What’s the white, cotton wool-like substance I’ve found on my leeks?
Stefan says: My first thought whenever I read of ‘cotton wool’ on the roots of any members of the onion family is white rot disease. It’s most serious on spring onions but almost all other kinds of allium can also be affected, and I fear that may be the case here. Unfortunately, you didn’t send in a photograph, but I suggest you look closely at the plant and see if you can spot very tiny black, seed-like bodies within the fluffy mould, because that would clinch the diagnosis.
If only one plant is affected, dig out a large clump of the surrounding soil and dispose of it, although not in your compost bin. White rot disease cannot be eradicated so eliminating any early outbreak is vital.
You wonder if pelleted chicken manure introduced it but that’s highly improbable.