and life cycle—Rust fungi
Rust fungi infect under mild, moist conditions. Rusts are spread primarily by windblown or water-splashed
spores. In addition to orangish pustules, many species also form black overwintering spores on leaves
in the autumn, which start the disease cycle in the spring. Each type of rust is specific to certain hosts.
Many species have complex life cycles, alternating generations between two host species. Certain rusts
that can kill pines alternate their life cycle between pines and other hosts, such as Ribes (currant
and gooseberry) or oaks. These rusts usually do not harm their alternate hosts. Others, such as the rose
rust, Phragmidium mucronatum, are apparently restricted to one host species.
of rose rust