This is caused by the fungus podosphaera leucotricha that produces a white powdery coating on leaves and shoots of apples, ornamental malus (crab apple), medlars, quinces and occasionally pears.
The fungus over winters in the shoots and terminal buds of the tree, and is then spread in spring by means of windblown white powdery spores that get onto the fresh young shoots.
Small gray or white patches of fungus develop on the undersides of leaves often causing them to become curled and distorted.
It can also cover entire terminals causing them to become brittle or even die.
Diseased flowers are brownish and shriveled and soon die.
When the fruit is infected, a network of lines, generally referred to as russeting, appears on the diseased area.
If not treated immediately this cycle will occur many times during the growing season, which can affect the fruit yield and the general health of the tree including flowering.
Where possible, remove and destroy badly infected shoots.
Alternatively, spray with a suitable insecticide at pink-bud stage in late April or early May and repeat at 7—14 day intervals until terminal growth ceases, this hopefully will limit the infection of following year’s buds.
Some varieties that are less susceptible to this disease are worth considering, check with your supplier prior to purchase as to what these varieties are.