A fungus that attacks apples and pears.
It sporolates and releases its spores during wet and warm periods....known as Mills Periods.
These happen predominantly in the spring to mid summer period.
There are no chemicals available that actual kill the fungi once it has infected the leaf or fruit.
There are a couple that have a 10 day or so ' kick back ' but I don't know if these are available to the ordinary gardener.
So ...if you feel you are at risk you have to protect the newly growing leaf and fruit during these muggy times. The sprays available just stop the spores that are falling on the tree from germinating. So you have to give a continual layer of protection onto the new leaf and fruit as they grow.
So a spray every ten days to two weeks needs to be applied during these periods.
Commercial growers would use these sprays to apply an anti mildew cover at the same time.
Some years are more worse than others and some varieties are far more prone to infection than others.
A lot of the more modern varieties like Gala can be very prone to infection.
If you are an organic grower then one of the best chemicals would be sulphur as this is recognised as an organic material.
Be aware though that it can cause russeting on some varieties.