Apple tree needs urgent help
 

I've just put my 5ft apple, pear, and plum trees outside and the pear and plum seem to have adapted well but the apple (golden delicious) tree has not; its leaves are going orange/brown and falling off (it doesnt look like rust, just rotting) and the green branches that grew are going orangey brown as well, should i take them back inside or is there anything i can do to help it?