Hi! I've been growing a couple avocado plants from the seed for a few months now. They were transplanted to soil in pots and placed in south facing window about 3 months ago. They were doing very well, and then suddenly the leaves started to have black spots, and started to wilt. I haven't changed anything in watering routine..should I? Any help would be appreciated.

They actually look worse now than these pictures I took a few days ago, turning black and falling off. However, at the very top of the stems, it looks like new leaves are trying to sprout. What is going on??