Why Is My Leek Collar Rotting at Soil Level?

Leek plants that are yellowing, wilting, and collapsing, with visible softening or rot at the base of the shank — at or just below soil level — have a collar rot problem. The collar (the junction between the buried shank and the above-ground leaves) is a vulnerable point where soil moisture, fungal spores, and bacteria can concentrate. Several pathogens produce collar rot symptoms; identifying which is present guides your response and informs rotation planning.

Fusarium basal rot

Fusarium culmorum and related species cause a dry brown rot that progresses from the root plate upward through the basal tissue. The rotted tissue is typically dry, brown, and fibrous rather than wet and slimy. Roots are brown and shrivelled. Fusarium is most active in warmer soil (above 15°C) and builds up in soil where alliums are grown repeatedly. Remove and dispose of affected plants; rotate alliums to a fresh bed for at least three years.

White rot

White rot (Sclerotinia cepivorum) produces a collar and base rot accompanied by the characteristic dense white fluffy mould with tiny black sclerotia. See the dedicated white rot page — this is the most serious possible diagnosis because of the 20-year soil persistence of the sclerotia. If you see the white fluffy mould with embedded black dots, treat the bed as permanently affected for alliums and begin long-term rotation planning.

Bacterial soft rot in wet conditions

Bacterial soft rot (Pectobacterium) at the collar is a wet, slimy, foul-smelling collapse with no fungal mycelium present. It is associated with waterlogged conditions and physical damage. Improve drainage; avoid working around leeks when soil is very wet (which creates splash and physical damage); remove affected plants and reduce overhead watering. Bacterial soft rot does not build up in soil like fungal diseases — improving conditions eliminates it effectively.

Preventing collar rot

The most effective prevention is good drainage. Collar rot pathogens thrive in wet soil with poor air movement around the base of plants. Raised beds, well-structured soil with generous organic matter, and adequate spacing all reduce collar rot incidence. Avoid planting leeks in the same bed year after year. Remove all plant debris at the end of the season to reduce pathogen carry-over.