The cedar quince/hawthorn rust (Gymnosporangium clavipes) is heteroecious, that is, it requires two different host species to complete its life cycle.
The cedar quince/hawthorn rust winters as a stem gall on a juniper or a cedar host – its primary host. These juniper galls can be perennial, persevering for several years depending on the weather. During wet weather, the galls produce orange yellow gelatinous growths. These growths produce teliospores that are scattered by the wind. If a teliospore lands on one of a wide range of rose family members, it can form galls on fruit, leaf midveins, stems, thorns and young branches if conditions are optimal. White, tubular aecia (up to a half inch in length) form on the galls of the secondary rose family hosts. The aecia resemble paper tubes and produce aeciospores. During dry weather the aeciospores are released and blow about in the wind. The cycle begins again if the aeciospore finds a suitable cedar or juniper host.
The pictured leaf and fruit cedar quince/hawthorn rust aecia infected the black hawthorn tree (Crataegus douglasii) discussed in my previous post (see Black Hawthorn on 08-01/2016). This tree was near the Hat Creek Riffle in Shasta County CA.