Powdery mildew
Powdery mildew fungi grow on the leaf surface, covering it in white or grey powder. Unlike most fungi they thrive when roots are dry and the air is humid. Courgettes, squash, peas and grapes are almost always affected by late summer.
Fungal diseases
What it looks like
It starts as spots and joins into a coating.
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White spots Small round white patches on the upper leaf. -
Powdery coating The patches merge into a floury layer. -
Yellowing leaves Leaves yellow and shrivel under the coating. -
Dusted fruit On grapes and peas, fruit and pods are coated too.
How to confirm it
Rub the white with a finger.
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It wipes off
The powder smears off the surface, unlike spray residue or natural bloom.
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Upper surface first
It starts on top of the leaves, unlike downy mildew.
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Dry roots
Worst on plants that have dried out.
What to do
Slow it down; late in the season, live with it.
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Water at the root
Keep the soil moist, and keep water off the leaves.
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Remove the worst leaves
Pick off heavily coated leaves to improve airflow.
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Thin crowded growth
Space and prune for air movement.
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Spray if early
A sulphur or potassium bicarbonate product slows an early outbreak on edible crops where approved.
Prevent it next time
- Grow resistant varieties of courgettes, cucumbers and peas.
- Mulch and water regularly.
- Avoid late, heavy nitrogen feeding.
What not to do
Do not compost heavily infected leaves in a cool heap; bin or burn them.
In the growing guides
Where the guides mention it, with plant-specific advice.
Sources
- Powdery mildews, Royal Horticultural Society
- Powdery mildew on vegetables, University of Maryland Extension