I would guess salt burn. How deep and how frequently do you water them? In addition to the comment of not bearing true from seed, be aware that a seedling takes 10-15 years to fruit, and while avocados may be self fertile a compatible cross pollinator (they have a designated A and B grouping with cross pollinators being from the other group) makes a huge difference in yield. On the other hand the advantage of a seedling is full size and it is a proven winner in your soil and environment by thriving. And that one in the photo looks very nice indeed. I would also get that trunk painted white. This post was edited by Fascist_Nation on Wed, Jan 8, 14 at 14:32

Grasshoppers, lady bugs, caterpillars.. Can't think of anything else at this moment. Grasshoppers are chowing down on my young mango tree this year! I read on another website that dusting some regular flour (not self rising) over the leaves could make it less appetizing to the bugs. The flour will make their mouths sticky. Spray it off after 2 days, do not leave on for more than 2 days. You can repeat this method after a week.

With a few on a small tree, a non-toxic option is to flick those weevils into soapy water, and then remain alert for newcomers.

My young avocados from four winds have some very obvious callus from grafting. So in my limited avocado experience I would guess that most if not all of them callus heavily. Only an educated guess.