Cane blight claimingroses in the Treasure Valley

April 12, 2013 

If you have rose shrubs in your yard, inspect them carefully. Rose cane bacterial blight is back and has killed a number of roses.

If you see dark red, purplish or black spots on rose canes, scrape through tender bark with your fingernail. If it's dry wood under the cane "skin," it's a sign of winter kill. If it's wet below the outer skin, it's bacterial cane blight.

If your rose is so infected that you can't prune at least a few inches below the blighted areas, your rose should be removed and sent to the landfill. Do not compost these infected shrubs.

Rick Freitag, in charge of the Julia Davis Park rose garden, noticed some shrubs were infected, so he and his crew set to work pruning at least two to three inches below evidence of blight, disinfecting their pruners with Lysol after pruning each shrub. They also used a copper-based spray.

Dr. Krishna Mohan, plant pathologist with the University of Idaho Parma research station, advises spraying a copper-based chemical in fall, after half of the leaves have dropped, and spraying again after the remaining leaves have dropped. Freitag said they didn't spray last fall because the leaves hadn't dropped.

Since the bacteria enter the canes at leaf scars and pruning wounds, it makes sense to wait until at least some of the leaves fall. If you pull the leaves off, you create more of an open wound that would admit bacteria than if you let leaves drop naturally.

Anju Lucas, head of perennials at Edwards Greenhouse, said several people have lost roses to the bacterial blight again this spring. She added that none of the dead roses brought to her have been David Austin English roses. Sandie Ford, rose grower and member of the rose society, said she had had no bacterial cane blight on her miniature roses. Apparently other varieties are more vulnerable.

This blight is a variation of the Pseudomonas syringae bacterial infection that has become common in this Valley but varies in intensity from year to year. Symptoms start as reddish-brown areas, turning purple-black and necrotic (dead). Be careful you don't spread the disease with pruning tools. Disinfect them before you touch another shrub.


You can prevent them or get rid of them after they arrive. Prevention is easier.

You can prevent weeds with thick mulch so their seeds don't have a chance to germinate. First, you need to remove visible weeds, use chemical germination prevention, or deny weeds the sunlight or moisture they need for their seeds to germinate.

You can deny them the proper dose of sunlight by using groundcovers, for instance. Those may be sweet potato vines, succulent vines, lamium, creeping thyme or even lawn substitutes. One thing to consider when selecting ground covers is whether there will be foot traffic on the ground cover. Not all can withstand that abuse.

Black weed-blocking cloth is available from garden centers, but it may be difficult to use in tight spaces.

Does the groundcover you're considering come in colors? If it's to be placed in a shady setting, will it turn the area even darker or will it lighten things up? Is it to cover a small area or a large one?

One groundcover variety that is colorful and variegated is Houttynia cordata 'Chameleon.' Some gardeners call it "hootenanny," and swear at it, not by it. Yes, it is hard to control, but it is edible. In Andrea Nguyen's "Into the Vietnamese Kitchen," she says it has a "slightly sour, fishy flavor. Some people love the tanginess, while others are put off by the unusual taste." It's also called vap ca and other Vietnamese names that require accent marks. She likes it with "boldly flavored grilled meat," and refers to it by a common name, "fish mint," although she doesn't specify its use on fish.

Lamium, or dead nettle, has in pale yellow ground-hugging leaves that can brighten a dark area of your garden. Some groundcovers, such as creeping myrtle (periwinkle), grow fairly tall (up to about 14 inches), and have pale purple flowers. Another hardy ground cover is ajuga, with blue-bronze leaves and insignificant flowers. Ice plant works, too.

Send garden questions to or Gardening, The Idaho Statesman, P.O. Box 40, Boise, ID 83707.

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