Apple Cedar Rust Gall

If you’re making a list of bizarre organisms, you need to add Apple Cedar Rust Gall. This formation is produced by Gymnosporangium juniperi-virginianae, a fungus that alternately lives on Eastern Red Cedar and various species of apple. The fungus spends the winter as an acorn sized mass attached to the cedar branches. The masses usually go unnoticed until spring rains cause them to explode with these fleshy orange spikes.

Who’s not going to notice the sudden appearance of something like this on their tree? This seems to be an off year for the fungus. These masses have been extremely rare this spring.

The galls affect the cedars very little. At most you may notice some die back of the needles or branch tips. It’s when the fungus transfers to apple trees that damage occurs. Blemishes to the apple fruit can severely reduce the market value of a crop. Leaf damage is sometimes severe enough to weaken the tree and make it susceptible to die back or infection by other diseases or insects.

Dimples on the surface of the gall open to allow extension of the filaments that will be the source of spores. Spores released from the gall must come into contact with an apple tree in order to survive and produce more fungus. Mature fungus on the apple tree will produce spores that will again infect cedars. In some ways it seems a somewhat risky survival strategy, but I guess it would be a fairly effective way of ensuring that the host plant was not overwhelmed and killed by the fungus.

It only takes a light rain to cause the dry projections to blossom into full spore producing appendages. Depending on the weather, the spore producing portion of the fungus may remain for as long as a couple of weeks. My apple trees sometimes play summer host to the fungus, but the blemished apples taste just as good.