Our six year old Cox's Orange Pippin flowered a bit late, leafed out as usual but leaves didn't darken like other apple trees, then set fruit which seems to be ripening about like usual. Down at the base of the tree i found strange vertical scraping marks, quite a few but not enough to girdle the tree. Reading other postings, could this be the "rodent damage" mentioned? Don't think we have rabbits in inner NE Portland - rats? Squirrels? I shall protect the trees with wire just in case, but wonder if anything should be done to injured parts? Wash? Press dirt on weeping scrapes? Sterilized (baked) dirt? Grafting seal? Would just air be best? Should our other young trees be protected too? (Pear, Katsura, Sophora, Crabapple?) What would be the best way to protect them - would chicken wire collars be best, or some wire/tar paper configuration? [/i]

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!

> strange vertical scraping marks

Cats. Your trees are nothing but scratching posts to them.
My neighbors cats used to scratch the plum trees in my front yard.
I took some spare sections of vinyl downspout, slit them along one edge, drilled many large vent holes, and put them around the tree trunks. No more cat problem.