With leaves still absent from many trees, it is not hard to see evidence of galls caused by insects, such as the oak apple gall, or by fungi, such as the knobby black protrusions on black cherry trees referred to as black knot. A gall is an abnormal outgrowth of plant tissue, which can be caused by all sorts of parasites, from fungi and bacteria to insects and mites. Another fairly common gall in this area is one called cedar-apple rust (Gymnosporangium juniperi-virginianae), which is likely to appear wherever apples or crabapples (Malus spp.) and eastern red cedar (Juniperus virginiana) coexist. Cedar-apple rust actually requires both hosts to complete its life cycle, similar to the witches’ brooms seen on high bush blueberries. Parasites like this rust fungus that require two hosts to complete their life cycle are called heteroecious.

On apple trees, the fungal infection occurs on leaves, fruits and young twigs, with small, pale yellow spots often appearing on the upper surfaces of the leaves, usually during late April or May. Over the summer, these spots will enlarge and darken, and by late summer tube-like structures will develop on the undersides of leaves, producing the spores that will infect the eastern red cedar. Cedar-apple rust takes on a different form when it infects eastern red cedars. On the cedar, the fungus produces reddish-brown galls up to an inch in diameter, with small round depressions covering their surface. In the center of each depression is a small bump that will elongate into orange gelatinous protrusions, called telial horns, later in the spring. This transition is brought upon by warm spring rains. Like the under-leaf tube-like structures on the apple tree, these protrusions produce the spores that will be carried on the wind to reinfect apple trees within several miles of the eastern red cedar host. This rust can be very obvious on eastern red cedars in the spring when the galls are covered with orange-brown gelatinous masses. I know I’ll be keeping an eye out for the transition.