Fat Maggots

Question

Nectarine growing in container, recently treated for peach leaf curl, began to wilt and leaves stopped growing. Soil was found to be full of horseshoe shaped maggots with brown heads

Answer 1

These are grubs. It sounds like the grubs of one of the many kinds of scarab beetles that plague gardeners (japanese beetles being among them). These articles will help:
http://edis.ifas.ufl.edu/lh037
https://www.gardeningknowhow.com/plant-problems/pests/insects/june-bug-beetles.htm