Certified GKH Gardening Expert
This is unfortunate, since this will be incurable. You can treat the disease, but eventually it will kill the tree.
This article will help: https://www.gardeningknowhow.com/plant-problems/disease/cedar-hawthorn-rust-disease.htm
As far as what to feed them... Any well balanced fertilizer will be fine once per year. This article will help: https://www.gardeningknowhow.com/ornamental/trees/hawthorn/growing-hawthorn-trees.htm