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Rust

Rust fungi are each specific to a group of plants: leek rust, mint rust, asparagus rust, bean rust. Pustules on the leaves release dusty spores. Most rusts weaken rather than kill, but mint rust can destroy a clump.

Fungal diseases
Allium leaves streaked with orange rust
Allium leaves streaked with orange rust

What it looks like

The spores come off like rust dust.

  • Small raised orange or yellow spots on leaves.
    Bright pustules Small raised orange or yellow spots on leaves.
  • Rubbing leaves leaves orange dust on fingers.
    Dusty spores Rubbing leaves leaves orange dust on fingers.
  • Late in the season, pustules turn brown or black.
    Dark later spores Late in the season, pustules turn brown or black.
  • Mint shoots twist and thicken in spring.
    Distorted shoots Mint shoots twist and thicken in spring.

How to confirm it

Rub a spot.

  1. Powdery dust

    Rust-coloured powder on your finger.

  2. Raised pustules

    They stand up from the leaf surface.

  3. Host-specific

    Only one kind of plant affected in the bed.

What to do

Reduce and move on.

  1. Remove affected leaves

    Pick off and bin infected leaves.

  2. Cut back mint

    Cut mint to the ground and bin the stems, or replant clean runners.

  3. Space and feed lightly

    Improve airflow and avoid excess nitrogen.

  4. Harvest early

    Lift garlic early; bulbs are usually unaffected.

Prevent it next time

  • Rotate alliums.
  • Grow resistant varieties.
  • Clear debris in autumn.

What not to do

Do not compost heavily infected material.

In the growing guides

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

Sources

  1. Rust diseases, Royal Horticultural Society
  2. Leek rust, Royal Horticultural Society
  3. Mint rust, Royal Horticultural Society