How to recognise it
Dry, white powder usually found on the upper surfaces of leaves in hot and dry conditions.
Powdery mildew survives as ‘mycelium‘ (fungal strands) in the buds overwinter. You can often identify it by the buds looking small and shrivelled and their accompanying twigs having a dried and silvery appearance. The mycelium then emerges with the bud in the spring and gives the young leaves a characteristic white bloom, which is the primary form of this mildew.
As the spring progresses, the fungus produces spores which spread via the breeze onto other leaves and develop into the secondary fungus, which is the external form causing most of the damage. This secondary attack appears as dry, white powder on the upper surface of leaves, the mycelium strands sinking small pegs into the leaf to draw out moisture.
As a result of the disease the blossoms wither and drop and, as it progresses, the leaves fall with only a few persisting (so you often see bare stems).
Apple fruit may have a rough skin as a result of the infection reaching the young fruit and very occasionally a powdery coating may be found on them.
In the autumn powdery mildew may be noticeable through its production of dark coloured, 1mm sized spore cases (called cleistothecia) on the leaf.
Why it’s a problem
Leaves may fall early in the season as the fungus leaches the water from them. As a result growth may be stunted and fruit can fail to set.
Fruits can become russetted (rough skinned).
Where you are likely to find it
All apples, including ornamental Malus and other fruits such as pears, quinces and medlars.
How to prevent it
Mulching around the plant to help it retain moisture and implementing a good watering regime (or an irrigation system) should prevent an infection (and also help an infected plant recover). In a glasshouse you can also ensure that there is a good level of humidity around the plants, without making it so humid that you risk encouraging downy mildew!
Some cultivars display some resistance to the disease, including ‘Discovery’, ‘Greensleeves’ and ‘Worcester Pearnmain’. However, some cultivars can be particularly susceptible, such as ‘Lane’s Prince Albert’, ‘Ribston Pippin’ and ‘Stirling Castle’.
How to get rid of it
In winter remove all buds and shoots which were distorted by the mildew and cut back any affected shoots to several buds below the signs of infection. With young trees you can continue to cut out the affected shoots throughout the season.
In spring, when the buds are pink, remove as many affected leaves (silvery looking) as possible taking care that the fungal spores don’t fall onto healthy leaves. At this point start to spray the systemic fungicide myclobutanil onto the plants fortnightly until late July (or as directed) to control the fungus, or use plant and fish oil blends or sulphur and fatty acid combinations. Other chemical options are available to professional horticulturalists, such as dinocap.
When pruning, ensure that your implements are thoroughly cleaned before moving onto other plants (as they could transfer the fungus) and destroy the cuttings (eg in municipal green waste bins, but not on domestic compost heaps which will not reach a sufficiently high temperatures to destroy the fungus).
Is it good for anything?!
Other useful information
Powdery mildew in various forms can be found on a large number of plants. It is distinguishable from downy mildew as it is found on the upper surface of leaves (downy mildew is usually on the lower surfaces) and being in hot, dry conditions (downy mildew prefers a humid environment).