The black stuff on the branches looks like sooty mold. I get it on some of my smaller avocado branches. It doesn't seem to harm them. You could try brushing it if off with a soapy tooth brush if it bothers you.

The leaves don't look good. There may be something going on with the roots. What type of soil did you plant the tree in? Has the soil been wet or dry?

Have you poked around in the soil and looked at it? Maybe the potting soil the tree came in has gotten all dried out.

Pathogenic root rot is caused by water molds - commonly referred to as "fungi" but actually a thing related to brown algae - that enter the roots and attack them. Once this happens there is nothing to be done for the plant.

As for roots and phosphoric acid:

Microsoft Word - B&B #2 - phosphate.doc (wsu.edu)

Thanks for the responses. I did clean the black spots with soapy water and most of them came off. However the tree has now dropped all its leaves.

The natural soil is mostly clay and I amended it with compost and soil I bought from the store. I planted the tree on a small mound. I poked around in the soil and the roots look pretty dark, however the soil is also dark so I'm not sure how much of that is the root vs the soil.

The symptoms of root rot and drought are the same so it is hard to tell which one it is without knowing if the soil the tree is planted in is wet or dry. Was the soil wet or dry when you poked around?

The general recommendation for planting avocados in clay soil is to put them on top of the native soil and build a mound around the tree.

If you've had a lot of rain since planting root rot is possible because amended planting holes dug out of damp claylike soils can become collection points for water as a result of the amending.

Also careless practices prevalent at retail plant outlets can result in more or less normal looking purchases being already infected before being brought home and planted. With root rot being historically a big problem for container production operations located in the southeastern US for instance. Because hot + wet = rot.