As the saying goes, “April showers bring May flowers,” but a rainy spring can also lead to leaf diseases on our trees and shrubs. Among the most colorful of these are rust diseases.
Rusts are named for the bright yellow or orange powdery spores produced by the various rust fungi. Rust diseases are also characterized by colorful yellow, orange or red spots that develop on the leaves.
One of the most well-known rust diseases of trees is cedar-apple rust. This disease affects junipers and apples, including crabapples. The “cedar” in the disease name comes from eastern redcedar, which is the common name for one species of juniper frequently affected by this disease.
Both host plants are needed for the disease to occur, and heavy infections of apples and junipers often occur when the two are grown in close proximity.
Junipers affected by cedar-apple rust develop woody brownish-green galls, usually ½ to 1 inch in diameter. During rainy periods in spring, the galls swell and produce orange gelatinous hornlike projections. Homeowners sometimes mistake these orange gelatinous galls for juniper blooms (note: junipers do not produce flowers). The gelatinous galls release microscopic spores into the air, which infect apple and crabapple leaves. Yellow to orange spots develop on the leaves in late spring to summer. The fruit may also be affected.
The rust fungus grows within the apple tissue and eventually forms more spores, which infect juniper trees, thus completing the cycle of the disease. In the absence of either host tree, the disease will not occur, but wind-blown spores can travel for miles. Most communities have an abundance of both crabapples and junipers.
More than a dozen rust diseases similar to cedar-apple rust occur on juniper and plants in the apple family. For example, hawthorn rust (or cedar-hawthorn rust) affects juniper and hawthorn, as well as apples. Hawthorn leaves develop yellow to orange spots and may drop prematurely.
Quince rust has a wider host range, including quince, flowering quince, apple, hawthorn, serviceberry, pear, cotoneaster and juniper. Fruit, twigs and leaves may all be infected, resulting in swelling, distortion and death of these tissues. Hawthorn and serviceberry fruits are often covered with pale pink tubular fungal growths.
Other trees frequently affected by leaf rust diseases include ash, cottonwood, poplar, aspen, willow and buckeye. Yellow to orange powdery leaf surfaces and clusters of tiny yellow to orange cup-like structures are typical signs of leaf rust infection.
Damage from leaf rusts varies depending on the specific rust pathogen, the susceptibility of the tree and weather conditions. In particularly wet springs and summers, some rust diseases may cause extensive damage or defoliation to susceptible plants. Often, though, rusts cause little damage to the overall health of trees.
Fungicide sprays may be needed for susceptible orchard trees. Disease resistant cultivars of apple, crabapple and hawthorn are available and may be good choices for new plantings.
The Nebraska Forest Service strives to enrich lives by protecting, restoring and utilizing Nebraska's tree and forest resources.