Handling Two Common Onion Fungal Diseases

For those of us who enjoy growing onions, leeks, and related alliums in our home gardens, there’s nothing more frustrating than having diseases ruin these slow-growing crops before they mature. Two of the most common problems are onion rust and onion white rot.

Onion Rust

The first gets its name from the orange pustules that form on the leaves of the affected plants. Eventually, those leaves may yellow and die, killing the plant. Fortunately, onion rust is easy to avoid. If you remove infected crop debris and make sure both nitrogen and potassium levels in the soil are nominal, the disease is unlikely to recur. Crop rotation every three or four years is also a good idea.

Onion White Rot

Onion white rot isn’t nearly as easy to deal with as onion rust. This ailment, which is characterized by a white cottony growth along the bases and sides of the onions, is caused by a persistent fungus that can survive without a host for as long as fifteen years.

If onion white rot strikes, your best bet is not to grow alliums in that particular piece of ground for 8-10 years. Since that’s a little extreme, you can try growing onions from seeds rather than sets (transplants) instead. That way, root development is minimal during the fungus’s most active period, early in the season. However, onions grown this way can take up to two seasons to mature, so this, too, is a long-term option.

You can also try planting your onions in widely-spaced clumps 12 inches or more apart, since onion white rot has a tendency to spread sideways along intertwined roots. This way, if one clump becomes infected, you can clear it out before others are stricken. If nothing else works and you need your alliums, try growing leeks. They’re least likely to be infected by onion white rot. Otherwise, you may need to switch to container gardening until your white rot problem clears up.

Want to learn more about onion fungal diseases?

Onion Rust from Pacific Northwest Extension
White Rot of Garlic and Onions from University of Maine Cooperative Extension