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xnese siimy orange excrescences are one of several sporeproducing stages of the cedar apple rust, Gymnosporangium juniperi-virginianae, a common fungal pathogen of red cedar (or common juniper, Juniperus virginiana) trees. The appearance of these orange globs on infected trees is as sure a fungal sign of spring as the more familiar and edible morels. Last year we began seeing them in May, but they’ve appeared early this year along with everything else.
And they would probably remain a mere curiosity if they stayed on cedar trees. Who cares if cedars sometimes get slimy growths on them? But as is typical of rust fungi, this pathogen has a complicated life cycle that involves two separate plant hosts, red cedars and one of the economic mainstays of the county
Apples. The life cycle alternates between red cedars and apples, hawthorns or crabapples, and the stages on each host tree are completely different. At first glance, no one would ever make the connection. But apple growers know all about it.
You can tell infected cedars by the presence of walnut-sized, dark-brown galls that develop on branches over the winter. Galls are fairly inconspicuous until the spring when they burst out in all their glory. The slimy tentacles produce masses of spores that cannot grow on cedars, but instead must find their way to the leaves of apple trees. You’ll know when the spores arrive on your apple trees because distinctive yellow lesions form on the leaves shortly afterwards. This stage of the fungus produces more spores that can only germinate on cedars.
The appearance of the slimy orange balls every spring is a warning that apple trees are susceptible to infection. Growers can take precautions by planting resistant apple varieties, treating their trees with fungicides and eliminating as many of the nearby alternate hosts as possible.
The cedar apple rust is one of about 7,000 species of rust fungi, all of them plant pathogens, and many of them with complex multiple-host life cycles. The most notorious of these is the black stem rust, Puccinia graminis, which attacks wheat and other cereal crops worldwide. The alternate hosts of this rust are barberry species, all of which have been eradicated in wheat-growing regions of the U.S. to provide some measure of control. Complete control is impossible, though, because many strains
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