This is my second posing of a question about my Robin Hill Serviceberry (first question/answer under “Serviceberry Blight”). The leaves have indeed developed bright orange spots, and the tips of some branches and also berry clusters are starting to drop. This isn’t “Fire Blight”. Do I treat it the same way, however? Pruning the affected branches, etc.?
Thank you for contacting Toronto Master Gardeners about your Robin Hill Serviceberry (Amelanchier x grandiflora ‘Robin Hill’) a cultivated variety of the hybrid between two native serviceberries, downy serviceberry (Amelanchier arborea) and Allegheny serviceberry (Amelanchier laevis) which are all members of the Rose family. Unfortunately, Robin Hill is susceptible to rust and leaf spots. See this useful article for description, advice and management strategies: https://www.mortonarb.org/trees-plants/tree-and-plant-advice/help-diseases/cedar-apple-rust
The Missouri Botanical Garden article: http://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/pests-and-problems/diseases/rusts/cedar-apple-rust.aspx provides clear information on the rust cycle which requires two hosts: a juniper and your serviceberry. The fungus responsible for infecting your serviceberry is cedar-hawthorn rust (Gymnosporangium globosum) While the common names of the various rust infections are termed cedar- the alternative host, eastern red cedar’s botanical name is Juniperus virginiana. The life cycle is explained and Integrated Pest Management Strategies are listed.
The Cornell University article http://plantclinic.cornell.edu/factsheets/cedarapplerust.pdf includes a graphic describing the disease cycle and management strategies.
Our long, cool, and very wet spring weather contributed to the various rusts infecting the various rosaceous species.
Be sure to purchase a product for rust control that is approved for Ontario use.
We wish you well in combating this distressing fungal disease