I germinated some Lychee seeds when it was really hot and humid in NYC this summer (~13 weeks ago). They grew really quickly and healthily until mid October when a new layer of leaves sprouted. The new ones struggled to grow and older leaves started to get brown spots. I thought it was either due to lower light levels or lack of plant nutrition. I bought an LED UV light and put some miracle grow plant food in their pots and now they seem to be doing ok. See pictures:
Does this look like a nutrition issue that i have solved or does it look like disease? should I expect the brown leaves to get healthy again? Should i prune them off or just leave them where they are? I have no gardening experience. No idea when pruning is required. Any background info on looking after a young tree would be great!