Pests & diseases
Powdery mildew
A fungal disease that coats leaves in a white, dusty bloom, common in late summer on courgettes, squash and peas when roots are dry.
What powdery mildew actually is
Powdery mildew is a very common fungal disease that leaves a white or greyish, dusty coating on the upper surface of leaves — it looks rather as though someone has dusted the plant with flour or talcum powder. It usually starts on the oldest, lowest leaves and spreads upwards and outwards across the plant.
It is caused by a group of fungi, and most are fairly fussy about their host: the powdery mildew on your courgettes is a different species from the one on your apples or your roses, so it rarely jumps from one type of plant to another. In a UK veg patch you'll most often see it on courgettes, marrows, pumpkins and other cucurbits, as well as on peas and a few other crops late in the season.
The dry-roots trigger
The big surprise for many beginners is that powdery mildew thrives in dry conditions, not wet ones. Unlike blight, which needs warm, soggy weather, powdery mildew takes hold when a plant's roots are short of water and the air around the leaves is dry but humid — exactly the conditions of a warm, rainless British August.
A plant under drought stress can't defend itself as well, and the fungus moves in. That's why it's so common on plants in pots, grow bags and free-draining beds that dry out quickly, and on big, thirsty courgette plants whose huge leaves lose a lot of moisture on a hot day.
It rarely kills the plant
The good news is that powdery mildew looks far worse than it usually is. It's unsightly, and badly affected leaves may yellow and die back, but it seldom kills an established plant outright. Most courgettes, squash and peas keep cropping perfectly well even when their older leaves are coated — by the time the mildew is widespread, many crops are near the end of their season anyway.
So unless a young plant is being swamped early in the year, there's no need to panic or to reach for chemical sprays.
How to slow it down
You can't really cure powdery mildew, but you can keep it at bay and slow its spread:
- Water consistently at the roots. Keeping the soil evenly moist is the single most effective thing you can do — a good mulch around the base helps lock that moisture in. Water the soil, not the leaves.
- Improve airflow. Don't crowd plants; give them space so the air can move and humidity around the foliage doesn't build up.
- Remove the worst leaves. Snipping off a few badly affected lower leaves and binning them (not composting) slows the spread and lets light in.
- Choose resistant varieties. Some courgettes, such as 'Defender', and many modern peas are bred to resist mildew — look for them in catalogues from UK suppliers like Suttons or Thompson & Morgan.
For a full walkthrough, see our guide to powdery mildew on courgettes.
In a UK garden
In UK gardens powdery mildew usually shows up from July onwards, especially in a dry spell or on plants in pots, where the soil dries out fastest.
Example
A white, talcum-powder dusting spreading across the older leaves of a courgette plant in August, while the plant itself keeps cropping, is classic powdery mildew.