Ask-a-Dave's-Gardener: Avocado tree stems turning black and leaves with brown spots, 3 by sunlightsolace

sunlightsolace wrote:
So we bought an avocado tree a few years ago (I believe a Hass avocado tree but cant remember). I was very busy at the time and kind of neglected it in a pot outside. Every once in a while it would get watered but for the most part it was left to the environment. Well now I am getting back into gardening and have started taking care of it. It has a few leaves, most of them with brown spots and are wilted. The stems look like are turning black. There is some new growth with fresh leaves I can see, and for the first time, little avocados! I would love to save this tree but need to figure out what plagues it first! Please see the below pictures. I live in central florida and the tree gets about 6 hrs of sunlight a day. Now that the summer is here for the past few weeks I have been making sure to water it about 3 times a week. It is still in a a large pot (with good drainage), and I know I should plant it in the ground but we plan on re-landscaping soon and would rather keep it in the pot for now. Though I do plan on transferring it to a larger garbage can sized pot. So the question is, what ails it and how can I fix it?! It seems to have withstand whatever it is for the past few years.

Thanks!