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How to recognise it
Small yellow spots will start to appear, followed by a white or off-white, powdery coating in patches on leaves (usually on the upper surfaces), then spreading to eventually cover the entire leaf. The foliage will become dull and slightly yellowed and will wilt in afternoon heat. The leaves may become tattered and frayed as the disease progresses.
Why it’s a problem
As the mildew spreads it will kill individual leaves and, in severe cases, the entire plant. Fruit production will be poor due to the reduced leaf surfaces available for photosynthesis.
Where you are likely to find it
Affects cucurbit plants including courgettes, marrows, pumpkins, squash, gourds, cucumbers and melons (although it is less common on watermelons).
It’s most prevalent in summer. In very wet seasons it is less likely to be a problem as it prefers dry and warm conditions.
How to prevent it
Ensure good ventilation in glasshouses and spread plants out (both indoors and outside) to maintain the air flow between them (high humidity increases the growth rate of the fungus). Ensure that plants are well watered and mulched (though don’t allow the mulch to touch the stems) to prevent the dry conditions which this disease prefers. Water plants from ground level, as using overhead watering can splash the fungal spores between leaves and plants.
Thoroughly clean your hands/gloves and equipment you’ve used when dealing with infected plants so you don’t spread the disease to other plants.
Clear up all the plant debris in the autumn to get rid of any overwintering spores.
Some varieties, particularly of courgette, claim to have resistance to powdery mildew.
How to get rid of it
Fungicides, including sulphur (although this can damage some plants), can be effective against powdery mildew and should be applied at least fortnightly in dry, summer periods.
You can remove the infected plant parts (particularly leaves) however you must be careful when doing this, so you don’t shower neighbouring leaves with fungal spores. An easy way around this is to carefully place a bag over the leaf to be removed before pruning it, so any spores fall into the bag. Any diseased cuttings should be destroyed (eg burned) and not put on the garden compost heap.
The fungus overwinters in plant debris so good hygiene practices (removing and destroying plant debris in the autumn) should prevent repeated outbreaks.
Is it good for anything?!
No.
Other useful information
The most common fungi causing this disease are Erysiphe cichoracearum and Podosphaera xanthii (which is also called Sphaerotheca fuliginea).
Powdery mildew in various forms can be found on a large number of plants. It is distinguishable from downy mildew as it is found on the upper surface of leaves (downy mildew is usually on the lower surfaces) and being in hot, dry conditions (downy mildew prefers a humid environment).
