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).
Some vocabulary, because this is complex:
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.
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.
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