Apple Powdery Mildew (Podosphaera leucotricha) is very common and widespread and one of the most serious apple disease. If your tree develops symptoms then try this:
1) Cut out any damaged material straight into a paper bag (i.e. do not let it fall onto the rest of the plant or the ground thus spreading spores). Put the refuse in the council waste do not attempt to compost or burn it.
2) Powdery Mildew is often a response to water stress, so once the damaged material is removed, make sure the tree is watered and mulched down to prevent it drying out.
3) Keep checking the tree and cutting out any recurrent symptoms.
4) If you are not adverse to using chemicals read the advice below, and apply as you see fit.
Most people recognise Stefan Buczacki as a leading plant disease expert.
He says (Collins Guide to the Pests Diseases & Disorders of Garden Plants 1981:
Apple Powdery Mildew Podosphaera leucotricha – Very common and widespread and one of the most serious apple diseases…
Symptoms (much the same as detailed below)
The first infections in spring arise from fungus that has lain dormant over winter following infection in the previous season. Conidia are produced on these primary infections and are then carried on the wind to secondary sites increasingly far from the original infected shoot. Such infections take place through the summer except when there is water actually lying on the leaf surfaces. Some buds may be killed by the disease but most survive to become the primary sources of the next year. Despite the very general development of mildew on shoots during the summer the disease dies down in the winter and the fungus in the buds is entirely responsible for infections in the following year. Early symptoms and effects vary slightly depending on whether the source bud was on a spur shoot or was terminal or lateral on the main shoot but the end result is more or less the same.
There are basically three main ways to treat mildew: in winter, in the spring and in the summer.
In winter remove all buds and shoots distorted by mildew and cut back any infected woody shoots to several buds below the limit of visible whitening. On small young trees the cutting out of affected shoots can be continued throughout the season.
During the early pink bud stage in spring carefully remove as much as possible of the affected leaves (the primary infections) but avoid showering spores onto healthy foliage. If done carefully and thoroughly on youg trees it is possible virtually to eliminate mildew for the entire season in tjhis way.
On large trees and as an added precaution on young trees spraying should also begin at the early pink bud stage and continue fortnightly to late July. The best chemicals available to gardeners are dinocap, and the systemic fungicides benomyl, thophanate-mthyl or carbendazxim which also have the merit of excellent control of scab and some control of red spider mite. Sulphur is the traditional remedy but the cultivars Beauty of Bath Cox’s Orange Pippin and Newton Wonder among others are damaged by it and it is probably better avoided.
Here’s what Garden Organic says:
Symptoms: This is a serious and common fungal disease of apples. It also infects pear, quince, peach, medlar. A white powdery coating appears on leaves and shoots, as well as flower buds in spring. Blossom may be affected, causing it to wither and drop. Leaves become distorted, narrow and folded, then turn brittle and fall. A harsh winter will reduce the risk of infection; it spreads most rapidly in summer when warm, sunny days are accompanied by humid nights.
Prevention and/or cure: Pruning is the best way to prevent infestation. In winter, cut out any shoots and buds that have been infected with mildew, they will appear silvery/grey, and buds distorted. In spring, carefully remove infected leaves and shoots. Prune directly into a bag to prevent spores from spreading. Check trees weekly through the season and carry on cutting out infection. On small trees this can be a very effective method of controlling mildew, if done thoroughly. Prunings should be buried in an active compost heap or sent to your local council’s green waste recycling centre.
Here’s what the Royal Horticultural Society says:
What is powdery mildew?
Powdery mildew is a fungal disease of the foliage, stems and occasionally flowers and fruit where a superficial fungal growth covers the surface of the plant.
Many common edible and ornamental garden plants are affected including apple, blackcurrant, gooseberry, grapes, crucifers, courgettes, marrows, cucumbers, peas, grasses (the powdery mildew fungi are major pathogens of cereal crops), Acanthus, delphiniums, phlox, many ornamentals in the daisy family, Lonicera (honeysuckle), rhododendrons and azaleas, roses and Quercus robur (English oak). Powdery mildews usually have narrow host ranges comprising of just a few related plants. For example, the powdery mildew affecting peas is a different species from the one attacking apples.
You may see the following symptoms:
White, powdery spreading patches of fungus on upper or lower leaf surfaces, flowers and fruit Tissues sometimes become stunted or distorted, such as leaves affected by rose powdery mildew In many cases the infected tissues show little reaction to infection in the early stages, but in a few specific cases, for example on Rhamnus, the infection provokes a strong colour change in the infected parts, which turn dark brown Sometimes the fungal growth is light and difficult to see despite discolouration of the plant tissues, e.g. on the undersurface of rhododendron leaves Control Non-chemical control Destroying fallen infected leaves in autumn will reduce the amount of infectious spores next spring. Mulching and watering reduces water stress and helps make plants less prone to infection. Promptly pruning out infected shoots will reduce subsequent infection.
Most powdery mildew fungi have a host range restricted to a relatively few, related plants, but these can include wild relatives which can be sources of infection, e.g. wild crab apples may be sources of infection for apple orchards.
Seed producers sometimes offer powdery mildew-resistant cultivars of both vegetables and ornamental plants, check catalogues for details.
Because most of the growth of powdery mildews is found on the plant surface they are easily targeted with fungicides.
Edibles: No fungicides are currently being produced for use by home gardeners against powdery mildews on edible crops. Gardeners in possession of the myclobutanil-containing products Systhane Fungus Fighter or Fungus Fighter Disease Control, which have been withdrawn and are no longer sold, can still use the products against labelled diseases (including powdery mildews of apples, blackcurrants and gooseberries) until the 30th November 2016. After this time it is no longer legal to use the products and any remaining stocks should be disposed of safely.
Ornamentals only: The fungicides tebuconazole (Bayer Fungus Fighter Concentrate), tebuconazole with trifloxystrobin (Bayer Fungus Fighter Plus), and triticonazole (Scotts Fungus Clear Ultra and Scotts Fungus Clear Ultra Gun) are approved for the control of powdery mildews on ornamental plants.
The following products contain a combination of both insecticide and fungicide, enabling the control of both insect pests and disease: myclobutanil containing cypermethrin (Bayer MultiRose 2, Doff Rose Shield, Vitax Rosegarde, Westland Rose Rescue); tebuconazole containing deltamethrin (Bayer Multirose Concentrate 2), and triticonazole containing acetamiprid (Scotts Roseclear Ultra and Scotts Roseclear Ultra Gun). When a proprietary product contains an insecticide as well as a fungicide it would be preferable to use an alternative product if pests are not a problem on the plants treated.
SB Plant Invigorator contains a blend of surfactants and nutrients and can be used on any edible or ornamental plants, with no harvest interval. It has a physical mode of action and may be used against powdery mildews, as well as a range of pests such as whiteflies, aphids, spider mites, mealybugs, scale insects and psyllids.