I Have Incense Cedar Rust!

Having moved to Vancouver, Washington two years ago, I’m running into some new and interesting things. Like this Pacific tree frog that amazed us about 5 feet up on one of our tomato plants. I’m surprised it survived the weeks of toxic smoke from forest fires that we had earlier in the fall.

But also interesting tree diseases. In front of our house was planted a flowering pear, Pyrus calleryana. It is one of the most common street trees around here, which is unfortunate because it is a rather ugly tree with lots of issues. The tree branches wildly and soon develops V-shaped forks with included bark. Then comes stem and branch decay. Frequently powdery mildew enhances the ugliness . . . . But I digress.

Gymnosporangium blasdaleanum on shoot and petioles of Pyrus calleryana (flowering pear). Tissues are swollen, distorted, and colored red to orange. Aecia (0.2-0.3 mm diameter) can be seen after peridia are mostly gone. A powdery mildew (Podosphaera leucotricha) can also be seen on the two newest leaves of the shoot. May 26, southern Washington, USA.

Early this summer, on the tree in front, appeared a rust unlike any I recall seeing before. The orange, swollen, distorted portions of leaves and new shoots bore short, cup-shaped aecia, almost like they were on a stroma. Though I am very sadly without a microscope these days, a little looking around in books and online led me to conclude that it was none other than incense cedar rust, also known as ‘incense cedar broom rust’ and, when the focus is on commercial pears, ‘Pacific coast pear rust’. The pathogen is Gymnosporangium blasdaleanum.

As you can guess from the disease names, it is a heteroecious rust, alternating between incense cedar (Calocedrus decurrens), where telia and basidia are produced, and a variety of trees and shrubs in the family Rosaceae, with spermogonia and aecia. I didn’t see the spermogonia, but they are inconspicuous and of course appear before the aecia.

By sheer luck, we have a young incense cedar in the back yard. It’s close to 2 m tall now and has very dense, full foliage (it is so well cared for ;-). I’ve been watching it for the rust, but so far no joy. If it is there, it should be easier to find in the Spring. If not, maybe I’ll shower it with aeciospores next year!

Elsewhere is a full presentation of the disease.

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