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Bacterial soft rot and blackleg

Soft-rot bacteria enter through wounds and thrive in warm, wet conditions. On potatoes, blackleg blackens the stem base and rots tubers; on brassicas, lettuce and ginger, heads and rhizomes collapse into a smelly rot.

Bacterial diseases
A potato stem blackened and slimy at the base
A potato stem blackened and slimy at the base

What it looks like

Soft, wet and smelly.

  • Potato stems turn black and slimy at soil level.
    Black stem base Potato stems turn black and slimy at soil level.
  • Leaves yellow and wilt above the rot.
    Wilting plants Leaves yellow and wilt above the rot.
  • Tubers and rhizomes rot into a wet mass.
    Mushy tubers Tubers and rhizomes rot into a wet mass.
  • A strong rotten smell.
    Foul smell A strong rotten smell.

How to confirm it

Smell and touch.

  1. Slimy and smelly

    Soft rot is wet and foul, unlike dry fungal rots.

  2. From a wound

    Often starts at damage or a cut.

  3. Warm, wet conditions

    Worse in wet spells and in store.

What to do

Remove and dry.

  1. Remove affected plants

    Lift and bin plants with blackleg or rot.

  2. Improve drainage

    Plant in well-drained soil and avoid overwatering.

  3. Handle gently

    Avoid wounding tubers and heads.

  4. Store dry and cool

    Keep only sound, dry produce in store.

Prevent it next time

  • Use certified seed potatoes.
  • Do not plant in cold, wet soil.
  • Rotate crops.
  • Dry harvested produce before storing.

What not to do

Do not store damaged or bruised tubers with sound ones.

In the growing guides

Where the guides mention it, with plant-specific advice.

Sources

  1. Bacterial soft rot and blackleg of potatoes, UC Statewide IPM Program
  2. Potato facts: blackleg and bacterial soft rot, University of Maine Cooperative Extension