Ever seen those odd-looking, broom-like distortions in a tree? Perhaps it’s one of yours or in a tree nearby. What are these and do they cause any harm? Keep reading to find out more about the signs and symptoms of witches’ broom disease.
What is Witches’ Broom Disease?
Witches’ broom is a symptom of stress found in woody plants, mainly trees, but it also affects shrubs as well. This includes deciduous trees and shrubs as well as conifers and evergreen varieties. The stress results in a deformed mass of twigs and branches, which appear broom-like in appearance. As brooms were once fashioned together from bundles of twigs and since witches were presumed to be responsible for anything unusual, these abnormalities became known as witches’ brooms.
What Causes Witches’ Brooms?
Witches broom disease is not caused by witches, however. They’re actually caused by stress that is brought on by pests or disease. This includes anything from mites, aphids, and nematodes to fungi, viruses, and bacterial organisms (phytoplasmas).
In addition, parasitic plants like mistletoe, which cause stress to host trees, can lead to the formation of witches’ broom. Environmental factors may also be to blame and some are caused by genetic mutations.
Generally, the type of tree/shrub is a good indicator of its causal agent. For instance, pine brooms are commonly caused by rust fungus. Fungal infections can also affect cherry trees and blackberry bushes, forming broom growth. Peach trees and black locust can be affected by viruses that can result in witches’ brooms. Hackberry trees can get brooms as well, and these are normally caused by both fungus and mites.
Mites can also be responsible for witches’ broom in willow trees. Aphids are generally to blame for these deformities in honeysuckle shrubs, while phytoplasmas lead to the disease in ash and elm trees.
Witches’ Broom Signs and Symptoms
Witches’ broom can be easily identified by the dense clusters of twigs or branches, which grow from a central source—resembling a broom. It is best seen on deciduous trees or shrubs when they are not in leaf. While needled trees, like pines, may consist of denser needles.
There may be only one broom seen, or in some cases, there may be many. Some may be large, while others may appear quite small and less noticeable.
Witches’ Broom Treatment
Witches’ broom can occur for several months to several years, and while it may be unsightly to some people, it really poses no serious threat to the tree or shrub affected. There is currently no cure or treatment for witches’ broom. You can, however, prune out the broom growth several inches (5 to 10 cm.) below the point of its formation, if desired.
How to Cure Witch's Broom on Roses
Roses are one the most popular garden flowers. Part of their popularity is the large variety of roses available. All roses (Rosa spp., hardy in U.S. Department of Agriculture plant hardiness zones 2 through 11) require maintenance and care to keep them healthy and protect them from disease, like witch's broom, also known as rose rosette, which mainly affects multiflora roses that thrive in U.S. Department of Agriculture plant hardiness zones 5 through 8, but can affect other rose varieties. The virus has no cure, but you can take steps to control it.
The symptoms of RRD are variable, depending on the cultivar of rose (Table 1). In the early stages, plants may display one or several elongated stems that may have unusual red or yellow color (Figure 1). It is important to note that many rose cultivars have normal red color on new growth, so this should not be confused with RRD (Figure 2). With time, normal new growth will develop a green leaf color, while foliage damaged by the disease will remain discolored and distorted (Figure 3). Infected shoots may be more succulent and pliable than normal rose stems. Excessive prickles (thorns) may form, which are initially soft and pliable and later may harden (Figure 4).
Rose Gardening Diseases
While roses often do succumb to diseases of various kinds, some varieties of rose are less susceptible than others. So, if you are planting a new rose garden try to stick to resistant species if you can.
Rose gardening diseases are generally categorized as being either fungal or viral however, there are also several deficiency diseases roses can get.
Deficiency diseases relate to the main nutrients all plants require, namely:
- nitrogen deficiency where older leaves turn light green to yellow,
- phosphorus deficiency where the leaves tend to be undeveloped and dark, with a purple tinge,
- potassium deficiency where stems are weak and leaves turn brown and papery around the edges,
- magnesium deficiency where patches of dead tissue form between the veins,
- iron deficiency where the young leaves turn yellow between the veins (usually because the soil is too alkaline), and
- deficiencies in trace elements that also cause weak, stunted growth of plants.
Common rose diseases
While deficiency diseases may be fixed with proper feeding, you need to be constantly on the lookout for other common rose diseases. If you can identify the most common rose diseases you should be able to control them and prevent the rose from deteriorating unnecessarily. These are the ones to look out for:
Powdery mildew is a fungal disease that attacks the young shoots, leaves and buds of rose plants. It appears as a white or grey powdery covering on new growth, usually when hot, dry days are followed by cool, damp or wet nights. New leaves and shoots often look deformed. While old leaves might appear normal, if you look on the underside, the telltale fungus will be there too.
Downy mildew is not nearly as prevalent as powdery mildew, but it does occur in some coastal areas during humid months of the year. It causes rose buds to turn brown, while a white down-like substance forms on the underside of leaves.
Black spot is a very common rose disease that often appears after the first rains of the season. Like powdery mildew, black spot is a fungus. Severe attacks can cause all the leaves to fall off (defoliation). Older leaves – particularly those towards the bottom of the plant – develop purplish-black spots (hence the name), and the area around the spots often turn yellow.
Rust, another type of fungus, causes small, reddish-brown (rust colored) pustules to form on the underside of rose leaves, and it cause yellow spots on the upper surfaces. It is usually more severe in rainy weather or where humidity levels are very high. In severe cases, rust can cover an entire plant and cause rapid defoliation.
Canker affects the stems or canes of roses, usually in summer in areas where the weather is hot and dry. If the canker (which is a fungus) forms right around the stem, the leaves will wilt from this point and you will need to cut the stem off at this point. The first sign of canker is small black spores that look like little flecks forming on the surface of the cane.
Mosaic is a virus that causes bright yellow, wavy patterns to form on the leaves of some rose varieties. It also sometimes causes leaves to be weak and stunted.
Rosette and witches broom causes stems to get long very quickly and then some of the branches of the plant get thick, thorny stems. Shoots are often deformed and shorter than they should be. Leaves appear to be red and leaves are tiny and also deformed. The shoots look like a witch’s broom, which is where the name of the disease originates.
Crown gall results in great bulbous bits of tissue on stems near to the ground. They vary in size from quite small swellings to irregularly shaped lumps that are several inches wide. Plants are stunted and they normally won’t flower, or if they do the flowers aren’t very pretty.
How to manage diseases that affect roses
Once you have identified the symptoms of a particular disease, you can treat it accordingly. But as always, prevention is better than cure, and it always pays to plant and maintain roses in such a way that diseases will be avoided.
Powdery mildew can be controlled by spraying with a fungicide or dusting with sulfur. If the disease gets out of control, you will have to cut away the affected parts of the plant. To avoid roses getting powdery mildew plant those that are particularly susceptible to the disease in shady areas of the garden that will dry out slowly in the mornings. Don’t plant hedges or large shrubs too close to these roses because this has the effect of restricting air movement around them.
Downy mildew can be controlled by spraying with a fungicide.
Black spot can also be controlled with a fungicidal spray program that should start when new leaves appear early in spring time. Roses that are susceptible to black spot should be planted in a sunny location where the plants will dry out quickly after rain or watering. It is also good practice to avoid splashing rose leaves with water. If the plants do get this fungal disease, rake up the leaves and burn them to prevent further infection. Be warned that the fungus does survive winter in fallen leaves.
Rust can be controlled by preventing leaves from remaining wet for extended periods. Plant roses in full sun and make sure there is plenty of air around the plants. Don’t water in the evening. A fungicide may be used as a last resort.
Canker must be removed by pruning and the diseased stems should be burnt. Don’t try to compost any diseased plant material. Protect plants, particularly in the winter months when it is very cold, because canker usually enters through ‘wounds’ in the stem. If you keep your roses well fed and healthy, they aren’t likely to get this disease.
Mosaic can be severe and if it is, the plants that are infected should be removed and burnt. You can prevent mosaic by controlling insects, especially aphids that spread this particular virus. If the disease is mild, mix a solution of 10% chlorine bleach and water and dip affected bits into the liquid.
Rosette and witches broom is a mysterious disease that cannot be cured. The other problem is that nobody is quite sure what causes it. It may be that it is spread by insects, so control these, and if you see that plants are affected, dig them out and destroy them immediately. If the disease spreads, all your plants will die.
Crown gall cannot be controlled chemically or otherwise. Try to keep all your roses clean and healthy and if you spot diseased plants, dig them out and burn them.
Popular disease resistant roses
When you buy roses, always ask your nursery which are the most disease resistant species they stock. Different species go in and out of fashion, so you may not always find every variety you want. However, here is a list of roses that are said to be resistant to black spot, the most common rose gardening disease of them all, taken from http://ohioline.osu.edu/hyg-fact/3000/3063.html:
Resistant hybrid teas include:
Carla, Cayenne, Charlotte Armstrong, Chrysler Imperial, Duet, Electron, First Prize, Forty Niner, Granada, Miss All-American Beauty, Mister Lincoln, Olympiad, Pascali, Peace, Pink Peace, Portrait, Pristine, Proud Land, Smooth Lady, Sutters Gold, Tiffany and Tropicana.
Resistant floribundas and grandifloras include:
Angel Face, Betty Prior, Carousel, Cathedral, Europeana, Fashion, First Edition, Gene Boerner, Goldilocks Impatient, Ivory Fashion, Love, Mirandy, Montezuma, Pink Parfait, Prominent, Queen Elizabeth, Razzle Dazzle, Red Gold, Rose Parade, Sonia and Sunsprite.
Resistant shrub roses include:
All that Jazz and Carefree Wonder.
Resistant miniature roses include:
Baby Betsy McCall, Gourmet Popcorn, Little Artist, Rainbow’s End and Rose Gilardi.
Resistant Rugosa hybrids include:
F. J. Grookendorst, Polyantha and The Fairy.
Are your ‘Knock Out’ roses down for the count? Here’s why
Posted: May 27, 2015 / 03:37 PM CDT / Updated: May 27, 2015 / 10:56 PM CDT
Knock Out Roses are usually a foolproof planting option that blooms all season and requires little maintenance or disease control. (PHOTO: David Wood, WHNT)
HUNTSVILLE, Ala. (WHNT) – Knock Out Roses are a hugely popular landscape specimen, especially here in north Alabama. The varieties are praised for their no-nonsense upkeep and disease resistance. But if you think your Knock Out Roses look like they’re down for the count this year, you may be right.
Here’s why. Although Knock Out Rose varieties are indeed bred to be tough, they can still fall victim to viruses like Rose Rosette Disease. The virus is also known as ‘witches’ broom,’ but there’s no magic involved here. The virus is hosted by a mite, almost invisible to the naked eye, which is literally blown onto perfectly healthy plants by the wind.
A Knock Out rose with Rose Rosette disease, or ‘witches’ broom’ (Click to see larger image)
Rosette Disease is not exclusive to Knock Out Rose cultivars, but is most noticeable in the variety because we are used to having no trouble with the prolific bloomers.
Rose Rosette Disease (RRD) was first found in California, Wyoming, and Manitoba, Canada in 1941 and has emerged as one of the most devastating diseases of roses. Symptoms of the virus include new growth that is bright red and thorns that begin appearing in large clusters and quantities on the plants.
Other symptoms include witches’ brooms or clustering of small branches small distorted leaf growth leaf reddening excessive thorn production and thickened stems.
Another photo showing Rose Rosette disease (Click to see larger image)
George Bennett with Bennett Nurseries in Huntsville tells us there’s only one way to get rid of Rosette once a rose plant is infected — pull it up from the ground and toss it.
Once you remove an infected rose bush, the soil will be fine to replant, but you will not want to choose any rose variety for the infected spot.
Take a look at the photos below from Delano Park in Decatur before and after infection with Rosette Disease:
- Knock Out Roses at Decatur's Delano Park looked healthy before infection with rosette disease. (PHOTO: Jim Jacobi, Alabama Cooperative Extension System)
- Knock Out Roses at Decatur's Delano Park after infection with rosette disease. (PHOTO: Jim Jacobi, Alabama Cooperative Extension System)
- Here's the Rosette Disease culprit -- the Eriophyid mite, only visible with a 10x or 20x hand lens, a little practice and patience. (PHOTO: Jim Jacobi, Alabama Cooperative Extension System)
Natural History A witch's broom on a sand pine in Ocala National Forest Photo credit: Niels Proctor
A witch's broom is an unusually dense and compact cluster of twigs and foliage formed on a woody plant. The mass of shoots comes from a common point, giving the growth a broom-like appearance. The witch's broom may last several years. Although witches' brooms might be considered unsightly, they seldom result in serious injury to the tree.
Witches' brooms can be caused by various sources including mites, viruses, fungi, mistletoes, insects, and nematodes. Knowing the type of host plant can help determine the cause of the witch's broom. Witches' brooms can also be caused by a genetic mutation in a plant. These "mutant brooms" are particularly important commercially because their new genetic makeup can lead to new plant cultivars. One of the most famous broom hunters was Dr. Sidney Waxman, a horticulturist at the University of Connecticut. His 2005 obituary in the New York Times described his work this way:
"As America oozed into tract housing after World War II, there was a demand for trees and shrubs that would fit in the small yards and not grow very much. Over 40 years, Dr. Waxman came up with 40 cultivars, as cultivated plant varieties are known, including the widely planted dwarf pines called Sea Urchin, Blue Shag and Sand Castle.
Todd Forrest, associate vice president for horticulture and living collections at the New York Botanical Garden, said Dr. Waxman's importance had transcended the popular plants he developed. Mr. Forrest said Dr. Waxman had 'systematically established' the method of getting seeds from witches' brooms, which often appear as large, tangled clumps of branches, sometimes with miniature leaves, on full-size trees.
Some of the clumps are caused by disease, but some are sports, or natural mutations, with desirable new genetic characteristics that are worth preserving and cultivating. Dr. Waxman was amused to note how many witches' brooms he found in cemeteries.
Dr. Waxman methodically stalked his prey, marking on a map of New England the witches' brooms he found particularly tantalizing, his wife said. Each year, usually in October, he would visit them. He sometimes used a .22-caliber rifle to shoot cones from high branches, assigning his wife to scramble through groundcover to find the cones.
She remembered the startled looks of passing drivers. 'What are those two nuts doing?' she imagined them saying."
- "Sidney Waxman, Innovator of Dwarf Pines, Dies at 81," New York Times, February 20, 2005
|Identifying the stress:||A witch's broom is easily identified as a dense cluster of twigs growing from a central source. Witches' brooms on deciduous trees are easiest to see when the leaves are not present.|
|Susceptible trees:||Most trees can exhibit a witch's broom.|
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