This is a fungal infection
attacks onions, shallots and garlic as well as leeks, and is most
noticeable in hot, dry summers.
Plants grown in overcrowded
conditions are particularly vulnerable. Infected leeks will have yellow
leaves, with a white or grey fungus covering the underground base of the
plant, they look like stands of white thread. The spores can remain
viable in the soil for at least eight years, so proper crop rotation is
the pest precaution against white rot. Leeks grown on fertile,
organically manured soil seem less vulnerable.