That is a type of rust disease that is common on Hawthorns and some other native trees. (Cedar-Hawthorn Rust, most likely, and not just because it happens to be on a hawthorn tree).
It is not concerning and you don't need to treat it.
It may show up on fruits as well, but they can still be appreciated by wildlife.
It mostly only impacts fruits and young twigs. Once symptoms manifest, treatment is not possible since fungicides are preventative measures only, not able to cure existing disease. 
In the past, preventative fungicidal sprays were used to suppress (never 'cure') and minimize infection, but they are generally not recommended, in part because the trees aren't really harmed and because fungicides are not curative and need to be repeated, and the residues might harm pollinators or other organisms.

The highlighted link above explains the unusual life cycle of rust diseases, which requires two host plants. In early spring, mild wet weather prompts infected junipers to release their rust spores, which then travel on the wind to certain rose-family plants that are vulnerable to rust. These include hawthorn, serviceberry, crabapple, apple, and a few others. Both the juniper, and the trees are good natives to have that have co-evolved along with other flora and fauna (perhaps even the fungus too) and are good for the environment. The problem is mostly cosmetic and it's increasingly important for homeowners to accept cosmetic imperfections in plants and landscapes.


Christine