These fungi are named for the dry to gelatinous, orange, reddish, or yellowish fruiting bodies and spore masses that many species form on infected tissue. Rusts infect many hosts, including birch, cedar, cottonwood, cypress, fuchsia, hawthorn, juniper, pear, pine, poplar, rhododendron, and rose.
Rust fungi and their damage usually occur on fruit, green stems, and leaves of broadleaf plants or on the bark or needles of conifers. The host species and characteristics of the damage and fungal fruiting bodies and spores are used to identify rusts. Many species of rust have four or five types of fungal spores that differ in appearance and the time of year when they are present.
Chrysanthemum rusts and rose rust cause discolored spots on leaves, which may drop prematurely. Gymnosporangium rusts on cedar, cypress, and juniper cause bunchy shoot growth (witches' brooms) and colorful raised growths or spots on foliage. On their alternate hosts in the rose family, Gymnosporangium rusts primarily cause leaf spots, but on pear sporadically damage fruit.
Pine rusts may discolor only the leaves or needles, cause symptoms mostly on bark, or affect both bark and foliage depending on the species of host plant and rust fungus. Western gall rust and white pine blister rust cause cankers, galls, ooze, and roughening on bark. Foliage outward or upward from infected bark can become yellowish, then brown or reddish and drop prematurely.
Rusts infect hosts when plant surfaces are wet and temperatures are mild, mostly during the fall to spring or almost any time of the year in coastal locations. Rust fungi are spread primarily by windblown spores and when infected plants (e.g., from a nursery) are moved. Each species of rust is specific to certain hosts.
Most species of rust fungi have several different-looking life stages and complex life cycles, alternating generations between two hosts in different plant families. Others, such as rose rust, Phragmidium mucronatum, and western gall rust are restricted to one host genus and spread, respectively, from rose to rose or pine to pine.
Infected foliage on broadleaf hosts may become spotted, turn yellow or brown overall, and drop prematurely. Infected conifers may develop bushy growth (e.g., witches' brooms), cankered, galled, and oozing bark and limbs, and spotted needles.
Many rusts have minimal affect on plant health, but certain rusts can kill their hosts. The importance of rust damage varies greatly, such as depending on local environmental conditions and the abundance and nearness of other hosts that are sources of infectious spores.
Avoid overhead watering. Collect fallen, infected leaves and needles and dispose of them away from host plants. Cut off and dispose of diseased shoots and branches as soon as they appear, except do not prune woody parts so extensively that plants are seriously damaged. Removing nearby alternate hosts of the fungus may help to reduce new infections in certain situations.
Plant rust-resistant cultivars if available, such as roses in coastal locations where rose rust is a chronic problem. Fungicides applied in the spring can prevent or reduce some rust diseases. The frequent applications required to provide good control of rust may not be warranted in many landscape situations. For more information on rose rust, see Healthy Roses and Pest Notes: Roses: Diseases and Abiotic Disorders.
Leaf spots and orange spores
Round western gall rust swellings
Orange, spore-forming bodies