Taisha is again the author, and she writes:
Long-time BPotD reader and occasional contributor Wouter Bleeker of Ottawa, Ontario, Canada sent along today's photograph. It shows the fruits of an Amelanchier canadensis plant infected by a species of rust fungus from the genus Gymnosporangium. Thanks for sharing again, Wouter.
Rusts (order Pucciniales) are obligate plant parasites, i.e., they require plant hosts. Species of rusts often have multiple means of reproduction, each associated with a different stage in the life-cycle. Up to five different mechanisms for bearing spores (spermagonia, aecia, uredinia, telia, and basidia) are known for some species. These species of rusts require two specific and unrelated host vascular plant species: an aecial host (for spermagonia and/or aecia) and a telial host (for uredinia, telia, and basidia). Rust fungi taxa that require two hosts are called heteroecious. Some taxa, however, carry out their full life-cycle on only one host species (macrocyclic). Other rust species simply reproduce asexually with a single repeating life stage. More variations on the pattern include microcyclic (having only the telial and basidial stages and live on aecial hosts of macrocyclic relatives) or demicyclic (missing the uredinial life stage).
The Gymnosporangium clade has an array of life cycles (gif), host taxa, and degrees of host-specificity. Gymnosporangium species have a demicyclic life cycle, with most alternating between members of the Cupressaceae as a telial host (telia are gelatinous due to long pedicels of teliospores absorbing water in spring rains), and species from the Rosaceae supertribe Pyrodae (PDF) as aecial hosts (see: Novick, R. (2008). Phylogeny, taxonomy, and life cycle evolution in cedar rust fungi (Gymnosporangium)(Doctoral dissertation). Retrieved from ProQuest Dissertations and Theses. (3317188)).
In the above photograph, the Gymnosporangium species is parasitizing a member of the Pyrodae, Amelanchier canadensis. This deciduous shrub species has a number of common names including shadbush, serviceberry, and juneberry. When members of the genus Amelanchier are infected by species from Gymnosporangium, symptoms include brownish-orange spots on leaves and distorted fruits with horn-like protrusions.
According to the Horticulture Diagnostic Laboratory at Cornell University (PDF), of the 36 species of Gymnosporangium that occur in North America, only three are significant in the northeast area to warrant concern. The three species includes the cedar-apple rust (Gymnosporangium juniperi-virginianae), quince rust (Gymnosporangium clavipes), and hawthorn rust (Gymnosporangium globosum). Unfortunately, we're unsure which species is shown in the photograph submitted, but it could be quince or hawthorn rust. This is inferred from Penn State's fact sheet on cedar apple and related rusts. Both publications mention that the best way of preventing cedar rust diseases is to simply avoid planting alternate hosts close together. Planting resistant cultivars is another strategy. These management means should be employed before considering the use of fungicides.