Mexican Avocado tree grown indoors 1½ years old has round spots on leaves

Question

I have a Mexican Avocado tree growing indoors in the winter (we live in the Northeast) outdoors in the summer. It has developed white round spots and the edges to the inside are becoming brown and the leaves are falling off. I do not see any sign of infestation anywhere. The soil looks clean, as do the leaves and I think it may be a disease but I do not know what.