Having grown up in Portland, I've always wondered what an Opossum would do to a fruit tree? Whatever it is - it needs to stop! I suggested the smearing of "roof patch" tar around the base of such trees. Some may find that "anti-organic" - I found it successful! It "sets up," so as not to run off, and I never smear it very thick, just enough for an ugly first bite.
Another old trick is White-washing the tree trunks... I believe this was done with a lime & water mix, to keep the "bugs" off and reflect the scalding sun. I've used interior white latex paint, from the ground up to the first limbs. They really look great, just like in old-time photo of a well kept orchard. I assumed it provided another nasty first bite!
Dealing with the damage... I've brushed on a brown petroleum based grafting compound in the past, but have read, and noticed, that "air" works best. I'd never jam "dirt" into an open wound ; I'd brush it out with water, maybe a "Clorox" mixture. The cambium will immediately begin to heal itself.
Close-mesh wire screen could work, if it's not a mouse or vole at work - they're so small they'd most likely evade it. My fear would be the potential for girdling - if forgotten. It would also make it nearly impossible to keep the grass away, and would have to be expanded fairly often - or at least monitored close (do keep any grass away).

Glad you caught it quick - I advise everyone to monitor their trees weekly, it's amazing what you will learn, and you'll stay on top of the problems!