Cedar Apple Rust

CEDAR APPLE RUST
By Bailey Walten 1993 Newsletter

When red cedar, apple, or hawthorn trees grow together in a small area, each may become infected with a disease known as CEDAR APPLE RUST. This “rust” is a fungus disease that affects the leaves, branches and fruit of the host tree; cedar, apple or hawthorn. If a cedar host is not within the infectious range, the apple or hawthorn will not be affected. The first year that red cedar becomes infected, the needles become covered with many galls, a swelling of the tree tissue, about an inch in diameter. The damage to cedar trees is not serious unless hundreds of galls are formed on the tree and terminal ends (current years growth) of the branches may die. On mature cedars, there may be a large swelling at the base of the trunk.

The second spring after infection, these galls form numerous long, yellow tongue-like outgrowths during periods of warm wet weather. The spores from these projections germinate to produce other small spores. These new spores, which are carried by the wind, will now infect nearby apples and hawthorns.

Damage to the apple trees is basically limited to pre-mature leaf dropping. Hawthorns may experience severe leaf drop, and deformation of the fruit and twigs. The fungus will form small cup-like structures on the twigs called cluster cups. Bright orange fungal spores, which are found on the fruit and foliage, are shed, carried by the wind to nearby cedars, infecting their needles and young twigs, completing the alternate host cycle.

Annual infections of hawthorn will cause disfiguration and eventually kill the infected trees. Controlling cedar apple rust can be somewhat difficult. The recommendation for the removal of all cedars within a 1 mile radius of apples and hawthorns is impractical. Chemical controls consist of 5 or 6 fungicidal applications per year providing less than adequate control, and it is not a guaranteed cure!