First published June 5, 2008

The aliens have landed. Bright-orange, jelly-like, horned monsters the size of baseballs have invaded the junipers in our gardens and are threatening to attack apple trees!

OK, perhaps that sounds a little too much like Orson Welles’ faux newscast that he “adapted” (meaning, stole) from the classic novel The War of the Worlds. Nevertheless, some strange-looking invaders have indeed landed on hundreds of junipers across the region, and they do look like something from another planet.

What’s causing all this flap? Cedar apple rust (Gymnosporangium juniperi-virginianae), a fascinating fungus that attacks junipers and apples. When the weather is dry, this fungus’ life cycle isn’t particularly noticeable. But give cedar apple rust a couple of days of wet and reasonably warm weather and this inconspicuous fungus develops a mass of strange, pointy horns with a texture reminiscent of orange Jell-O…gone terribly, terribly wrong.

It takes two
One of the reasons that cedar apple rust is so interesting is because it’s a heteroecious fungus, meaning it requires two hosts to complete its reproductive life cycle. For cedar apple rust fungus, that entails growing strange, gelatinous, baseball-like structures on junipers during wet weather and then shedding spores that hope to find their way to apple trees. If the rust spores find apple leaves or fruit that are moist for several hours, those spores can then germinate and develop in the apple tissue. However, because the rust fungi are heteroecious, the shed spores from the apple tree cannot continue on and invade another apple tree. Instead, the spores must find another juniper to complete their lifecycle before being able to attack again.

Now if you’ve connected the dots, you’ll have figured out the solution to keeping your apple trees free of cedar apple rust: get rid of your junipers. Junipers need apples and apples need junipers to perpetuate the rust fungus, which is why commercial apple growers keep junipers as far away from their orchards as possible. Unfortunately, separating plants from each other becomes a more difficult challenge in the city. Because rust spores have been known to travel as far as a kilometre, it’s no surprise that the fence between your yard and your neighbour’s doesn’t intimidate them. Even in the case of commercial orchards, many growers resort to spraying on a weekly basis with a fungicide (from the time of flowering to fruit set) to prevent rust from becoming established.

The good news is that cedar apple rust hasn’t really caused much of an apple problem in our region; perhaps our typically drier prairie weather and lower humidity has kept it at bay. Just the same, I have seen enough infested junipers this year to feel the need to offer some preventative measures, starting with a recommendation to forget the sprays. Instead, I suggest pruning out the swollen, jelly-like growths as soon as you see them on your junipers. Be particularly vigilant in the spring after a few days of showery weather, and keep the pruners handy. To dispose of the fungus, simply bag it and toss it. By keeping it out of the compost bin, you prevent the spores from travelling any farther.

Last but not least
One other thing that you might want to do is show your friends and kids this remarkably impressive alien invader. Once one gets beyond an initial fear and loathing of this cold, clammy, oozing ball, your revulsion might be tempered with just a touch of marvel.