Q What is powdery mildew?
A A group of related species of fungus causes powdery mildew on different plants. All plants can be affected, and it is particularly prevalent in late summer. A white, powdery covering to leaves, stems, flowers or fruit gives the disease its common name. Sometimes, though, the first symptoms you notice may be yellow or purple discolouration, falling leaves or general poor growth and distortion. Powdery mildew on one type of plant won’t generally spread to other unrelated plants.
Caption: Powdery mildew is common in late summer/autumn
Q How do I spot powdery mildew?
A 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.
Q Which types of powdery mildew am I likely to see in my garden?
A 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.
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.
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.
Caption: Powdery mildew tends to affect courgette plants in late summer and early autumn
Gooseberry powdery mildew (Sphaerotheca mors-uvae)
Turns shoots white and fruit brown and felty.
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.
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.
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) White, powdery deposits on young stems, buds and leaves, followed by stunting and dieback with foliage loss.
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.
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.
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.
Q How does powdery mildew spread?
A Mildew can be introduced with new plants; low levels of infection may show no symptoms but develop when conditions are favourable. However, the spores are also spread far and wide by the wind.
Q How can I prevent powdery mildew?
A 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. Incorporating organic matter in autumn and using mulches in spring 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.
Caption: Mulching helps keep the soil moist and prevent powdery mildew
Q How do I treat plants affected by powdery mildew?
A First remove the worst-affected parts. If herbaceous plants such as pulmonaria 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).
Thirdly, spray with a suitable fungicide. Scotts Fungus Clear can be used on ornamentals; Bayer Garden Systhane Fungus Fighter or Westland Plant Rescue Fungus Control on ornamentals and some fruit. On vegetables and fruit, use Vitax Organic 2in1 Concentrate or Vitax Organic 2 in 1 RTU.
Q What should I do with plant debris infected by powdery mildew?
A 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 disinfectant.