Certified GKH Gardening Expert
This is definitely a fungal issue! Most likely Anthracnose. Do you know how this tree was propagated? Starting them with toothpicks is one of the most common causes of introducing infections, and they may not show until later.
If it is no known how the tree was started, then it can be chalked up to an infection already present in the soil.
I would recommend applying, both, Dolomitic lime and wettable sulfur to correct the issue. Unfortunately all old leaves will retain the damage until they drop, but after treatment, within a few weeks, you should start to see new unharmed growth.
If this does not correct the issue, more drastic treatments may be necessary.
This article will give you more information: https://www.gardeningknowhow.com/edible/fruits/avocado/avocado-anthracnose-treatment.htm