It's in a pot with soil on bottom and some mulch on top. I water it once or or twice a week, I use tap water and drip the water around the tree and out towards the edges of the pot. And once in a while I will mist the leaves at night. I live in texas but it's an indoor plant unless it's hot and sunny. I just moved a few weeks ago and that's when the first leaf turned black, not the whole leaf just the tips. I can take a picture later if that would help. It sprouted late last year around september or october time.

Mexicola is dark skinned also Naomi

I've had only a few avocado, the Florida type many seem to not like, but I like them a lot. I'm no expert by any stretch and not familiar with problems as mine have all been trouble free. Anyway, re-potting has shocked mine, but ultimately they like being in the ground. Leaf drop NEVER happened to me when they were in the soil. You didn't mention watering but I never watered mine excessively till they fruit then they like a bit more water but too much has caused fruit drop. Your tree is pretty big, I'd think cutting the top would force branching. I ALWAYS cut my trees a bit high because I'd rather walk under than around them.

Backyard growers don't need to worry about a second tree. You will get plenty of avos without it.