University of Manitoba - Faculty of Agricultural & Food Sciences - Rusts

Problem type: Disease

Name of problem:

Plant name(s): Most deciduous and coniferous trees and shrubs, herbaceous plants and field crops

Symptoms / Characteristics:
There are numerous fungal species causing rust diseases in many plant species. Each rust species infects its host or hosts differently. Rusts are usually associated with the orange spores contained in the infected area, though not always visible. On evergreen hosts, rusts can produce galls, which can vary greatly in shape and size. They can also create stem swelling, witches brooms and can cause dieback of twigs and branches. On deciduous hosts, colourful spots can develop. Swellings on leaves, fruits and twigs, and distortion or dieback of these infected parts can also occur. Powdery orange-yellow pustules can be found on plant parts and, when ruptured, release spores that can infect new tissue.

Some rust species need two hosts to complete their life cycle. If either host plant is not present, the fungus will die. One example includes cedar-apple rust, where the fungus initially infects susceptible cedars and junipers, and in the following year, infects its alternate host, which is an apple or crabapple tree. Smooth, round galls can be found on juniper twigs and branches. It takes over a year for galls to mature and release spores for further infection. During warm, wet weather, hornlike, gelatinous material emerges from the gall, which produces spores for infection. Galls and slimy tissue will eventually dry up and die but can remain attached to twigs for some time.

Western gall rust and other closely related rust species are among the most destructive rust diseases affecting forest trees. Many of these rusts infect pine trees but they also infect oaks and other deciduous trees and shrubs. These diseases cause gall and canker formation, dieback, deformity and death of branches or entire trees. Western gall rust infects numerous pine species producing globular woody galls on branches and main stems. These galls enlarge each year, reaching up to a foot in diameter until they or the branch above the gall dies. When spores inside the gall break through the surface, yellow-orange spores can be seen covering the gall, which can then infect new tissue. Cool, wet weather favors disease development.

Another common rust occurs on roses, affecting all of the plant parts, except the roots. All rose rusts produce yellow to bright orange powdery masses of spores. Rust appears in the spring and intensifies throughout the growing season. Powdery orange masses can be found on the underside of leaves and on stems. In severe cases, leaves may develop reddish brown spots above the pustules. Leaves can yellow, brown, shrivel and drop off the plant.

All rusts are favored by wet conditions but can also be dispersed by wind. They over winter in infected stems, branches, galls and fallen debris.

Control / Preventions:
Remove infected plant parts and destroy. In cedar-apple rust, junipers should be inspected in the fall and any galls removed before the spore horns develop in the spring.

In the cases where two hosts are needed for fungus survival, one of the hosts can be eliminated. The tiny spores responsible for infection are capable of traveling far distances, so eliminating one host may not be possible or easily done.

Fungicides can be used to combat rust diseases. Sprays should be applied to deciduous trees in the spring at bud break. Evergreen trees and shrubs can be sprayed throughout the summer during spore production.

Resistant cultivars are available for most plant species and can be planted in place of susceptible ones.