This is a very common fungal infection called rust, named for the rusty-orange spores produced when the fungus is ready to spread. Serviceberry and its relatives (members of the rose family, in this case, mainly trees more than shrubs) are common victims of infection, and the fungus is unusual in that it requires two different host plants to complete its life cycle. The other is our wild Eastern Redcedar (Juniperus virginiana). Spores produced on Serviceberry are no longer infecting the Serviceberry and instead are being dispersed in the wind or rain to any Redcedars within a few miles. In spring, infected Redcedars produce spores that travel to Serviceberry.

There is no fungicide treatment for rust that will be effective once spores are visible. Redcedars are not treated, and usually, infected Serviceberries are not significantly harmed by infection, though it can be a nuisance aesthetically and "ruin" a crop of its berries for the year, though it's also common for some fruits to ripen normally while others succumb to rust.

The type of rust causing the twig deformities, called galls, is Cedar-Hawthorn Rust. While the gall can kill the tip of the twig it forms on, it won't spread down the branch very much, unlike other kinds of fungal or bacterial infections of branches. All you need to do is to trim off those galled twigs and dispose of them (don't compost).

While fungicide use might protect uninfected growth when used starting in early spring, its use is generally not recommended, in part because of the hassle and expense of annual treatments (several sprays may be needed for about a 8-10-weed period each spring), and partly because of the potential risk to pollinators and other organisms, since spraying would overlap with the tree's flowering. (Fungicides might not be targeting insects, but their use can still cause them harm.) Since it's impractical or impossible to remove all junipers within the immediate surroundings of a Serviceberry in order to interrupt the rust life cycle, there's not much else that can be done about this disease. Plant infection spread and severity can be highly dependent on weather conditions, so a heavy rust outbreak one year won't necessarily repeat itself to the same degree the next year.

Miri

Thanks Miri. Very helpful. There are a lot of cedars around us, but I’ve not noticed any of them being infected. I’ll cut off what I can reach.

You're welcome.