Onion white rot

White rot is a serious and devastating fungal disease of the allium family, particularly important on edible crops – onions, garlic and leeks.

The first visible sign of attack is when the leaves start to turn yellow and then the plant starts to wilt, as the roots rot and are killed. Wilting is more pronounced in dry weather. In wet weather, plants may not wilt.

With no roots to support them, plants often become loose in the soil and even topple over.

When lifted, you will probably see a white, fluffy fungal growth around the base of the plant. Later in the year, this is accompanied by small, black, round structures. These are the sclerotia – resting spores that can remain in the soil for up to 12-20 years.

Onion white rot attacks are more severe in cool, wet summers.

Affected onions will deteriorate and rot further in storage, so should be used immediately.

Onion white rot is sometimes confused with onion neck rot disease, which is caused by Botrytis allii. This disease of onions and garlic starts, as its name suggests, at the top or neck end of the bulb. Although infection starts while the plants are growing and during harvesting, symptoms only appear during storage.

• Onions
• Leeks
• Garlic
• Ornamental alliums

The onion white rot fungus remains dormant in the soil as small, black, round resting structures (called sclerotia). When alliums are grown in the soil near them, these germinate, and the resulting fungal growth invades the roots and develops inside the plant.

Later in the year, as the affected bulbs rot, new sclerotia are formed and remain in the soil until the next allium is close by. They can remain dormant in the soil for up to 12-20 years.

Try to keep plants growing as strongly as possible to help them fend off disease attacks. However, this is unlikely to stop the disease, especially in years of ideal conditions – cool (under 20C/68F) and damp or wet weather.

Because the resting sclerotia can remain dormant in the soil for a long time, it is not feasible to practice crop rotations to starve it.

Affected plants should be dug up and disposed of. Similarly, the soil around them should be removed.

Since the sclerotia only germinate once and in the presence of alliums, it may be possible to “clean” the soil of sclerotia by growing a crop of chives, which are then disposed of. Or even using a solution made from onions watered onto the soil.

Unfortunately, there are currently no chemicals approved for home gardeners to use to treat onion white rot.

As the disease can remain dormant in the soil for up to 12-20 years, it is important not to introduce contaminated soil into the garden or allotment. And clean tools and footwear if they have been in contact with contaminated soil, which can also spread the disease.

• Vegetable seeds