My Avocado tree is sickly
I'm very new to gardening. I helped my grandparents pick and put up tomatoes on their farm in upstate NY every summer when I was a kid. I also would help my dad with his veg garden when I'd go home to visit him many, many years ago in the Poconos.
We were living in Palm Beach County for the last ten years. We lived on the coast less than a mile from the ocea and I couldn't get anything to grow. Previous owners filled in the swimming pool and graded the yard with fill dirt which consisted of mostly crushed cement. Nothing, and I mean nothing, would grow in it. I got very sick in 2010 and started to consume more organic vegetables and hormone free meats. We moved to south central FL a year ago (90 miles inland) and after I got the house all painted inside and everything put away, I began to work on the outside. First I fenced a portion of our yard so my jack russell terrorists would have space to run wild. After that, we set about getting the first round of trees planted. That was the first weekend in February. The avocado we picked was the brogdon variety, which I understand handles the very occasional cold snap well. For the past few months, it has been coming in nicely. The other day I noticed that the leaves looked spotty brown, wilted and would fall off with the slightest touch. I don't know what is wrong with it. The people who owned the property before us apparently didn't believe in growing anything but grass with the exception of one lovely gardenia bush. The other day I noticed that the gardenia had a thick coating of black on the leaves that would scrape off easily with my fingernail. I looked it up and found that it has been affected by sotty moss. Well I pruned it way back and we purchased some insecticide for killing aphids on rose bushes. Since I planted the avocado, I have sprayed it 3 or 4 times with a combo of insect soap/water/neem oil. I've been trying to go the natural route. I read that avocados can also get sotty moss but I could not find any pictures of what that would look like. Has anyone here ever had this problem? I'm a little anxious because I paid well over $100 for this tree and have grown attached to it. Like I said, I'm new to all of this and am at a loss for what to do. Any help would be greatly appreciated. Just bear in mind that I'm a noob and don't get things easily when they're too technical.