Posted: 28/08/2013 at 20:01
The stuff on the apple tree is a lichen as others have said. However, even the fastest growing lichen only grows at 1cm per year, so I think the other plants you mention are covered in something else - most probably powdery mildew.
Lichen will only grow where the air is of excellent quality as it is killed by pollution so having it is a good sign.
Powdery mildew on the other hand is a pain: