Avocado and Citrus Fruit Tree Health Problems?

I live in southern Florida. My avocado is now only growing little acorn size fruit, that then drop, and leaves. The same is true of my citrus trees which are always covered with a black substance. This has been going on for the past 2 years. It is a shame as they were doing so well.

I've found that my fruit trees only survived if started in passive shade and my avocado was producing many huge size fruit, but not now. I have been giving them expensive fertilizer lately, but no change. My mango is no problem it is a factory. I don't eat much, but I enjoy giving the fruit away and have felt like I been doing battle to keep them alive.