There's a fungus among us, one that is both fascinating and destructive. It's called cedar apple rust (Gymnosporangium juniperivirginianae), and it requires two different host plants to complete its life cycle.
This fungal organism spends part of its life on a member of the juniper family, most commonly the eastern red cedar (Juniperus virginiana), although a few other species of juniper are occasionally used, including Chinese juniper (J. chinensis), creeping juniper (J. horizontalis) and low juniper (J. communis var. depressa).
The other part of this disease's life cycle is spent on apples, crabapples and hawthorns.
On the juniper host, the fungus looks otherworldly; it starts as a brown, swollen gall on the twigs, but as it matures over the next two years, orange, jelly-like “horns” come out of holes in the gall. These bright orange growths occur in the spring and look like 2- to 3-inch-wide sea anemones hanging off the branches. Cedar apple rust doesn't cause major harm to its juniper host, but that's not the case with the apple trees it infects.
Spores are dispersed from these orange fungal growths on eastern red cedars in the late spring. After the spores are released, the growths fall off the cedar and the plant remains unharmed. The spores travel on the wind or on birds to nearby apple, hawthorn and crabapple trees.
Because apples are an important crop, cedar apple rust control is generally targeted at apples, and breaking the two-host disease cycle is the best way to suppress future infections. Commercial orchards remove all potential juniper hosts from fields and field edges to break the disease cycle for up to a mile around the orchard, if possible.
Once apple trees are infected with the fungus, yellow, circular lesions appear on the upper leaf surfaces shortly after bloom. Wet conditions are particularly conducive to this disease.
As the infection progresses, the undersides of the leaves develop distinctive, raised orange, thread-like structures that ooze in late summer. The blossom end of fruits may show similar signs of infection as well. Eventually, the structures turn black and disperse a new set of spores that go on to re-infect the juniper host, thus completing the two-host cycle.
Though eastern red cedars and other infected junipers can live for many years with the disease, it's best to try to control it, whenever possible, to prevent spreading the disease to nearby commercial orchards where the negative effects can be more significant. Infected apple trees may be disfigured and yields reduced.
To manage cedar apple rust, immediately prune out all galls on eastern red cedars and other infected juniper species as soon as they are noticed. The orange, anemone-like growths are unmistakable.
Be aware, however, that once the orange structures are present, spores may already be travelling in the air. Spores can travel on the wind for several miles. Preventative fungicides do work, but by the time symptoms appear, it's often too late to control with fungicide applications.
Some varieties of apples are more susceptible than others. Good resistant cultivars include: “Redfree,” “Liberty,” “Winesap,” “Jonafree” and “Freedom.”
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