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Blue Sky Science: What is cedar-apple rust disease?

Lindsey Janda

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What is cedar-apple rust disease?

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, crabapple 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.

In the springtime around Madison you can see huge gelatinous orange masses that form on the 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 and crabapples and hawthorns.

So the fungus bounces back and forth between two different hosts.

There’s usually not a big impact on either host. The disease has more cosmetic effects than health effects. But this year, several hawthorns around the UW-Madison campus had multiple infections per leaf. This caused a bit of leaf browning, and really severe disease can cause leaves to fall off the trees earlier than they normally would.

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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