Congratulations to Roseanne Saalfield, the first of several readers who correctly identified the Mystery Photo as a stage in the life cycle of Cedar-Apple Rust (Gymnosporangium juniperi-virginianae Schwein). This rust is a member of the family Pucciniaceae, a group of fungi that contains many species that usually require plants from two different families (usually within a mile of each other) in order to complete their life cycle: one plant from the Cupressaceae family (eastern red cedar, juniper) and the other from the Rosaceae family (crabapple, apple, hawthorn, serviceberry).
The fungus assumes very different forms on each host. On rose family plants, the fungus can be present on the leaves (orange spots on the surface of the leaves and tiny projections beneath them) as well as the fruit. On cedars and junipers, brown spherical galls produce orange, fleshy projections.
For those readers who wish to know the fairly involved details of the life cycle of this fungus, read on: This rust produces four kinds of spores: basidiospores, teliospores, spermatia, and aeciospores. Teliospores are produced on orange, gelatinous telial horns (see bottom photo) which originate from hard, brown galls on red cedars or other junipers, usually in the spring when it’s been raining. Teliospores germinate to form basidia. Basidia produce basidiospores that are released into the air, blown two to three miles potentially to an apple or hawthorn leaf or fruit. They germinate and form a yellow or orange spot on the leaf or fruit (see photo). These spots produce spermogonia that in turn produce spermatia. The spermatia are released into a sticky liquid attractive to many insects. As insects carry spermatia from one spot to the next fertilization takes place. The fungus grows on the fruit or through the leaf and produce aecia on the underside of the leaf (see photo). The aecia produce aeciospores that are windblown back to the red cedars. They then germinate and start the formation of galls that in the following year will produce telial horns to start the process over again. (U.S. Forest Service)
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