Cedar-Apple Rust (Basidiomycetes)

Cedar-apple rust (Gymnosporangium juniperi-virginianae) is caused by a fungus in the Basidiomycota phylum. The disease life cycle involves two types of trees – apple or crab apple (Malus genus), and red-cedar or juniper (Juniperus genus). The apple or crab apple is an aecial host (the host plant on which heteroecious rust fungus produces aecia and pycnia); and the red-cedar or juniper is the telial host (the host plant on which a heteroecious rust fungus produces telia, and sometimes uredinia).



Telial gall of cedar-apple rust on cedar, with spore horns withered after spore germination. Photograph courtesy C. W. Mims from the APS slide collection, Introduction to Smut and Rust Fungi.

Some vocabulary, because this is complex:

• Aecia – the fruiting body of a rust fungus in which the first dikaryotic spores (aeciospores) are produced. Aeciospores infect the alternate host (in this case, the juniper). Dikaryotic spores have two sexually compatible haploid nuclei per cell and divide simultaneously.
• Pycnia – the flask-shaped haploid fruiting body of a rust fungus bearing receptive hyphae and sexual spores.
• Telia – survival spore, or teliospores, produced by rust fungus in which germination occurs.
• Uredinia – asexual dikaryotic, often rust colored spores, capable of infecting the host species on which it is produced.

Basidiospores form on the germinating teliospores of a red-cedar or juniper gall. The basidiospores infect apple leaves to produce pycnia. Aeciospores are produced in blisterlike aecia on the lower surface of the apple leave. The aeciospores infect junipers only (not the apple tree). The disease is monocyclic, meaning one life cycle per growing season (versus polycyclic which is even more complicated)!

For management tips, see the Integrated Pest Management (IPM) site. Prevention is the best strategy – remove junipers that are within five miles of apple orchards, and avoid planting junipers near apple trees in the landscape (although damage is not extensive to apple trees, except in very wet years). Because the disease is monocyclic, you can effectively apply fungicide applications timed to protect the apple trees during the time when basidiospores are released. Plant apple and crabapple trees that are disease resistant.