Cedar apple rust, caused by _Gymnosporangium juniperi-virginianae_, is
important commercially in the apple-growing regions of the Virginias,
Carolinas, and the Mississippi Valley. The alternate hosts of this rust
are eastern red cedar and several species of junipers.
[Illustration: Fruiting galls containing spores on cedar tree.]
[Illustration: Fruiting on apple leaf the alternate host.]
Cedar “apples” or galls are the characteristic signs of the fungus on
cedars. Cedar needles are infected in the summer by wind-borne spores
from apple leaves. By the next spring or early summer galls begin to
appear as small greenish brown swellings on the upper needle surfaces.
By fall, the infected needle turns into a chocolate brown gall covered
with small circular depressions. The following spring, orange jelly-like
tendrils protrude from the galls producing an attractive ornament for
the cedar tree. Spores produced from these orange spore masses are then
capable of reinfecting apple leaves, thus completing the fungus life
cycle.
No practical control of the rust on cedars is available because of the
low value of cedar. However, considerable effort is expended to protect
apple trees. Where apple is to be protected, cedars should be eliminated
in the vicinity or, rust galls should be picked or cut off cedars before
the galls mature.