My yellow Knockout roses are blooming like crazy, but they could be doomed. Five other KO bushes have already died, victims of the dreaded Rose Rosette disease. Until recently, I didn't know this scourge existed, but it's been around since the 1940s. It was rarely seen until decades later, when Knockout roses became an ubiquitous--and beloved--feature in gardens. And why not?
Knockouts have a long, lush blooming season, creating a dazzle of color from spring till frost. They are resistant to black spot. However, they aren't immune to Rose Rosette, also known as Witches' Broom Disease--an apt name because the infected cane resembles a long, tapered broom. The petals are red and misshapen, as if a sorceress has cast an evil spell.
Rose Rosette disease is caused by a virus, one that is transmitted by a woolly mite. These microscopic bugs are impervious to miticides. Worse, Knockouts aren't the only victims. Any rose cultivar is susceptible. And, according to our local UT extension agent, the disease is spreading. So far, no cure exists. The mites can be dispersed by wind, rain, and pruning shears.
What are the symptoms?
* deformed canes: long, wispy branches that are markedly different from the rest of the bush, resembling a narrow witch's broom.
*red leaves--abnormally shaped; the growth stays red and distorted
*buds are deformed or don't unfurl; stunted blooms
*an abundance of thorns (and the thorns can be somewhat softer than normal).
Sometimes it's difficult to nail down a diagnosis because non-infected roses produce red growth. The different is, healthy leaves and canes will turn green, and the small leaves will acquire a normal shape. Diseased canes and leaves remain red, apparently.
Normal thorns aren't bunched up together.
Here, you can see a small witch's broom, clustered with small, misshapen leaves. Even though many leaves have turned green, the shape and size are distorted.
When Knockouts are planted in hedge rows, they make a statement, but the proximity reduces air flow, creating a hotbed of trouble. Many plant pathologists believe that the over-planting of Knockouts led to the rise in this disease.
I'm keeping a close eye on other Knockouts.
If one part of the bush is infected, the whole bush is infected, root and all. I have read about cases where the diseased cane are cut off (and burned). I spoke to a plant pathologist, and he didn't believe that pruning, even in the early stages of the disease, would save the bush. His recommendation was to yank up the bush, root and all, and torch the whole mess, a botanical version of a Salem witch hunt. However, my yellow roses are so lush and full, I might clip the offensive canes.
It goes without saying that all clippers should be disinfected in 10% bleach. Our extension agent recommends removing the soil after a diseased bush has been pulled up. He doesn't recommend planting a new bush in the old spot, as the mites can live in the soil, even after you've removed some of it. (I've also read reports that you can plant a new rose after a certain amount of time, but this wasn't presented as an option at the workshop. The consensus was: wait until a cure is found.)
If you suspect that one of your roses has RRD, call your local extension agency and follow their instructions. They keep track of statistics, and the disease might not even be in your county. Your rose may not have this disease. Extension services are researching the virus. Call your agent for more information or check their Facebook page. Sometimes the state agencies do updates on diseases in your area. Also, check newspapers for Rose Rosette seminars.
Example of a Knockout that succumbed to RRD.
I can't decide if this pretty rose has RRD or if the new growth will eventually turn green. I'm waiting and watching.
Remember--it might not be a mite!
When I attended another workshop, I jotted down a few notes about basic rose care. Beyond that, I'm no help at all. For me, it's a work in progress. If you're a seasoned gardener, just skip this part. If you're a newbie like me, I hope my notes will be helpful.
Happy roses need:
* 6 hours of sun daily
*pH of 5.5 (this requires a soil sample test)
* regular pruning
* regular watering and feeding to reduce stress
*detective work: check leaves and canes for signs of disease
While RRD isn't a new disease, it's new to me. I know that many of you have gorgeous roses, and I'd appreciate any advice or tips. Have you dealt with RRD? And, when it comes to basic rose care, how often do you fertilize? When you prune or weed, do you wear protective gear (long, long gloves, for example?).