioned, sclerotia germinate when they sense compounds released by alliums in the soil.

The possibility of tricking the sclerotia to germinate by saturating onion or garlic juice into the soil has been thought of.  There is no commercial product available but if a home gardener could get enough garlic or onion juice to saturate a garden bed, it would be worth a try.

The sclerotia would die after it germinated since there would be no plants to infect.  When alliums were planted the following year in that bed – the results should be plants free from white rot.

Final Thoughts

Working with nature and attention to what’s going on in your garden are your best ways to cope with almost any garden problem.

It’s how I minimize white rot that has the potential to destroy an onion or garlic crop.

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Suggested Reading:

Defense Against Garden Pest and Disease

Organic Pest Control Disease Management/Working on the Cause Rather Than the Symptom

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