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Keywords: galls, Cytisus scoparius, Rosa nutkana, Insect-plant relationships, Invasive plants--Control

I work on a restoration site and this fall I have been noticing weird fuzzy growths on many of the Nootka roses (Rosa nutkana) growing there. Do you know what is causing this? And is it related to similar strange growths on all of the Scotch broom? In the case of the broom, it actually kills them completely—they turn brown or black, and their roots are pretty much non-existent, which makes them very easy to uproot (which is what we are trying to do). I just don’t want to lose the roses or other desirable plants on the site.


What you are describing on the roses sounds like mossy rose gall (Diplolepis rosae). Washington State University Extension's HortSense page says that these galls which are caused by cynipid wasps will not harm the host plants. You could picky them off the roses, but that seems impractical in a restoration site, and besides they are fairly benign and attractive curiosities. The particular species of cynipid wasps which cause it are unlikely to affect plants which are outside the rose family.

Your other question about dying Cytisus scoparius (Scotch broom) is especially interesting. I think what you are seeing on those browned and blackened plants is also a gall, caused by the Scotch broom gall mite (Aceria genistae). This insect is apparently on the cutting edge of controlling invasive broom. According to this informational page from University of California Statewide Integrated Pest Management Program, the mite was first seen on broom plants in Tacoma, WA and Portland, OR in 2005. It has since spread through the Pacific Northwest. An abstract of an article entitled "The Scotch Broom Gall Mite: Accidental Introduction to Classical Biological Control Agent?" (J. Andreas et al.) appeared in the 2011 XIII International Symposium on Biological Control of Weeds. Studies are underway to see if the mite affects non-target plants such as lupine. For now, you can rejoice in the fact that the mites are curbing the growth and reproduction of the broom, and making your work a little bit easier!

Date 2017-11-03
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August 01 2017 12:36:01