Ask Mr. Smarty Plants

Amelanchier are susceptible to several foliar diseases. The two most likely in your case are Entomosporium leaf spot (Entomosporium sp.) and Cedar-hawthorn rust (Gymnosporangium globosum) or the related Cedar-quince rust (Gymnosporangium clavipes). On servicerberries Gymnosporangium rust appears as brownish-orange spots on the leaves and distorted fruit showing horn-like protrusions.   Entomosposium usually appear as small, dark circular spots on the leaf often with a distinct halo outline. It is recommended that you try cultural practices (raking fallen leaves, avoiding overhead watering) first to help reduce your leaf spot problem since removing the alternate host of the Gymnosporangium is not possible. If this is not effective and the disease is severely defoliating your serviceberry, there are some copper or sulphur fungicides that are registered for use on rusts that might be an option.

George M. Dickert, Horticulture Extension Agent, Spartan County, Clemson University has prepared an information sheet online for the Serviceberry. Under problems he discusses several leaf foliar diseases ... As serviceberry is in the rose family (Rosaceae), it is susceptible to many of the same disease and insect pest problems seen in other species within the family (e.g. apples and pears). Entomosporium leaf spot (Entomosporium sp.) may cause serious spotting and partial defoliation on some selections, especially following rainy weather. Cedar-serviceberry rust affects twigs, buds, fruit and foliage and can disfigure these plant parts or result in witches’ brooms. Other minor diseases include powdery mildew and fire blight. Proper selection of resistant cultivars and good cultural practices can often prevent these problems from becoming serious.

In addition, Gary W. Moorman, Professor of Plant Pathology, Penn State Extension has published an interesting article on Gymnosporangium that will provide insight into the life cycle of this rust that is the likely cause of your leaf spots.
Cedar Apple And Related Rusts on Ornamentals
Cedar-apple rust and closely related diseases, cedar-quince, cedar-hawthorn, and Japanese apple rusts are caused by fungi belonging to the genus Gymnosporangium.
These fungi require two different living host plants in order to complete their life cycles. If either host plant is not present, the fungus dies. Juniper, sometimes called cedar, is one host while alternate hosts include apple, hawthorn, quince, and shadbush or serviceberry, depending upon the species of fungus involved.
Life History
Rust overwinters as mycelium in gall tissue on junipers. During warm, rainy spring weather, gelatinous horn-like protrusions emerge from the gall. Teliospores produced on the horns germinate and give rise to another spore type, basidiospores, which are carried by wind to fruit or leaves of the broadleaf host. Infections occur if moisture is available on the leaves and temperatures are adequate. Later, pycniospores formed on the upper leaf surface of the broadleaf host allow the fungus to mate. Aeciospores that form in the same spots as the pycniospores but on the underside of the leaf of the broadleaf host are carried by the wind back to junipers in the summer. The spring after the juniper is infected, gall tissue begins to form. The second spring after juniper infection, spores are released from the mature gall. Thus, 18 to 20 months are required for the gall to form on junipers, mature, and release spores.
Management
In most cases in the landscape, damage to either host is not severe enough to warrant fungicide applications. However, susceptible varieties of the broadleaf host can be heavily infected and defoliated. Fruits of the broadleaf host can be severely deformed as well. Significant damage to junipers may occur if the broadleaf host is close by. Unsightly galls forming on the twigs and branches reduce the value of the plant and twig dieback can occur if the disease is very severe. In many cases, the disease is not noticeable on junipers except in the spring when the galls are producing the bright yellow-orange spore horns. Junipers should be carefully inspected during each dormant season and any gall tissue should be pruned and destroyed before the spore horns develop. In the nursery, apply a fungicide in the summer. Avoid planting the alternate hosts close together. If possible, remove susceptible non-crop plants from the vicinity. If a broadleaf host is the desired plant, remove wild junipers.

Lastly, Kristan Crouch, Tiffany Maughan and Brent Black at Utah State University Extension have published the following information about problems of Serviceberry in the Garden that discuss control suggestions for Entomosporium. Entomosporium leaf and berry spot is one of the most common diseases of serviceberry plants. Symptoms include small, angular brown discolorations on the leaves, often with a yellow ring around the spot. Low humidity helps keep disease occurrence low, but in rainy years or if over-watered, it can still be a problem. Keeping an open canopy through proper pruning, removing leaf litter in the fall and avoiding irrigation techniques that would wet the leaves will help control for Entomosporium leaf and berry spot. Infected fruit will have gray spots and will be disfigured. Prune out diseased wood 12 inches below the infected section and sterilize the shears between each cut. Maintain an open canopy to aid in preventing disease occurrence. Saskatoon-Juniper rust can be another problem for serviceberry production. Yellow spots and swellings first develop on leaves and fruit, followed by characteristic yellow, spiky outgrowths from these locations. As the name implies, the life cycle of the rust includes stages on juniper plants. To avoid the disease, it is best to avoid planting serviceberry next to juniper plants.