Tuesday, May 3, 2016

Game of Thornes: Rose Rosette Disease

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. 

If you lop off an infected cane, then continue pruning the bush, the contamination can spread. 

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

*red stems

*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?).

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  1. Never dealt with RRD, it looks like you'd probably have to rip up the whole plant. Loved the title - perhaps this disease was cast by the red witch. (GoT fan here)

  2. Good to know. We have several knockouts!

  3. Several years ago a mildew was attacking impatiens plants. I spread easily like this RRD too. They recommended removing the plants and putting them in the garbage- not to put them in a mulch pile. They also recommended removing the soil too because the spores can stay alive and carry on to the next year. I admit that when I trim my plants I rarely clean my clippers but it would be wise. I haven't put any knockout roses in so I am not familiar with this problem as of yet. What a shame if you loose some or all of yours. Every year there is something that challenges the gardener.

  4. I have never heard of this disease but will be on the look out for it. I too am a work in progress when it comes to growing roses and I have had some old roses dying which is unusual but I am quite certain it was not this disease. I'm very glad to have read all the information that you have shared, very informative.

    Carolyn/A Southerner's Notebook

  5. I have never heard of this disease but will keep my eyes open for it. I don't have any Knockout roses, but I have some that have gone back to the original rose stock that they were started from. Good luck with your research and please let us know if you find more information on this.

    Happy Mothers Day to you.


  6. I have trouble with my KO roses and little holes on the leaves. I am guessing some bug is nibbling them...and so far not sure if my rose spray is working. Boohoo. sheila

  7. Crikey! My husband planted Knockouts after many of my hybrid teas went by the wayside. We've had them for quite a few years and have only had to deal with Japanese beetles and I've never heard of RRD and I hope not to have to deal with it. Good luck with your plants!

  8. Great information. I've just learned a lot. I've 2 roses, a climbing rose, and one of another variety. I will be on the lookout. Thank you for the info, this is new to me. The pictures are a great help.
    On a different note, Happy Mother's Day.

  9. Michael, we have hybrid teas and knockouts. We haven't had rrd on any of our knock outs, but we have had to remove two hybrid teas over the years. Your photo doesn't look like rrd did on our hybris, but I would definitely seek a professional decision as soon as possible. I am a firm believer in remov9and destroying if you have it.

  10. The disease is a real problem where I live, Michael Lee, and three of my knock outs have it! They'll be removed soon. I enjoyed them while they lasted.

  11. Aaack! How horrid... I only have two KO roses that I planted last year by my shed. I noticed have a couple of canes with some spindly red growth. I'll have to keep my eyes on them. It seems everything that is over planted succumbs to a disease. Our neighbors lost their stand of Leyland Cypress and I remember the Bradford Pears dying a number of years ago from something as a result of over planting too.

  12. Thank you for posting so much information about RRD. I went to a seminar here in Okc at the botanical garden and the speaker talked about how the disease is spreading and she advised against planting knockouts. Made me sad..because I have quite a few in my garden and so far only one has RRD. I pruned it last fall and since it was a more expensive tree form that was in a pot I moved it out behind our garden to the greenbelt. It has come back beautifully this spring and was loaded with blooms but it did have one branch with the witches broom deformity and I cut that part off. I guess it will eventually succumb, but right now it looks so pretty I don't have the heart get rid of it.