Powdery mildew control: symptoms, treatments and resistant varieties

Val Bourne / 04 June 2020

During dry weather you might find powdery mildew - a white, dusty powder, on the foliage of plants like cucumbers and tomatoes. Find out what causes powdery mildew and how to avoid it.



How to recognise powdery mildew

You've probably noticed that some of your plants develop white, dusty looking foliage, especially during dry weather. This is a fungal disease called powdery mildew, but there are several different types and they each attack a very narrow range of plants. If you get mildew in your peas, for instance, it will only attack a narrow range of leguminous (pea-like) plants. The mildew on your apple tree is likely to only affect other apple trees. In other words, don’t panic!

How to prevent powdery mildew

We don’t tend to resort to pesticides in the same way as we used to, because we know they are harmful to us and the environment, so it's worth improving your husbandry rather than using toxic substance and a sprayer.

Try to plant susceptible vegetables, fruit and perennials in airy, light positions. This will improve airflow and cut down on humidity so the fungal growth will struggle.

Keep the plants irrigated early in the year. Don't just dribble the hose over them, try to water them thoroughly and mid-morning is best. Always to wet the foliage because the spores don't like being in contact with water.

Try to avoid early morning and evening watering because this is likely to create a damp atmosphere and encourage the fungus. Mid-morning is apparently best because the water has plenty of time to evaporate. Find out what you need to know for smarter watering techniques in your garden.

If there’s a wet spring it's worth mulching perennial plants and roses with garden compost, or you could use green waste. This has to be done in warm, wet weather and the idea is to trap moisture at the root.

Avoid nitrogen-rich feeds

If mildew is a recurring problem in your garden you must also avoid nitrogen-rich feeds. These produce soft leafy growth that’s more attractive to fungal spores and aphids. Use high potash feed such as Vitax Q4 because this makes the foliage more leathery. It also toughens the stems.

Find out more about the best time to use different plant feeds.

How to control powdery mildew

When autumn arrives make a special effort to collect up all the foliage and then mulch again to break the cycle of spores being washed up from the soil by the rain.

Keep as much airflow as possible through affected plants.

Powdery mildew on roses

To prevent powdery mildew on roses prune them carefully. When you prune roses concentrate on four of five good stems and aim for a cup shape, so that the air can circulate. Take out any branches in the middle, if you can. You can also thin out the stems on perennials to allow air to pass through the clump.

Choosing mildew resistant plants

You will often find that only certain plants are affected. In the flower garden it tends to be border phloxes, North American asters and prairie plants, acanthus and delphiniums.

Mildew resistant perennials

When it comes to asters it’s far better to grow drought-tolerant European asters rather than thirsty North American asters. Recent name changes have made this easier because European asters have kept the Latin name of Aster, but most North American asters are now called Symphyotrichum.

Aster x frikartii 'Mönch'
The longest-flowering aster of all, this is glorious from July until September. Large lavender-rayed daisies sit above green foliage on slightly floppy stems, this is one for the front of the border. The perfect partner is brown-centred yellow daisy called Deam’s cone flower - Rudbeckia fulgida var. deamii. Both 3ft/ 90cm

Aster amellus 'Veilchenkönigin'
This deep-violet form of the Italian aster has lots of petals look like eyelashes. It needs a front of border position, because it's fairly short, but it flowers for many weeks even in dry weather without suffering from mildew. 12in/ 30cm

Symphyotrichum novi-angliae ‘Helen Picton’
New England asters are the most drought tolerant of the North American asters and this one is a deep purple. They all flower in September and produce stiff, tall stems, which can get a little ragged, so it's best to hide them in the middle of the border. They persist for many years without being divided and they are the best asters attracting butterflies. It is a strong purple, but there are many good pink forms on offer. They also cut well.

Helenium autumnale ‘Sahin’s Early Flowerer’
Heleniums often suffer badly or refuses to perform well in drier soil. However there is one earlier flowering drought tolerant hybrid that will perform and that doesn't normally get affected by mildew. It's ‘Sahin’s Early Flowerer’ and it’s a dazzling plant, with vivid orange flowers that are all slightly different in form.

Monarda ‘Scorpion’
Monardas or bee balms can be equally troublesome, but nurseryman Piet Oudolf selected a purple form called ‘Scorpion’ for its resistance. This metre-high monarda is a strong grower.

Phlox paniculate ‘Monica Lynden-Bell’ and ‘Alba Grandiflora’
Border phloxes are notorious of going down with mildew and if you have a dry garden it's probably best to avoid most of them. There are two exceptions. The pale-pink ‘Monica Lyden-Bell’ was discovered in a Hampshire garden on chalky soil and it seems to do well even in dry summers. The airy, white ‘Alba Grandiflora’ will also grow in dry gardens.

Mildew resistant fruit and vegetables

When choosing fruit trees it's worth using a specialist nursery and acquiring East Anglian varieties because they are likely to do well in drier summers because that's what they’re used to. Keeper’s Nursery will offer advice.

The following vegetable varieties are less likely to suffer from mildew.

Pea ‘Hurst Greenshaft’ very available
Spinach F1 ‘Amazon’ (T&M)
Courgette F1 ‘Tarmino’ (Suttons)
Broccoli F1 ‘Monclano’ (Kings Seeds)
Cucumber F1 ‘Bella’ (T&M)

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