How to treat powdery mildew on plants

 A white, powdery covering on leaves, stems, flowers or fruit gives powdery mildew disease its common name. All plants can be affected, and it is particularly prevalent in late summer.

Sometimes, though, the first symptoms you notice may be yellow or purple discolouration, falling leaves or general poor growth and distortion. 

The good news is that powdery mildew on one type of plant won’t generally spread to other unrelated plants. 

Breathe new life into your outside spaces. Sign up for our Gardening newsletter, it's free monthly

How do I spot powdery mildew?

The first, barely visible, signs are yellow spots or tiny blisters on the leaves or flowers. Beneath these, the infection develops and white patches start appearing on leaves, stems, flowers and fruit. This fine, powdery covering is actually the fungal mycelium throwing out countless spores. Later, whole leaves are engulfed and much foliage is lost. Leaves can be distorted or discoloured, and fruit may crack and split.

How can I prevent powdery mildew?

Lush growth, dry roots, cool temperatures, damp air, poor air circulation and too much shade all encourage powdery mildew. 

• Avoid giving susceptible plants too much nitrogen-rich fertiliser (either organic or inorganic) which encourages lush growth. 
• Ensure plants, especially those in containers or growing close to walls, fences or trees, have adequate water. 
• Shade the roots of climbers such as honeysuckle and clematis so their roots stay moist. 
• Covering the soil with a 2-3cm-deep layer of soil improver in autumn will help. 
• Avoid overcrowding and reduce competition by keeping plants further apart, or thinning out crowded growth. Under glass, make sure there is a good flow of air at all times.
• Prune shrubs, such as roses, so that the plant has an open centre and there's good air circulation within the plant.

Discover our recommended soil improvers

Get bloomin' great results

Our product tests & advice help you get the garden you want. Subscribe for only £4.99 a month or £49 a year.

Join Which? Gardening

Cancel anytime

How do I treat plants affected by powdery mildew?

First remove the worst-affected parts. If herbaceous perennial plants develop mildew after flowering, shear off all the leaves and water well. A new flush of healthy leaves should develop. 

With woody plants, remove the worst affected leaves and shoots. Do this slowly and carefully, putting the diseased material straight into a container, to avoid shaking spores on to healthy shoots. 

With annuals you may have to destroy whole plants to prevent the disease spreading.

Secondly, improve the plant's growing conditions to reduce the factors that encourage mildew (see above).

Try the best garden hoses and hose guns from our expert tests

What should I do with plant debris infected by powdery mildew?

The resting spores of some types of powdery mildew overwinter in plant debris, so clear infected plants away. With strawberries, for example, shear or burn off the foliage in late summer. 

Other spores overwinter in infected buds of perennial plants. Cutting out sick-looking material at the end of the summer and in autumn may help reduce the level of infection the following year. 

After getting rid of infected material in the greenhouse, clean up thoroughly using a household disinfectant.

Which types of powdery mildew am I likely to see in my garden?

Here are some of the commonest and most damaging species, with tips on identification and individual treatment.

Apple powdery mildew (Podosphaera leucotricha)

Young shoots turn powdery white, older leaves show distortion and white patches. Infected blossom can cause reduced crops. Some varieties offer disease resistance, such as eating apple: 'Discovery' and 'Egremont Russet' and cooking apple: 'Bramley's Seedling' and 'Reverend W Wilks'.

Learn more about how to grow fruit

Aster powdery mildew (Erysiphe cichoracearum)

Upper leaf surfaces turn powdery white, followed by yellowing and foliage loss. It is difficult to treat; keep plants well-watered and choose resistant species such as Aster x frikartii.

Begonia powdery mildew (Microsphaera begoniae)

Ruins indoor plants, with spots appearing on infected leaves that turn brown and dry or yellow and soggy. Control can be difficult. Remove affected leaves, keep roots moist and improve air circulation.

Brassica powdery mildew (Erysiphe cruciferarum)

Causes a white covering on leaves and stems, especially of swedes, turnips and Brussels sprouts where the buttons are spoilt. Water in dry spells and clear up crop debris. Try resistant varieties, such as Brussels sprout 'Nautic'.

Cucumber powdery mildew (Erysiphe cichoracearum and Spaerotheca fuliginea)

Starts off as round white spots on the upper surface of the foliage, enlarging to cover the whole leaf, which withers and dies. It can also attack courgettes, squashes and pumpkins outdoors. Improve ventilation under glass, and clear up crop debris. Plant more resistant varieties, such as cucumber 'Carmen' and courgette 'Defender'.

Learn more about how to grow cucumbers

Gooseberry powdery mildew (Sphaerotheca mors-uvae)

Turns shoots white and fruit brown and felty. Try resistant variety 'Hinnonmaki Yellow' or 'Hinnonmaki Red'.

Grape powdery mildew (Uncinula necator previously known as Oidium)

Starts on the underside of leaves, then covers foliage and attacks bunches of grapes both indoors and outside. For prevention, thin out growth in winter and ventilate grapes grown under glass. Try varieties with resistance, such as white grapes 'Phoenix' and 'Solaris' or red grape 'Regent'.

Pea powdery mildew (Erysiphe pisi)

Affects late-sown peas, covering foliage and pods with moulds. Usually only occurs at the end of the season so has little effect on cropping. Keep plants well-watered. Try resistant varieties, such as 'Eddy' and 'Sweet Horizon'.

Rhododendron powdery mildew (Erysiphe species)

Yellow blotches on upper surfaces with matching light-brown, felty ones below. Keep plants well-watered and mulched. Pick off severely affected leaves and avoid the most susceptible varieties. 

Rose powdery mildew (Sphaerotheca pannosa)

The symptoms appear mainly on the younger shoots. The leaves become distorted and puckered, and buds fail to open. A greyish, powdery deposit appears on the leaves, stems or buds, and can turn the whole shoot a dirty white. 

Mildew can weaken roses, and ruin their appearance, but is unlikely to kill them. Water well in dry spells but avoid wetting the leaves. You could also spray with a suitable fungicide. Try resistant varieties, such as climber 'Alibaba'.

Learn more about how to grow roses

Strawberry powdery mildew (Sphaerotheca macularis)

Purple areas form on upper leaf surfaces with the tell-tale white covering beneath the leaves. Flowers and fruit can also be affected. 

Choose resistant varieties such as 'Cambridge Late Pine' and 'Red Gauntlet'. Avoid 'Aromel' and 'Elsanta' which are very susceptible. 

Cut off and burn diseased foliage at the end of the season. It can also affect raspberries, blackberries and loganberries.

Learn more about how to grow strawberries

Tomato powdery mildew (Leveillula taurica)

Occurs mainly on greenhouse crops. It starts as pale, light-grey patches on the upper leaf surface. These are followed by the powdery stage with a yellow surrounding area.

Learn more about how to grow tomatoes