Leaves on Avocado Plant Are Dying?

I grew a avocado plant from a pit. When it was rooted I put it in a pot. Once I put it outside, when it got about 2 ft. tall, it began to have tiny round brown spots on it. Under the leaves were similar, but light white. The leaves started dying from the bottom then it started on the new leaves as well. I'm about to trash it and start over. What can I do?