Onion White Rot Fungal Disease, its Recognition and Control

The resting bodies (sclerotia) of onion white rot can survive in the soil without a suitable host for fifteen years. This serious fungal disease can affect any member of the Allium family including onions, spring onions, chives, shallots, garlic and leeks. In order to live with onion white rot in the soil the gardener must first of all be able to recognize it before taking measures to reduce the devastation it can cause. Overwintered onions and garlic are particularly susceptible to onion white rot.

How to Recognize Onion White Rot

• Evidence of this dastardly disease shows up when the leaves turn yellow and die back.
• Plants will sometimes keel over as the roots rot.
• A few plants may be affected at first, but this disease often spreads to infect whole rows of plants.
• Upon lifting affected plants, white fluffy fungal growth, a bit like cotton wool, can be seen around the bulb with tiny black globules, like poppy seeds among the fungus.
• These black globules are the resting bodies or sclerotia of the white rot fungus.

Measures to Reduce the Impact of Onion White Rot

The sclerotia fall into the soil where they wait for their next victim. When the temperatures reach 10-18 degrees centigrade during April the Allium roots stimulate the previously comatose fruiting bodies to germinate and infect new plants.

• The gardener who must grow onions in infected ground will get better results by growing onions from seed rather than sets, because sets have well developed roots when temperatures trigger disease activity.
• Onions grown from seed have smaller roots and are therefore less likely to encourage attack.
• White rot spreads sideways through the soil and intertwined roots encourage rapid travel along the rows of plants. Space plants widely to slow the speed of onion white rot spread.
• Removing the infected soil by taking out holes 10cm in diameter and replacing with uncontaminated soil before planting garlic may help.
• Clean tools and boots well after cultivating contaminated ground.
• In most cases a worthwhile crop of leeks can be grown on land which is badly infected with white rot.
• Begin harvesting onions as soon as the bulbs are fully formed rather than wait for them to ripen.
• Use any onions which shows signs of the disease immediately, whilst they are still edible, and only store those bulbs which are completely free of the disease.

Elimination and Prevention of Onion White Rot

There is no control for this disease other than to avoid growing members of the Allium family on infected land for at least eight years. Unfortunately like clubroot it can persist for much longer. It therefore makes sense to try to avoid letting the soil become infected in the first place by practising strict crop rotation and following a strict policy of garden hygiene including cleaning tools and boots after working on contaminated soil and never importing infected soil into uncontaminated ground.