cedar apple rust
Gymnosporangium juniperi-virginianae
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DESCRIPTION This fungus is an example of a rust in which there are two alternate hosts. Colorful spots of red, rust and yellow appear on leaves of susceptible rosaceous species (here, apple or crabapple trees). Sometimes other parts of the these primary hosts are affected. This can cause distortion and premature leaf drop, as well as poor quality fruit. The secondary host, eastern red cedar or another juniper, is where the disease will overwinter in a gall and ooze a gelatinous mass in the spring. The mass contains teliospores which produce basidiospores that are windblown to the apple tree.
The spores on the apple leaves grow into a spermacia that in late summer produce an aecium that protrudes through the lower surface of the leaf, releasing aeciospores that will be windblown back to a juniper. Hawthorn and quince are two of the other plants affected by particular Juniper-Rosaceous rusts. |
| Aeciospores are released from aecia on lower leaf surface |
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| Gall on juniper with orange gelatinous mass | The dried gall later |
HOST PLANTS Rusts have very specific hosts, this one, the two mentioned above. Rusts won't infect plants that are not their particular host.
CONTROL Good air circulation is an important deterrent for all fungal pathogens. When planting apples and crabapples look for resistant varieties. Consider removing junipers from the area, especially if they have galls, or attempt to remove the galls. This may not be the answer because the spores can be windblown many miles. Systemic fungicides may help to prevent infection but must be used in spring and autumn and thoroughly sprayed on the undersides of the leaves.