Here is a link about poisonous fruit trees for dogs. I was shocked to read about apple trees. We have one in the back yard and Sophie strips the apples off as high as she can reach. My husband has trimmed branches on our mulberry tree and I was thankful to see it is ok, since Sophie has been dragging whole branches around and chomping on them. He didn't trim the apple tree, thank goodness!
Fruit Trees That Are Poisonous to Dogs | Garden Guides