Prunus insists on good drainage. It's very possible that your soil is breaking down and holding too much water. Make sure the lower part of the pot is quite (but not completely) dry between waterings. If you find the soil remains wet for more than a couple of days, repot in spring before budswell. Could also be a nutrient deficiency. Are you by any chance using a fertilizer with a high second number? Check carefully for insects. Many of the sucking insects can cause leaves to curl & yellow. I'm thinking particularly of red spider mites here.

Al

Thanks Al. I stopped watering it for awhile and started spraying it with neem oil. It's now got a lot of new green leaves. This suggests to me overwatering and/or insects were a problem.

I have yet to fertilize it, though. What do you recommend?

If you want to keep the tree compact, fertilize with a balanced blend when branch extension stops after initial spring flush of new foliage. This will be when leaves are mature & new leaves have stopped appearing at branch apices (growing tips). Another application of the same product or 0-10-10 around mid Aug. should be all you need. Prunus does very well in a spare soil. If you want to maximize growth (not necessarily the healthiest thing for the tree), follow fertilizer manufacturer's recommended dosage & dosage intervals.

Al