What Is Rose Rosette Disease: Control Of Rose Rosette And Witches Broom In Roses


By Stan V. Griep
American Rose Society Consulting Master Rosarian – Rocky Mountain District

Rose Rosette disease, also known as witches’ broom in roses, is truly a heartbreaker for the rose-loving gardener. There is no known cure for it, thus, once a rose bush contracts the disease, which is actually a virus, it is best to remove and destroy the bush. So what does Rose Rosette disease look like? Keep reading for information on how to treat witches’ broom in roses.

What is Rose Rosette Disease?

Exactly what is Rose Rosette disease and what does Rose Rosette disease look like? Rose Rosette disease is a virus. The effect it has upon the foliage brings about its other name of witches’ broom. The disease causes vigorous growth in the cane or canes infected by the virus. The foliage becomes distorted and frazzled looking, along with being a deep red to almost purple in color and changing to a brighter more distinct red.

The new leaf buds fail to open and look a bit like rosettes, thus the name Rose Rosette. The disease is fatal to the bush and the longer one leaves it in the rose bed, the more likely it is that other rose bushes in the bed will contract the same virus/disease.

Below is a list of some of the symptoms to look for:

  • Stem bunching or clustering, witches’ broom appearance
  • Elongated and/or thickened canes
  • Bright red leaves** and stems
  • Excessive thorniness, small red or brown colored thorns
  • Distorted or aborted blooms
  • Under-developed or narrow leaves
  • Perhaps some distorted canes
  • Dead or dying canes, yellow or brown foliage
  • The appearance of dwarfed or stunted growth
  • A combination of the above

**Note: Deep red colored leaves may be totally normal, as the new growth on many rose bushes starts out with a deep red coloration and then turns to green. The difference is that the virus-infected foliage keeps its color and can also become mottled, along with vigorous unusual growth.

What Causes Witches Broom in Roses?

The virus is believed to be spread by tiny mites that can carry the nasty disease from bush to bush, infecting many bushes and covering much territory. The mite is named Phyllocoptes fructiphilus and the type of mite is called an eriophyid mite (wooly mite). They are not like the spider mite most of us are familiar with, as they are far smaller.

Miticides used against the spider mite do not appear to be effective against this tiny wooly mite. The virus does not appear to be spread by way of dirty pruners either, but only by the tiny mites.

Research indicates that the virus was first discovered in wild roses growing in the mountains of Wyoming and California in 1930. Since then it has been a case for many studies at plant disease diagnostic labs. The virus has recently been placed into a group known as Emaravirus, the genus created to accommodate a virus with four ssRNA, negative-sense RNA components. I won’t go further into this here, but look up Emaravirus online for a further and interesting study.

Control of Rose Rosette

The highly disease-resistant knockout roses seemed to be an answer for disease problems with roses. Unfortunately, even the knockout rose bushes have proven to be susceptible to the nasty Rose Rosette disease. First detected in the knockout roses in 2009 in Kentucky, the disease has continued to spread in this line of rose bushes.

Due to the huge popularity of the knockout roses and the resulting mass production of them, the disease may well have found its weak link to spreading within them, as the disease is readily spread through the grafting process. Again, the virus does not appear to be able to spread by pruners that have been used to prune an infected bush and not cleaned before pruning another bush. This is not to say that one does not need to clean their pruners, as it is highly recommended to do so due to the spread of other viruses and diseases in such a manner.

How to Treat Witches Broom on Roses

The best thing we can do is to learn the symptoms of the disease and not buy rose bushes that have the symptoms. If we see such symptoms on rose bushes at a particular garden center or nursery, it is best to inform the proprietor of our findings in a discreet manner.

Some herbicide sprays that have drifted over onto rosebush foliage can cause foliage distortion that looks very much like Rose Rosette, having the witches’ broom appearance and the same coloration to the foliage. The tell-tale difference is that the growth rate of the sprayed foliage and canes will not be extremely vigorous as the truly infected bush will be.

Again, the best thing to do when you are certain a rose bush has the Rose Rosette virus is to remove the bush and destroy it along with the soil immediately around the infected bush, which could harbor or allow overwintering of the mites. Do not add any of the infected plant materials to your compost pile! Be vigilant for this disease and act quickly if observed in your gardens.


How to Manage Pests

Witches' broom

Witches' brooms typically are dense clusters of twigs or thickened stems that develop on the branches of woody plants. Affected leaves and shoots may become discolored, distorted, and dwarfed.

Identification

The specific appearance of witches' brooms varies with the particular cause and host plant(s). Causes and their hosts include

  • cedar, cypress, and juniper rusts, Gymnosporangium species
  • dwarf mistletoes, Arceuthobium spp. that infect conifers
  • fasciation, which is caused by microorganisms and unidentified causes possibly including genetic factors numerous plant species are uncommonly affected
  • glyphosate herbicide injury and certain other herbicide phytotoxicities
  • powdery mildew (PDF) fungus of oaks, Cystotheca (=Sphaerotheca) lanestris
  • rose rosette disease, a mite-vectored virus
  • western gall rust, Endocronartium (=Peridermium) harknessii, a pine disease
  • yellows, or phytoplasmas that mostly affect herbaceous plants
  • zinc deficiency and possibly other nutritional disorders on most any woody species

Life cycle

The biology and development of witches' brooms varies with the particular situation. Oaks, for example, commonly develop witches' brooms after spring weather is cool and moist and in coastal areas where fog occurs during the growing season. The witches' brooms become apparent on oak terminals by late spring or early summer.

Rose rosette disease that causes witches' broom in roses is caused by an RNA virus in the genus Emaravirus. The virus is spread by a rose-feeding, wind-transported eriophyid mite, Phyllocoptes fructiphilus. This virus is common in the United States east of the Rocky Mountains but in California has only been reported in the Fresno area. Most rose viruses do not threaten plant survival they only slow plant growth and mar the aesthetic appearance. Rose rosette virus, however, generally kills infected hosts.

Solutions

Provide plants with proper cultural care and a good growing environment to keep them vigorous. Where they are aesthetically objectionable, prune out witches' brooms at least several inches below where they form. This is commonly the only available management strategy, such as for most fasciations and western gall rust.

Removing affected roses and replanting using clean plants is the only management likely to be effective for rose rosette disease. Infected roses cannot be cured. Control of the eriophyid mite vector of this virus can reduce the risk of the pathogen spreading to uninfected roses. But good control of eriophyids using miticides is difficult to achieve in most gardens and landscapes. For more information see Five Ways to Manage Rose Rosette Disease and Rose Rosette Disease Demystified (PDF) .

Adapted from the publications linked above and Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).


Dense, bushy foliage caused by juniper rust.


Small, yellowish almond leaves in tufts due to zinc deficiency.

Statewide IPM Program, Agriculture and Natural Resources, University of California
All contents copyright © 2021 The Regents of the University of California. All rights reserved.

For noncommercial purposes only, any Web site may link directly to this page. FOR ALL OTHER USES or more information, read Legal Notices. Unfortunately, we cannot provide individual solutions to specific pest problems. See our Home page, or in the U.S., contact your local Cooperative Extension office for assistance.

Agriculture and Natural Resources, University of California


What causes rose rosette?

Researchers have suspected that mite damage, phytoplasma, or a virus causes rose rosette disease.

  • Eriophyid mites

Transmission experiments using eriophyid mites collected from asymptomatic roses did not result in appreciable rose rosette symptoms (Armine et al,1988), making it unlikely that the eriophyid mite’s feeding causes the damage all by itself.

Phytoplasma (a specialized group of bacteria that infect plants) has long been considered a major candidate for the cause of the disease. Research articles from Poland (Kaminska et al, 2001), India (Chaturvedi et al, 2009), and China (Gao et al, 2008) demonstrated the presence of a phytoplasma (from the aster yellows family) causing rose rosette-like symptoms But, there are no reports of phytoplasmas in symptomatic roses in the United States. In an experiment where symptomatic plants were treated with antibiotics rose rosette symptoms persisted (Epstein and Hill,1995). Antibiotics should have killed or suppressed the phytoplasma

In 2011, a research group from the University of Arkansas detected a new virus, an Emaravirus negative strand RNA virus), in symptomatic roses. The virus occurred in 84 out of 84 symptomatic plants (Laney et al, 2011). This study also resulted in a genetic test to detect the virus. However, the procedure can be tedious.

Several diagnostic clinics, including the Texas Plant Disease Diagnostic Lab, are testing a modified,easier-to-use detection method. The Oklahoma Plant Disease and Insect Lab and the Texas Plant Disease Diagnostic Lab are two National Plant Diagnostic Network-affiliated labs that can test for the rose rosette virus using PCR methods.


What we do not know:

  • How long you must wait before planting new roses back into the same soil? I hear varying recommendations from within a couple of weeks to one or two years. Honestly, if this were attacking my roses, I’d put any new rose plantings in fresh soil somewhere else in my garden, at least for now.
  • Specifically, which varieties are vulnerable? Certainly the Knockout® family has been hit the worst, but that’s probably since they have been planted so extensively, often in masses in large beds. Other roses are also susceptible. Research is underway to determine which types are most and least likely to develop the problem.
  • Where will we find a remedy? Breeders, I am told, are working with the genetics of their roses, even to the level of work within laboratories, to find resistance. I’m not an expert in this field, but I have heard no mention of a consumer miticide to control the mites. Honestly, I’m not sure anyone knows where we will find help for our roses, but the nursery industry has invested millions of dollars into the research projects.

Rose Rosette Disease

Rose rosette is a viral disease that has been increasingly common on many rose species and cultivars in North America. The disease causes growth distortions, resulting in a range symptoms that may include: rapid shoot elongation, ‘witches broom’ growth pattern (tight clustering of leaves and small stems around a parent stem), deformed leaves deep red in color, malformed flowers, canes that produce no flowers, unusually thick or succulent stems, and lack of winter hardiness. The most recognizable symptom is a proliferation of thorns, found on stems in much greater density than normal. These numerous thorns are often succulent and more pliable than normal thorns, and may appear red or green in color.

The disease is caused by a virus that is spread by tiny arachnids called eriophyid mites which, at 1/200 th of an inch long, are tiny even by mite standards, and are light enough to be carried from plant to plant by the wind. The mites feed off plant sap in young stems, transmitting viral particles in the process. Humans can also transmit the virus by contaminated pruners and through grafting infected material. The virus moves through the entire plant quickly, so pruning off only the symptomatic tissue will not help. Like all viral infections, there is no cure for infected plants, and most roses will succumb to the disease in several years.

In principle, controlling the vector could inhibit the spread of the disease, but there are currently no effective miticide or insecticide treatments for controlling eriophyid mites: application of insecticides would only harm beneficial insects. If you observe symptoms of rose rosette disease, the best control strategy is simply to remove the infected plant(s) from your yard. Bag the infected plants and dispose of them, or burn them if burning is permitted in your area. As long as infected roses remain in your landscape, they can be a source of inoculum for future infections.

The wild multiflora rose (Rosa multiflora), which was brought in from Japan for breeding ornamental rose rootstocks, is particularly sensitive to rose rosette disease. Muiltflora rose was once widely planted for erosion control and for hedging, but has now become invasive in much of North America and may be a source of inoculum for the rose rosette virus. Multiflora rose can be removed from your property, but make sure it is not the similar, native Carolina rose (Rosa carolina), which is an important pollen and food source for native wildlife.

All ornamental roses are susceptible to rose rosette disease, including the popular Knock Out® cultivars. Research in breeding resistant varieties is ongoing. Fortunately, the virus only seems to infect ornamental roses, so you may plant many other shrub species in your former rose bed should you have an infestation.

If you think some of your roses have rose rosette disease, send a photo to your local N.C. Cooperative Extension horticulture agent. For more information, visit the Rose Rosette website.


Watch the video: Rose Rosette Disease Witches Broom Virus


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