Skip to content
growing.guide
Menu

Downy mildew

Downy mildews are water moulds rather than true fungi. They thrive in cool, wet weather, turning leaves yellow in patches bounded by the veins, with a grey or purple down underneath. Basil, lettuce, brassicas, onions and grapes each have their own.

Fungal diseases
Basil leaves yellowing and blotched with downy mildew
Basil leaves yellowing and blotched with downy mildew

What it looks like

Turn the leaf over.

  • Yellow areas bounded by the leaf veins.
    Angular yellow patches Yellow areas bounded by the leaf veins.
  • Fuzzy grey or purplish growth on the underside.
    Grey down below Fuzzy grey or purplish growth on the underside.
  • Patches brown and leaves collapse.
    Browning leaves Patches brown and leaves collapse.
  • Seedlings and young plants stall.
    Stunted plants Seedlings and young plants stall.

How to confirm it

Look underneath in the morning.

  1. Fuzz underneath

    The down is on the underside, opposite yellow patches.

  2. Damp weather

    Outbreaks follow cool, wet spells.

  3. Not powdery

    Unlike powdery mildew, the top of the leaf is not coated.

What to do

Remove and dry out.

  1. Remove infected plants

    Pull and bin badly affected basil and lettuce; do not compost.

  2. Water in the morning, at the base

    Keep leaves dry overnight.

  3. Improve airflow

    Space plants and ventilate greenhouses.

  4. Grow resistant varieties

    Resistant basil and lettuce varieties are available.

Prevent it next time

  • Rotate crops.
  • Avoid overhead watering.
  • Buy clean seed and plants.

What not to do

Do not keep harvesting from infected basil; the disease spreads to the whole crop.

In the growing guides

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

Sources

  1. Downy mildews, Royal Horticultural Society
  2. Downy mildew of basil, University of Maryland Extension
  3. Brassica downy mildew, Royal Horticultural Society