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Pest Watch

Powdery mildew

 

Symptoms of powdery mildewPowdery mildew is a fungal disease that attacks the foliage, stems and sometimes the flowers and fruit of a wide range of plants. The symptoms are a superficial whitish-grey fungal growth that covers the surface of the affected part of the plant.

Powdery mildew attacks a wide range of plants. These include ornamental plants: acanthus, clematis, delphinium, lonicera (honeysuckle), phlox, rosa (roses), rhododendron, quercus (oak); and edible plants: apple, blackcurrant, courgette, cucumber, gooseberry, grape, marrow, pea.

There are numerous different powdery mildews, and each one attacksone or just a few related plants; for instance, the powdery mildew affecting roses is a different species from the one that attacks clematis.

Symptoms

White, whitish-grey, powdery spreading patches of fungus on upper or sometimes lower leaf surfaces, stems, flowers &/or fruit.

Affected parts sometimes become stunted or distorted.

Control

To prevent the disease spreading, affected parts of the plant should be carefully removed and destroyed. This is particularly important in the autumn as destroying fallen infected leaves will reduce the amount of spores the following spring.

The problem is always worse on plants that are under water stress, especially when growing in dry soil. Mulching around the plants and regular, correct watering at times of warm, dry weather reduces water stress and helps make plants less prone to infection.

As the disease spores need moisture to germinate and grow, only water the soil not the plants - keep the soil moist but the leaves dry.

As most powdery mildew fungi have a restricted host range, but these can include wild relatives that can be a source of infection, remove closely related plants - especially weed species.

Some varieties of some plants have powdery mildew resistance, so check plant catalogues for details.

Chemical control

There are a number of chemical controls for powdery mildew. Most can only be used on ornamental plants, but there are one or two that can be used on some edible plants; always check the label before use.

Garden fungicides always work better as a prevention/protection rather than a cure, so always spray a fungicide before the disease gets hold.

Scotts FungusClear Ultra (triticonazole) and Westland Plant Rescue Fungus (difenoconazole) can be used on ornamental plants.

Bayer Garden Systhane Fungus Fighter and various other products that contain the active ingredient myclobutanil can be used on ornamentals, apples, blackcurrants, gooseberries and pears.

Organic gardeners can use plant and fish oil blends (Vitax Organic 2 in 1) on all plants, including edible crops.

I've also tried Plant Invigorator SB with good results. This is not a pesticide per se, as it protects/controls by physical methods.

Although it is illegal for me to recommend it, research has shown that a 10% mixture of full-fat milk applied twice a week is an effective control. Which? Gardening magazine had a report on this in its October 2010 issue.

Use plant protection products safely. Always read the label and product information before use.

Suppliers

Buy a range of pest control products from my affiliate companies, Harrod Horticultural and Greenfingers.com

 

If you want to know more, or if you've got a gardening problem you need help with, then send an e-mail to: info@gardenforumhorticulture.co.uk

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