Row of serviceberries along the northern coast of Québec, rusticity zone 3a, subarctic climate. Full sun. Planted 50m from the coast. Bog-like soil (field of blueberries and cranberries adjacent). Planted more than 5 years ago. Many dead stems and branch tips, with these swollen branches at the juncture. Leaves yellowing and many brown spots. I would like to know the species of parasite if possible. Thank you!
Thank you for contacting the Toronto Master gardeners concerning your ailing Serviceberries. From your photo I believe that your serviceberries are suffering from a type of rust disease. Serviceberry is susceptible to several types of rust diseases. This disease caused by Gymnosporangium spp. requires two different plant families to complete it’s life cycle: one plant from the Cupressaceae family (red cedar, juniper) and the other from the Rosaceae family (apple, crabapple, serviceberry etc.). It is advisable to not plant these families within a few hundred yards of each other. Depending on the type of rust diseases you have the symptoms are very different on each type of plant.
The swollen stems of your serviceberry could leads me to believe that your serviceberry is infected with either quince or hawthorn rust
The fact that your serviceberries are planted adjacent to blueberries and cranberries I do not recommend using any fungicide to treat this disease. Instead, good cultural practices should keep this disease in check. Pruning out the infected area when first observed will help reduce the spread of the spores Careful removal of any dropped leaves & debris is critical now and into the fall (do not home compost any of this material), but place the infected material in a garbage bag for disposal.
Note: the fungicides described in the Minnesota document is NOT permitted for use by home owners in Canada.
Consulting an arborist or tree care specialist is another option.
Good Luck with your serviceberries