Fungi in the Gymnosporangium genus are members of the Basidiomycetes, fungi that produce basidia, small club-like structures. They have a complex life cycle that requires them to alternately infect two different hosts: a conifer and a deciduous tree in the Rosaceae family.
They reproduce both vegetatively (thread-like mycelia) and sexually (spores). Depending on their development stage, the fructifications produce different types of spores: aeciospores, teliospores and basidiospores.
The complete life cycle of cedar-apple rust (G. juniperi-virginianae) takes two years and requires two host trees: a juniper, where the fungus spends 18 to 20 months, and an apple or crabapple tree, where it spends 4 to 6 months.
The fungus overwinters in the form of mycelia inside galls on junipers. The galls form in the spring and summer of the first year of an infection.
The following spring, when conditions are favourable, the galls swell and split, giving way to orangish jelly-like fructifications. These produce spores, first teliospores and then basidiospores, over the next few weeks. The basidiospores can be carried several kilometres by the wind before landing on their alternate hosts, apple or crabapple trees.
Over the summer, the basidiospores germinate and the mycelia invade the young leaves and growing fruit. At first, yellowish spots with small dark specks mark the upper leaf surfaces. Long, pale fructifications then appear on the underside of leaves and on fruit and twigs, producing large quantities of bright orange spores (aeciospores).
In late summer, during dry, windy weather, the released spores are carried by the wind to juniper needles and twigs, where they cause new infections.