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Q What is cedar-apple rust disease?

— Lindsey Janda

A Brian Hudelson, director of the Plant Disease Diagnostics Clinic at the University of Wisconsin-Madison:

Cedar-apple rust is one of several plant diseases that are all caused by different species of a fungus called gymnosporangium. All of these diseases are referred to as gymnosporangium rust diseases.

Sometimes you see the fungus infect hawthorn fruits, and the normally red, round berries become spiny and discolored. On apple and crab apple leaves you might find little jellyfish-like tendrils that are spore-producing structures.

This fungus is interesting because even though it can be found on an apple, crab apple or hawthorn plants, they’re not the only plants it potentially infects.

In order for this organism to complete its entire life cycle, it needs another host. Spores from infected apple or hawthorn trees spread to juniper trees called red cedar, hence the name cedar-apple rust or hawthorn-apple rust.

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In the springtime, you can see huge gelatinous orange masses that form on junipers. They look like alien creatures, big orange tentacles on the branches. Those big orange structures on the junipers are producing spores that reinfect the apples, crab apples and hawthorns.

So the fungus bounces back and forth between two hosts.

There’s usually not a big impact on either host. The disease has more cosmetic effects than health effects.

If you’re setting up a new landscape in a yard, try not to combine things like junipers and crab apples. You want to keep the two hosts as far apart as possible, at least 100 feet, if not more.

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Blue Sky Science is a collaboration of the Wisconsin State Journal and the Morgridge Institute for Research.

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