I only have a few fruit trees and I used to have problems with possums, raccoons and squirrels. Last two years I have been tying wild blackberry canes to the trunks of the fruit trees, because I read of someone else using that solution. About 3 or 4 canes per tree will keep them from climbing the tree. So far I have had great success. The vines are free, and even they they die and turn brown, they are effective for at least a year. If the fruit tree is small enough, I even plant a wild blackberry plant next to the tree in the spring. By the following year the wild blackberry plant will send up new canes, and I just tie these new canes close to the trunk. That way I don’t have to search for wild canes to tie to the trunk, because the living plant provides me with fresh canes that I need. Has anyone else used this solution?
Not intentionally but I have this happening naturally at my place in that I notice my fruit trees growing amongst blackberry canes are free of deer pressure. Natural fencing.
That is a really interesting idea! 
Never would have thought of this but it makes total sense. The deer-deterrent effect you mention rings true too, I’ve noticed my trees on the boundary of a bramble patch never get browsed while the open ones do.
Shoot…plant some if Clark’s blackberries and they’ll alsi keep bear and moose away. Lol ir Col
This is a fascinating idea.
I wonder if wild black raspberries would be equally effective (I prefer the berries over blackberries) or if they would need blackberries’ more vicious thorns to make a noticeable impact…
Another thing I would wonder is if this would add risk of rusts/blight for other rose family members (ie apples, pears) in a humid region like mine?
I have pretty high deer pressure and I’m not sure this would work without expanding my mechanical barriers to include the berry canes, but it is definitely an idea I’m curious to consider since the neighborhood squirrels are about the size of housecats and I’m sure will be an issue in a year or two once my trees start to produce… it would give me a use for the several volunteer wild blackberries that appear to be sprouting at my place this year instead of just mowing them down… 
Edit: how do the blackberries cope with the shade? I know the local wild raspberries tend to grow in part shade happily, but the local blackberries are most often growing in more open/recently disturbed areas so I’m not sure how well they cope long term with shading.
Another edit: I also wonder how practical/useful it would be to plant something like a trifoliate orange very close to more preferred and taller fruit trees for a similar effect or if the animals would just slip between the orange thorns since they wouldn’t be secured directly to the larger tree’s trunk?
I think the area around my trees would turn into a weedy mess if I tried this.
I have thought of making one side of my fence a blackberry hedge though and I think it would work but seems like more maintenance and work than the actual fence
Deer eat even my thorny raspberries, black raspberries, and blackberries (except Clark’s). Even the Clark’s were browsed the first year and I had to put them in a cage till they were larger.
I’ve been told by extension that the fire blight that affects berries is different from the apple/pear ones and that there are even strains which preferentially will attack pear, Asian pear, or apple (though there is crossover with those). I found at least one research article which seemed to confirm this.