We have an orchard about 3 yrs old 1 tree has a lot of apples [not sure if they are eaters or cookers] they are covered in a wooly substance, and when rubbed off, are perfect underneath. it was the same last year, good with flowers , no use with fruit trees. sorry if the question seems rather silly. plums were fantastic last year, not good this year.
The wool is a protection for the aphids, which are greenflies. After the fruit is picked, thoroughly spray with a garden insecticide and again next spring if there are signs of the wooly growths on branches.