How can you stop white rot? Anne offers sage advice
QA growing number of gardeners I know are having trouble with white rot that has stopped them from growing onions and garlic. Are there any steps I can take to avoid this disease, and does watering onions during dry spells increase the risk? Sheila Macreith, Salisbury, Wiltshire
AOnion white rot is indeed a horrible and depressing disease of the onion tribe. Once the sclerotia or resting spores are in the ground, they can persist for 15 years or more, even if no onions are grown!
These dormant spores resemble tiny black poppy seeds and slumber on until plants from the onion tribe (Alliaceae) are detected. Alliums give off volatile chemicals and the spores respond by germinating to infect the roots. Whether from seeds or sets, onion foliage turns yellow, the plant comes loose and, on inspection, roots have rotted and white fluffy fungal growth shows on the bulb base. Black resting spores are produced and fall into soil.
Since our garden became infected with white rot some 10 years ago, I’ve given up on onions, garlic and shallots, but we still enjoy good crops of leeks and chives. Ornamental alliums don’t seem affected, either. Maybe their chemical signal is weaker and fails to wake up the resting spores.
Once in the soil, white rot is nigh on impossible to get rid of, so avoiding it in the first place is paramount. Number one is to avoid putting parings from shopbought onions into your compost heap, as I have spotted white rot on the base of these. Be cautious about tools and the boots and shoes of visiting gardeners as they may carry soil from infected sites. The feet of birds and animals, sadly, we can do nothing about.
White rot won’t germinate when temperatures are above 68°F (20°C), so a pattern of cooler, wetter summers in recent years will have helped it to spread. I think it’s unlikely that watering onions encourages white rot, as it has to be present to start with. However, watering during high temperatures and holding back during cool spells might be sensible, especially on heavier soils.
Having wilted, loosened and toppled in the ground, an infected onion shows signs of white fungal growth at the base As we have white rot in our soil, I’m sowing spring onions into a crate of clean compost. Sow thinly and evenly, and they will not require thinning and will crop next spring