Margaret Lauterbach

Your roses have many enemies, so keep a close watch

Margaret Lauterbach
Margaret Lauterbach

How are your roses doing? We’ve learned to keep a lookout for their health, whether they’re hybrid teas, miniatures, ramblers or climbers, any of which could be a victim of a devastating disease.

Long-time rose growers in this valley are aware that a new-to-us rose disease appeared in the Treasure Valley in 1996, and in 2011 caused a great deal of damage. That year the rose garden in Julia Davis Park lost 500 shrubs to this disease. Krishna Mohan, then plant pathologist with the University of Idaho’s Parma station, looked into the problem and identified it as Pseudomonas syringae, unknown pathovar. Apparently the disease occurs elsewhere in the country, but people have just shrugged and replaced their shrubs. Krishna wrote a paper on it and came up with a way to control it: Phyton 27, applied at the right time.

It’s also known here as bacterial cane blight. Since it apparently enters canes at the site of wounds, standard advice for rose growers here is not to prune roses in autumn unless a cane is long and will blow in winds, wounding other canes. We used to think the disease appeared sometime in winter or early spring, but careful observation in the rose garden indicated that it might be infecting canes in mid- to late summer, and we didn’t notice until much later. Rose growers can manage this disease with timely application of Phyton 27, a copper sulphate-based pesticide. If you begin to see red blotches on canes, spray if outdoor temperatures are amenable to that treatment.

David Austin roses appear to be resistant to this disease, according to Anju Lucas, head of perennials at Edwards Greenhouse. Those English roses are a cross between once-blooming antique roses and modern multiblooming shrubs. Austin, who passed away last month at age 92, bred shrubs for vigor, fragrance and abundant, voluptuous blossoms. The robust health of those shrubs apparently protects them from this bacterial disease.

I know it’s still winter, but look carefully at your rose shrubs. If you see black canes, scrape with a fingernail to expose the material under the black coat. If it’s dry wood, at least that cane or part of a cane has been killed by cold. If it’s reddish in color and oozes moisture, it’s due to bacterial cane blight. Prune off that cane so that only healthy inner colors are exposed (light tan and green), and spray if the pesticide instructions allow in the weather we have when you discover the problem.

If you think you are going to have to replace rose shrubs or want to enlarge your rose bed, you can save 10 percent of the cost at Edwards Greenhouse by selecting your new shrubs before Feb. 15. They’ll keep your shrubs in their greenhouse until it’s time to plant them. Other locally owned nurseries may offer similar discounts. I know Edwards has nearly 200 different varieties of roses, including nearly 40 David Austin-bred rose shrub and climber varieties. Many of the varieties carried by Edwards are on their own roots, so if the shrub appears to be killed and comes back from the roots, you’ll get the rose you planted. If it’s a grafted shrub, you’ll find you’re growing something different.

Austin’s efforts have brightened ornamental beds around the world since he started this career in the 1950s. His passing doesn’t mean the end of David Austin roses, since his son (also David) and grandson, Richard, are active in the business.

El Nino tips

If you’re new to this area, we are having an El Nino winter, with mild temperatures and less precipitation than usual. In spring, we usually have cool, breezy days that make outdoor activities less than pleasant. It’s also our rainiest season, a great time to transplant seedlings outdoors. Make sure the soil is warmed to at least 50 degrees, preferably 60, and wear rain gear. If you transplant on a drizzly, overcast day, you may get away with a shortened hardening-off (acclimating) time for seedlings.

Don’t walk on wet garden soil, for it compacts, expressing oxygen out of the soil. If soil is wet you can go into the garden by walking on boards, so that your weight is more widely distributed than if it’s footprints alone.

Send garden questions to melauter@earthlink.net or Gardening, The Statesman, P.O. Box 40, Boise, ID 83707.

 

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