Answers from the Wild Side

What the heck is this alien-looking growth on a West Tisbury tree?! — Photo by Danielle Zerbonne

The great outdoors can produce baffling mysteries. MVTimes Wild Side columnist Matt Pelikan tries his best to solve them. Got a question for the Wild Side? Send it here.

Dear Matt,

I saw this growing on a cedar in West Tisbury. They seem prone to strange orange growths;  I have noticed similar ones before.  Alien looking thing! Any idea what it is?

-DZ, West Tisbury

Dear DZ,

Appearances to the contrary, your cedar trees have not been infected by space aliens! Those knobby growths, sprouting orange tentacles under a mild spring rain, are a common fungus known as cedar-apple rust.

As the name of this odd organism suggests, it’s a parasite that alternates between two different species of plants as hosts. Spores produced in the orange fingers on the knobs on your cedar drift through the air (potentially as far as several miles!) and, if they’re lucky, land on a tender, young leaf on an apple tree that is starting to break dormancy. If conditions are warm and wet enough to suit the fungus, it infects the apple leaf, producing reddish lesions that persist through the season and may eventually kill the leaf.

Triggered by warm spring rain, the production of spores by the cedar-dwelling generation of cedar-apple rust is timed to coincide with the period when apple leaves are most susceptible to infection. When mature, the fungal growth on the apple produces spores that, echoing the springtime process, can infect red cedars, completing the fungus’s complicated life cycle.

Cedar-apple rust doesn’t seem to hurt cedar trees much. But in its apple-eating avatar, this fungus can defoliate and kill apple trees, or mar the fruits and make them unmarketable. Apple varieties differ in how well they resist the fungus, so choosing stock carefully can help avoid problems in areas where cedar-apple rust is a problem. And orchard owners can also treat their trees with fungicide in spring, when incoming spores are prevalent. In theory, removing all the cedars (or apples) in an area would also eradicate the fungus by breaking its life cycle, but this solution is rarely practical. I remove the fungus from the cedars in my yard, at least the ones I can reach, to try to reduce the risk that a neighbor’s ornamental crab apple (highly susceptible!) will get infected. But I’m not sure this has any real effect on the local prevalence of the fungus.

Such dual-host life cycles, called “complex” or “indirect” by biologists, may seem implausible but are actually pretty common in the parasite world. Indirect life cycles evolve to help a parasite get past a difficulty posed by its primary host. I’m just speculating here, but in the case of cedar-apple rust, the fungus may essentially be a disease of apple leaves which evolved the ability to use cedar as a refuge in winter, when the deciduous apple trees have no leaves on them for the fungus to inhabit.