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

1st picture -- possible mealybugs or a woolly aphid. Rub them off; squish; or wash them off. 2nd photo -- an overwhelming case of spider mites. Wash them off; repeat as needed.

It may be a good idea if you can post a photo of the pests. It's better to have a closer look so appropriate action can be suggested. How is the tree is doing now? Ankrara's Hobby Corner

Aphids. They are a sucking bug that feed off trees vascular system. Take your garden hose on gentle shower and thoroughly wash them off daily. You can also use water & mild soap like ivory in a solution or a neem oil mix, to make things a little more inhospitable for them. Lots of ideas on the www using google. They tend to be worse when the weather is dry but an infestation can be present even with rainy weather.