apple trees

Look closely at the 'fur'' Is there are greenfly like creature hiding underneath it? If so then you have an attack of woolly aphid (or American blight). The wool is in fact wax which keeps the creture safe from water and predators and sadly insecticide.
If there is only a small amount or it is a small tree then wipe over the wax with methylated spirits. This will kill the inect. If it is a big tree or a lot then the next best thing is to wash the tree down with a pressure hose, set so that it dislodges the pest, but does not damage the bark or buds or fruit. You could add some horticultural soft soap to the water. (sold as Winter Tar Wash).
Good luck it is not the easiest thing to get rid of.