juniper hawthorn rust (49901 bytes)

Juniper-Hawthorne Rust

By Nancy Downs, Colorado Master GardenerSM, Colorado State University Cooperative Extension, Denver County

After The Rains Came

It was the Sunday the rain ended. We were headed down Montview, enjoying the sunshine and walking the pooch, when, lo and behold! There, oozing all over a juniper hedge, were great gobs of slimy, orange, gelatinous goo. It was disgusting! It was revolting! It was...cool! if you happen to be a horticulturist.

What the Heck Was That?

I had seen it once before. That was back in 1993. I was working at the Denver Botanic Gardens, and was privy to all manner of disgusting creatures and gross disease symptoms. We’re not talking magazine pictures here. We’re talking business. Sometimes, it’s not a pretty picture.

So, back to the orange goo. My friend recoiled in fear, but I recognized it for what it was: ‘ah, hah,’ I exclaimed, ‘it’s the spore horns of juniper-hawthorn rust!’

Plant Disease Triangle

What makes the fungal disease juniper-hawthorn rust so entertaining to plant pathologists, other than that its most obvious symptom looks like something you would find in a Halloween fun house, is that it requires not one but two host plants to work its black magic.

See, in order for any disease to occur, all 3 factors of the plant disease must be present:

1) a susceptible host plant,

2) the disease organism, and

3) a conducive environment.

But in the case of juniper-hawthorn rust, it takes two hosts to tango. The primary host is a juniper (either J. scopulorum or J. virginiana will do) and the alternate host is a hawthorn (a mountain ash (Sorbus, spp.) or an apple/crabapple (Malus, spp.) works, too). Given those two planted near each (the susceptible plant hosts), an infected juniper (the presence of the disease organism) a good spring rain (the conducive environment) and bingo! Galls on the juniper become active and spew out billions of disease spores which are carried by the wind over to the hawthorn to complete the cycle.

The obvious defense against the disease is to avoid planting the two susceptible hosts near each other. However, in neighborhoods where every house seemingly has one or the other, it is kind of beyond your control. For the homeowner with the revolting hedge along Montview, I’d recommend spraying - the hawthorn, not the juniper - every seven to ten days beginning at blossom time. Then next year, to break the cycle, I’d recommend removing the galls from the juniper in March or April before the gooey spore horns emerge.

rust spore horns on juniper (43461 bytes)

Of course, if they took that advice, they would risk missing out on a pretty cool sight.

For more information on Juniper-Hawthorne Rust, see CSU factsheet 2.904.

Photographs of Juniper-Hawthorne Rust on Juniperus communis by Judy Sedbrook.

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Date last revised: 01/05/2010