Certified GKH Gardening Expert
I see several things going on here!
The container is quite large for a small root mass. This means that the soil may not lose moisture fast enough, leaving it soggy and lacking oxygen.
It is important to let the soil dry, completely, down to about 2 or 3 inches before watering again. This will ensure that the soil remains aerated and free from infection.
From here, you will have to treat the upper portions of the plant with a spray fungicide. This article will help: https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm
You can also treat the soil with a handful of DOLOMITIC LIME and just a teaspoon of WETTABLE SULFUR. Do this once or twice per year to keep the soil conditioned.
Here is a couple of articles that will help you to grow avocado in container and grow them in general: https://www.gardeningknowhow.com/edible/fruits/avocado/indoor-avocado-plant-care.htm