This is very likely cedar-hawthorn rust, given the host plant species and the presence of the symptoms on the fruit (it can also infect leaves and shoots). The disease is similar to cedar-apple rust in that it uses Juniper plants as a host for part of its lifecycle. If there are any Junipers in the vicinity (~2 miles) of the hawthorn, it may be a persistent problem because the fungal spores disperse by wind. Rust diseases can be more common in years with wet spring weather and the severity can vary from year to year.

Although the symptoms of rust can be somewhat unsightly, most landscape trees can tolerate some of this disease without long-term harm. A fungicide will not cure any of the symptoms that are already present.
At this point, nothing can be done. The berries will eventually fall off. For a longer-term approach, we recommend selecting/planting disease-resistant trees when feasible.

Rust Diseases of Trees

Christa