Powdery mildews

Powdery mildews are common plant diseases. They all produce a white or greyish, dusty surface coating on leaves, stems, flowers and sometimes fruit.

Each species of powdery mildew usually has a narrow range of plants it will attack, which is made up of one species or a few closely-related plants. So, rose powdery mildew won’t attack clematis, cucurbit powdery mildew won’t attack rhododendrons, for example. This means each one doesn’t spread as far as many people think.

Commonly affected plants include the following.

Ornamental plants

Clematis
Crab apple
Delphinium
Honeysuckle
Lilac
Monarda
Phlox
Rhododendron and azalea
Rose

Edible plants

Apple
Blackcurrant
Courgette, cucumber and marrow
Gooseberry
Grape
Pea

• Trees – including fruit trees
• Shrubs
• Herbaceous perennials
• Annuals
• Vegetables
• Houseplants

Powdery mildews are caused by several different, closely-related fungi. Each one attacks a different species, or closely-related species, of plants. They are spread from leaf to leaf and from plant to plant by airborne spores.

Powdery mildews are always worse on plants that are under stress, especially water stress, and during warm, dry weather.

They start to become active in spring and the spores are easily carried by the wind, spreading from plant to plant over very large areas.

They either overwinter on fallen leaves, releasing spores to infect plants the following spring, or as dormant infections on the plant.

Try to keep plants growing as strongly as possible to help them fend off disease attacks. However, when large outbreaks of powdery mildews hit your garden, it may not be possible to do very much about it.

Carefully pick off and dispose of affected leaves when the disease first strikes to reduce or slow down its spread. This is highly unlikely to actually eradicate the disease. Try not to disturb the spores, or you will be spreading the disease further.

Prune out badly infected growth and dispose of it by either burning it or put it in the council’s green waste bin. Don’t compost it.

Similarly, carefully pick up or rake up affected fallen leaves – especially in autumn – to try and reduce carry-over through the winter.

Old clumps of herbaceous perennials are always affected more than new plants. Lift and divide old clumps in autumn or spring and replant in well-prepared soil with lots of added bulky organic matter.

You can help protect new growth and prevent the disease – and to a slightly lesser extent, control existing diseases – by spraying ornamental plants with a good garden fungicide.

Unfortunately, there are currently no fungicides generally approved for home gardeners to use to treat diseases on edible crops. Some are approved for use against powdery mildew on courgette, cucumber and summer squash when grown in the greenhouse.

“Plant Invigorator” products, can also give excellent results. These are not pesticides per se, but plant growth stimulants that also control pests and diseases by physical methods. They can be used on edible crops.

Sulphur was one of the active ingredients in suitable fungicides, but this lost its approval some years ago. Sulphur is also a key nutrient needed for healthy plant growth and, if there is a sulphur deficiency, plants may become more susceptible to diseases.

Use plant protection products safely. Always read the label and product information before use. Plants in flower should not be sprayed due to possible danger to pollinating insects. Either spray early in the morning or late in the evening when pollinating insects are less likely to be active.

Since powdery mildews are worse on plants that are under stress, especially those that are short of water at the roots, make sure the soil or compost is kept evenly moist – especially during periods of warm, dry weather. Adding a water-retention gel will help even out periods of soil drying.

Adding a 5-10cm (2-4in) thick mulch of organic matter to the soil around the plant will help conserve soil moisture and help prevent it drying out.

Some varieties are listed as being powdery mildew resistant. Bear in mind that over time such resistance can break down.

• Fungicides
• Water-retention granules
• Mulches