A cup of fertilizer at planting, depending on type, could be part of the issue. I apply little or none at planting. If it is a salty type fertilizer that can burn the roots. That said your tree doesn't look too bad. Keep the soil moist not wet. And if you haven't watered enough since fertilizing to run water out the bottom, do that to remove excess salts. But don't drown the plant in an attempt to rescue it. A couple good flushings is enough. There are other possible causes: Too wet, too dry, soil that doesn't drain and others I may not be aware of related to avocado.

If that was my tree I would spray it with a copper fungicide because from the picture it looks like a fungal disease. Avocado trees can be prone to this problem. Be sure to not over water the tree especially this time of year. I see that you are in 9b. You have the tree planted out in the open, not in a warm microclimate. At this size, avocado trees aren't very cold hardy. I also suspect that it got stressed from the cold snaps that we had in earlier in the winter. Check out my thread on cold protection with directions and pics on how to make a mini-greenhouse for your tree. It won't take much of a freeze to kill that young tree. You must protect it from freezes until it gets much older. If that is too much work, pot the tree up and bring it in the garage during freezes. Replant the tree in a more protected area. The best place is close to the southeast part of the house. Also, some of the remaining leaves have brown ends. Possible cause is over fertilizing. So next year, fertilize frequently but VERY LIGHTLY. It looks like the tree will recooperate if you give it the extra effort. Christine

As a general rule with trees, if the leaves fall off, they'll come back. If the leaves die but stay on the tree, the tree is dead.

I've been trying for years to help my parents in Walnut Creek, CA grow avocados bc we see and know of nearby trees, including a friend's tree that survived the all time record freeze of 1990, and they've lost several trees, always during the summer, due to these same issues of leaf browning, blackening and shriveling of stems and trunk, etc. They grow sapote, guava, and macadamia in the same part of the yard, but those darn avos won't make it. With low watering, high summer heatwaves, and clay soil, it sounds like we're getting to the bottom of things. If I can persuade them to try another Mexicola Grande, then with more water and lots of gypsum, maybe theyll have better luck. Thanks for the tips!