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Crown gall is a common plant disease caused by the soil-borne bacterium Agrobacterium tumefaciens. It is found throughout the world and occurs on woody shrubs and herbaceous plants including grapes, raspberries, blackberries and roses.

Crown gall symptoms include round, wart-like growths — 2 inches or larger in diameter — that appear at or just above the soil line, or on lower branches and stems. Plants with several galls may be unable to move water and nutrients up the trunk and become weakened, stunted and unproductive. Young plants can be killed by developing gall tissue.

The bacteria responsible for crown gall can persist in the soil for many years and are released when galls become saturated with moisture or as older galls decompose. Susceptible plants are infected through fresh wounds or abrasions, many of which are a result of pruning, freeze injury, soil insects, cultivation and other factors that may damage plants. Nursery stock is often infected through grafting and budding scars.

Treatment

  1. Select resistant cultivars when possible and purchase plants from a reputable nursery.
  2. Do not buy plants that shows signs of swelling or galling.
  3. When caring for susceptible plants, avoid injury or pruning wounds that may come in contact with the soil.
  4. Use Tree Wrap to protect against string trimmer damage and keep your garden tools clean.
  5. Provide winter protection with natural burlap so bark does not crack.
  6. In many cases, existing galls can be removed with a sharp pruning knife. Destroy the infected plant tissue and treat the wound with pruning sealer. If the plant does not recover, remove and destroy it.

Tip: To get rid of this problem on roses, remove the infested plant and prune out gall tissue. Soak the entire root system and damaged areas for 15 minutes in a solution of 2 level Tbsp of Actinovate per 2-1/2 gallons of water. Replant in healthy soil and apply 1/2 Tbsp per 2-1/2 gallons of water as a foliar spray at weekly intervals.

How to Manage Pests

UC Pest Management Guidelines

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Floriculture and Ornamental Nurseries

Crown Gall

Pathogen: Agrobacterium tumefaciens

(Reviewed 3/09, updated 3/09)

In this Guideline:

  • Symptoms
  • Comments on the disease
  • Management
  • Control
  • Publication
  • Glossary

SYMPTOMS AND SIGNS

Crown gall is a disease resulting from infection by a bacterium that causes galls to form at the base of the stem or root crown or on other plant parts. The bacterium infects only through fresh wounds.

COMMENTS ON THE DISEASE

A wide variety of both woody and herbaceous plants are susceptible. The disease is most damaging to trees because the galls are perennial and increase in size with growth of the tree. Galls may occur on roots, stems, and even leaves. Aerial galls are common on grapes and caneberries. Under moist conditions aerial galls are often seen on chrysanthemum.

Gall tissue is disorganized growth with an enlarged cambium layer and irregular vascular tissue. Movement of water and nutrients is severely impaired by galls. The early stages of gall formation can be difficult to distinguish from normal callus tissue. Isolation of the pathogenic bacterium is the most common method of confirming that the bacterium is present. Callus tissue, which is soft and easily wounded, can be a common site of infection.

The bacterium Agrobacterium tumefaciens is common in many agricultural soils. When the plant is wounded, the bacterium attaches to an exposed plant cell and transfers a portion of its genetic material, DNA (deoxyribonucleic acid), into the cell where it is incorporated into the genetic material of the host cell. The host cell is induced to become a tumor cell and also to produce a unique substance (opine) that only the crown gall bacterium can readily utilize. Agrobacterium tumefaciens is then able to multiply between cells and in cracks of the gall with somewhat less competition from other microorganisms.

The only useful method of treating soil for crown gall pathogen is with heat. The common soil fumigants reduce the population of bacteria but do not result in satisfactory control of the disease. Steam (at 140°F for 30 minutes) or solarize (double-tent at 160°F for 30 minutes or 140°F for 1 hour) the soil. For flower production in open fields, solarization in warmer climates has been successful for control of crown gall. Steaming and solarization are acceptable for organic production.

Sanitation is very important in a control program, especially where cuttings are produced. Rose propagative material and work areas are often soaked or cleaned with hypochlorite solution to kill any bacteria that may be present on the surface. Grape propagative material and perhaps some others have also been treated in this manner. In some plants, such as grape, the bacterium may occasionally enter the xylem. Cuttings taken from such plants may develop crown galls.

Tools and surfaces that contact the propagative material should be cleaned and periodically treated with a disinfectant.

CONTROL

The K-84 strain of Agrobacterium tumefaciens (formerly A. radiobacter), which is available for use in preventing infection by the crown gall pathogen, is an excellent biological control agent.

Galls on many woody plants can be treated with a mixture of chemicals that are toxic to and kill crown gall tissue but are safe on uninfected woody tissue. The mixture, which is currently marketed under the name Gallex, was previously sold as Bacticin. It has been used with success on rose crown galls.

Common name Amount to Use R.E.I.+
(trade name) (hours)
When choosing a product, consider information relating to environmental impact. Not all registered pesticides are listed. Always read label of product being used.
A. AGROBACTERIUM TUMEFACIENS K-84#
(Galltrol) Label rates 12
. . . or . . .
AGROBACTERIUM TUMEFACIENS K-1026#
(Nogall) Label rates 4
COMMENTS: Products may list bioagent under former name A. radiobacter. Prevents infection by the crown gall pathogen if it is applied to fresh wounds. It must be applied as soon as possible after wounding; i.e., within 24 hours. It has been used with success on Prunus spp. and Rosa spp.
B. GALLEX Label rates 0
COMMENTS: For killing of existing galls; apply directly to galls winter through spring.
# Acceptable for use on organically grown ornamentals.
+ Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing.

PUBLICATION

UC IPM Pest Management Guidelines: Floriculture and Ornamental Nurseries
UC ANR Publication 3392
Diseases
S. T. Koike, UC Cooperative Extension Monterey County
C. A. Wilen, UC IPM Program, UC Cooperative Extension San Diego County
Acknowledgment for contributions to Diseases:
R. D. Raabe, (emeritus) Environmental Science, Policy, and Management (ESPM), UC Berkeley
A. H. McCain, (emeritus) Environmental Science, Policy, and Management (ESPM), UC Berkeley
M. E. Grebus, Plant Pathology, UC Riverside

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Controlling crown gall disease

Crown galls develop following cold injury to trunks on Riesling vines in the Finger Lakes region of New York. (Courtesy Thomas Burr)

Researchers are making strides in efforts to combat crown gall disease and to develop clean plants free of the pathogen that causes it, but there’s still much to be learned about how to prevent, diagnose and eliminate it in grapevines.

That said, if there’s any place that should be able to grow disease-free vineyards, it’s Washington, with its desert, sandy soils and lack of the wild grapevines that serve as reservoirs for the disease elsewhere, said Dr. Thomas Burr, Cornell University professor of enology and viticulture.

“You have a real advantage in Washington,” he told attendees at the inaugural Ravenholt Lecture Series at the Ste. Michelle Wine Estates WSU Wine Science Center in Richland in January. “If you can plant a vineyard with clean plants, you certainly are not going to eliminate the presence of crown gall forever, but if you can get a healthy vineyard started, you’re way ahead.”

The pathogen that causes crown gall disease, Agrobacterium vitis, can live silently in the vines until an injury initiates an infection, most commonly those caused by freezing temperatures or at graft unions.

The infection causes galls that crush the vascular tissue of the plant, restricting the movement of water and nutrients, and impede the vine’s ability to heal the wound. The infection also can cause necrotic lesions on roots.

The disease kills young vines that develop it at graft unions and can stress older vines, depending on the level of infection, significantly reducing vigor and yield. Once established in a vineyard, there is no chemical control.

New knowledge about A. vitis

Researchers at Cornell took cuttings from 10 different grapevines in the winter of 2014, sampled from nodes and internodes, starting at the base and working their way out through the cane.

They hypothesized that the pathogen would be at its highest level at the cane base and at nodes, Burr said.

They were wrong. It turns out the bacterium can be randomly distributed in dormant grape canes.

“It’s not only at nodes, but it’s also at internodes. It’s not only at the base, you can find it way out,” he said. “It’s not what we wanted, obviously. If you’re going to be sampling a certain block for a grower, you want it to be simple.”

That means that if a sample is positive, researchers can positively say that crown gall is present. But if it’s negative, they can’t say for certain that it’s not present somewhere else in the vine. “The more we study this bacterium, the more places we find it,” he said.

That includes in wild grapevines. In another study, researchers sampled wild grapevines in New York in 2013 and 2014 for tumorigenic strains of the pathogen, finding that roughly one-third were positive for it — 18 of 54 samples in 2013, and 12 of 41 samples in 2014.

Similar tests of wild grapevines in riparian areas in California showed 24 of 87 positive samples in 2014 and 2015, he said.

“It’s a bit shocking, but good to know that it is an important source of the pathogen in the environment,” he said.

But what researchers still don’t know, he added, is whether it can survive in other places in the environment, such as water, soil, weeds or other plants that could serve as reservoirs for it.

Finding and controlling A. vitis

Researchers have been studying this pathogen for more than 30 years but have never had a good method for detecting the bacterium that causes it. Tests to ensure plants are clean and to identify infected plants can take weeks.

However, a new test developed at Cornell University by plant pathologist Kameka Johnson would shrink that time to just three to four days.

Magnetic capture hybridization involves probing material for a specific DNA sequence to determine if the bacterium is present. For crown gall, that involves the virD2 gene in the bacterium, which is essential for infection.

“This technique can only be used if you’re looking for a specific target,” Burr said. “Because agrobacterium needs virD2 to form galls, it really was just the perfect method for detecting crown gall in grapevines.”

The assay avoids the detection of bacteria that are present in vines and do not cause crown gall, Burr said.
Researchers also see gains in biological control of crown gall.

Another strain of A. vitis, F 2/5, is a non-tumorigenic strain that originated in South Africa. Researchers discovered that applying F 2/5 along with a tumorigenic strain to inoculate the plant, crown gall would not materialize.

However, F 2/5 still causes necrosis of the roots, so researchers worked to see if they could develop a derivative of F 2/5 that was necrosis-free.

They knocked out the gene tied to causing the necrosis and found that it still maintains the ability to knock down the tumorigenic strain of A. vitis. They are working to develop the practice for commercial use.

“This is something that could be quite useful, both for treating grafts and treating dormant cuttings before rooting, two sites that could be infected with crown gall,” he said.

Advice for growers

In the meantime, growers should take steps to try to prevent crown gall in vineyards, Burr said.

These include planting certified clean material and varieties that are more tolerant of the disease — all Vitis vinifera are susceptible, but some more so than others — in sites with good air circulation and well drained soil.

Two sites in the National Clean Plant Network provide the best options for plant materials for Washington growers: Clean Plant Center Northwest and the Russell Ranch in California, which is at lower risk for the disease than other out-of-state centers but is not risk free, said Michelle Moyer,

Washington State University plant pathologist. That’s because plants that are produced in a warm location could be infected and not show symptoms until they are exposed to a freeze after being planted in Washington.

Growers should reduce likelihood of cold damage by hilling around the trunks and train with multiple trunks, Moyer said. “The galls don’t appear until after bloom — after you’ve pruned — so keep some of those suckers as an insurance policy if you’ve had a cold snap,” she said.

Burr added that growers should manage vine growth with minimal amounts of water, while keeping vines in good shape heading into winter to protect against freeze damage as much as possible.

Kevin Judkins of Inland Desert Nursery made a similar point. Some growers are pushing the limit on what they can grow where — such as Cabernet Sauvignon in too cold an area.

They need to be more selective about pairing growing sites and varieties and follow good management practices to prevent cold injury, he said. •

– by Shannon Dininny

Agrobacterium tumefaciens – Crown Gall

The bacterium Agrobacterium tumefaciens is a soil borne pathogen that causes crown gall on many types of plants. In greenhouse crops, Crown Gall has been diagnosed in recent years on mums, argyranthemum, osteospermum and lobelia.

Symptoms of Crown Gall are white masses of callus tissue or small swellings appearing on roots, at the base of the stem and occasionally on leaves or anywhere wounds occur. Gall formation may be seen about 8-12 days after infection.

The bacterium can be moved by water splash, but wound or natural opening is required for the bacterium to penetrate the plant tissue. Propagation tools may move the bacterium between plants and also provides wounds for bacterium to enter. Infection can also occur through the roots through infested substrate or irrigated from contaminated water source.

Crown gall management begins by carefully monitoring plants for symptoms and practicing good sanitation techniques if the disease is found. Discard infected plants as soon as disease has been diagnosed. Do not compost since the bacterium can survive in the soil. Use disinfestants to clean surfaces and any tools used on the crops. Never use cuttings that have galls and do not propagate from plants with galls. If one cutting in a tray has symptoms, others in the tray could be infected especially if the plants have been pinched or sheared. It is recommended to bag and discard the entire tray, substrate and cutting.

There is no cure for infected plants and chemical prevention is ineffective.

Resources

Crown Gall Agrobacterium tumefaciens, Colleen Warfield, Ball Horticultural Company

Crown Gall: Still Confounding Scientists and Growers Alike, Grower Talks, Sept. 2015

Photos: Crown gall on many crops, Oregon State University

By: Kevin Ong and Ashley Brake

Crown gall, a bacterial disease that occurs throughout the world, infects several different plant hosts. In particular, it is a devastating disease in the Rosaceae (rose) family.

The specific bacterium, Agrobacterium tumefaciens, causes crown gall by inserting a tumor-inducing gene into the plant genome. Scientists have extensively studied this bacterium and used it for introducing desirable traits into many cultivated plant species. Since the 1970s, genetic engineers have exploited the bacteria’s ability to insert genes into other plants, and the agricultural industry has benefitted from creating genetically modified plants with crops that grow faster and bigger and are resistant to insects and other diseases.

Symptoms

Initial symptoms on roses appear as a small, light green spherical swelling around the crown of the plant. This swelling also occurs below the soil line (Fig. 1) on the roots as well as on higher branches, depending on the infection site. These growths should not be confused with normal callus growths that commonly form at wound and grafting sites.

Once the infection worsens, the shape of the galls becomes uneven, and they harden into a dark, woody mass (Fig. 2). In heavily infected plants, secondary tumors may develop near the first gall. There may be several tumors affecting a single plant in any given infection. Gall formation interferes with the plant’s ability to transport water and food supplies, producing other symptoms such as

  • Weakness
  • Stunting
  • Discoloration of leaves
  • Dieback of shoots
  • Increased susceptibility to winter injury or secondary infection
  • Wilt and, eventually in severe infections, death

It is easy to confuse crown gall with symptoms of hairy root, another root pathogen caused by the bacterium Agrobacterium rhizogenes. Hairy root causes the same gall formations; however, they are not woody and produce fibrous roots that project from the galls, giving them a “hairy” look.

Cause

The bacterium that causes crown gall lives in the soil that surrounds the host—in this case, roses. This bacterium can live in the soil as a decomposer for years without infecting a living host. When a plant is injured—either by mechanical transmission, insect feeding, or naturally—the damaged cells release compounds into the soil, attracting the bacterium to the wound site. Once inside, the bacterium replicates rapidly, forming the tumor-like gall by integrating some of its DNA (deoxyribonucleic acid), which is contained in a circular plasmid, into the host’s DNA. A plasmid is a small DNA molecule that is isolated from chromosomal DNA and can replicate on its own. Once the bacterial genomic material is incorporated into the host’s genome, the normal plant cells are altered. They multiply and form the gall structure.

Only the strains of these bacteria that contain these tumor-inducing plasmids (Ti plasmid) can cause disease. Some strains of A. tumefaciens lack the specific plasmid and remain in the soil without causing disease. When the outside tumor cells shed into the soil, replicated bacteria can live in the soil and be carried off by water to infect neighboring plants.

Environmental Factors

A.tumefaciens lives primarily in the upper layers of the soil closest to the plant (rhizosphere). It lives for years off of decaying organisms until conducive conditions allow it to inhabit a host plant. The bacterium can enter a plant only through a wound site or natural opening. Following favorable rose-growth conditions, A. tumefaciens becomes dormant during the cold winter months and is most active in summer months.

Managing crown gall disease includes, but is not limited to, these preventive cultural practices:

  • Purchase healthy-looking plants.
  • Avoid injuring the plant (especially around the roots and crown) while planting, and try to reduce the impact of chewing insects that can cause wounds.
  • Plant in soils with no previous record of crown gall.
  • Sterilize all pruning utensils before and after use.
  • If symptoms develop after planting, dig up the whole plant and dispose of it properly, including the surrounding soil.
  • Use insecticides to reduce the wounds on plant tissues that insects can cause. Be careful when undertaking any chemical control measures.
  • Soak seeds or bare-rooted plants in a solution containing a closely related species, Agrobacterium radiobacter, as a preventive treatment. The bacterium
  • A. radiobacter does not induce disease in plants; it uses similar resources and can prevent crown gall disease by typically out-competing A. tumefaciens.

Sources

Londeree, N. 2013. Garden Bad Guys–Crown Gall.Marin Rose Society. ttp://www.marinrose.org/crowngall.html.

Download a printer-friendly version of this publication: Crown Gall of Roses

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Contact Your County Office

Bacterial Crown Gall of Roses Caused by Agrobacterium tumefaciens1

Kamil Duman, Susannah da Silva, Fanny Iriarte, Barron Riddle, Gary Knox, Matthew Orwat, Shawn Steed, E. Vanessa Campoverde, Jeffrey Jones, and Mathews Paret2

Introduction

Crown gall is a bacterial disease that causes large, tumor-like swellings (galls) that often occur at the crown of the plant, just above the soil line (Figure 1), resulting in plant disfigurement and eventually plant death. The pathogen Agrobacterium tumefaciens infects hundreds of plant species including roses. It also survives in soil for up to three years, which greatly affects disease management practices.

Figure 1.

Later stage crown gall symptoms at the base of a rose plant caused by the bacterium Agrobacterium tumefaciens.

Credit:

Kamil Duman, UF/IFAS

Galls result from uncontrolled growth of undifferentiated host plant tissue at the infection site of Agrobacterium tumefaciens. Infection only occurs through wounds; these can result from horticultural practices such as pruning, or from environmental damage such as freezing. Infection of rose commonly occurs at the soil line (crown) (Figures 1 and 6), but can also occur on branches or roots (Figure 2, 3, 4, 5).

Figure 2.

Galls can appear on roses infected with A. tumefaciens within 2‒3 weeks.

Credit:

Mathews Paret, UF/IFAS

Figure 3.

Galls on roses turn dark in color as they age.

Credit:

Kamil Duman, UF/IFAS

Figure 4.

Galls on primary and secondary roots of roses.

Credit:

Susannah da Silva, UF/IFAS

Figure 5.

Gall formation at pruning sites indicates inoculation of the plant during pruning.

Credit:

Kamil Duman, UF/IFAS

Figure 6.

A plant with a mature gall in contact with the potting soil.

Credit:

Susannah da Silva, UF/IFAS

Galls are initially light in color (Figure 2), but over time the outer plant cells die and the galls darken (Figures 3, 5, and 6). Sometimes galls are indistinguishable from normal plant tissue due to pruning scars or other environmental factors leading to gall-like appearance. The bacteria can invade wounds (injuries) on the branches or roots (Figures 3 and 4). Swelling or gall formation can be seen as early as 14 days or as late as 6 months following the entry of the bacterium into the plant. The tissue near the gall suffers a rapid cell proliferation. If vascular tissue is affected, wilting can result from the restricted water movement. However, this is not a common symptom.

Pathogen: Biology and Spread

The crown gall bacterium, Agrobacterium tumefaciens, is a soilborne bacterium. It can enter the plant from the soil through wounds on the roots, lower stems, or from the branches during plant pruning. Symptoms are caused by the insertion of a small segment of transfer-DNA (T-DNA) from its plasmid into the host plant cell, which is incorporated into the plant genome. Once the T-DNA integrates into the plant chromosome, the altered plant cells start dividing rapidly and uncontrollably, and the infected tissue develops a tumor-like swelling. Galls can range from pea size to softball size. Tiny cracks from freezing temperatures or wound sites can be the sites for gall formation.

One of the common ways the disease spreads is by pruning healthy plants with the same non-sanitized pruning tool used on an infected plant, causing galls to form at the pruning site (Figure 5). Also, following pruning, gall tissue from infected plants can land in the soil and the pathogen can persist for several years (Figure 6). The bacterium can spread easily during field preparation, pruning, and irrigation. Insects, nematodes, and grafting materials can also transfer the bacterium. However, from an agricultural standpoint, the bacterium is found mostly on pome and stone fruit trees, roses, raspberries, blackberries, and grapes. We currently do not know whether the strains isolated from Florida and Georgia roses infect other plant species.

The crown gall disease cycle highlights how this pathogenic bacterium is disseminated and infects roses (Figure 7).

Figure 7.

A summary of the disease cycle of crown gall, caused by Agrobacterium tumefaciens.

Credit:

Kamil Duman, Susannah da Silva, and Mathews Paret, UF/IFAS

Disease Management

  • Plant only pathogen-free roses. When you receive new plant material, check very carefully for galls on the crown, branches, and roots.

  • Plant in clean soil (avoid locations with a history of crown gall infestation for at least three years).

  • Avoid locations with heavy infestations of root-attacking insects and nematodes.

  • Select well-drained soil and irrigate from clean water sources (i.e., water that is treated, or is not known to have contact with this pathogen).

  • Disinfect pruning tools between plants and budding/grafting tools before and after use. Bleach (10%; equivalent to 0.6% sodium hypochlorite) or quaternary ammonium-based sanitizers are effective disinfectants. Make sure to prepare fresh stock regularly.

  • Keep grafts and buds well above the soil line.

  • Rogue infected plants, as the likelihood of accidental transmission to healthy plants is high. Destroy infected plants through burning to prevent further contamination.

  • Avoid mechanical injury to plants from pruning and tillage.

  • Provide winter protection to rose plants to avoid damage to bark.

  • While there are biological control strains and chemical compounds available for crown gall management, their efficacy on roses is not known.

  • If crown gall plants are noted, please contact your county Extension agent for information on management.

Footnotes

This document is PP343, one of a series of the Plant Pathology Department, UF/IFAS Extension. Original publication date November 2018. Visit the EDIS website at https://edis.ifas.ufl.edu for the currently supported version of this publication.

Kamil Duman, visiting scientist, UF/IFAS North Florida Research and Education Center (NFREC); Susannah da Silva, planty pathology agricultural/food scientist II, UF/IFAS NFREC; Fanny Iriarte, plant disease diagnostician, UF/IFAS NFREC; Barron Riddle Sr., biological scientist, UF/IFAS NFREC; Gary Knox, professor, Environmental Horticulture, UF/IFAS NFREC; Matthew Orwat, Extension agent, UF/IFAS Extension Washington County; Shawn Steed, Extension agent, UF/IFAS Extension Hillsborough County; E. Vanessa Campoverde, Extension agent, UF/IFAS Extension Miami-Dade County; Jeffrey Jones, professor, Bacteriology, Plant Pathology Department; and Mathews Paret, associate professor, Plant Pathology Department, UF/IFAS NFREC; UF/IFAS Extension, Gainesville, FL 32611.

The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other UF/IFAS Extension publications, contact your county’s UF/IFAS Extension office.
U.S. Department of Agriculture, UF/IFAS Extension Service, University of Florida, IFAS, Florida A & M University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Nick T. Place, dean for UF/IFAS Extension.

Crown gall is a bacterial disease caused by Agrobacterium tumefaciens (syn. Rhizobium radiobacter), which produces tumour-like swellings on the bark of trees and plants. It can infect a wide range of dicotyledonous (broad-leaved) plants, particularly members of the Rosaceae (rose) family such as roses, raspberries, almonds, cherries, peaches, pears and apples.

These disease-causing bacteria live in the soil and only enter fresh tree wounds (less than 24 hours old), which are often caused by insect and frost damage, mechanical damage from mowers and lime trimmers, pruning, or transplanting damage.

Crown gall bacteria are able to detect the various phenolic compounds which wounded dicotyledonous plants release as chemical signals in their defence and repair process, enabling them to home in on the wounds and infect the damaged plant cells.

They don’t just infect the plant though, the Crown gall bacteria insert some of their own bacterial DNA into the plant cells, altering their functioning and causing them to produce unusually large amounts of plant hormone, which in turn stimulate the plant cells to multiply and enlarge, leading to tumour or gall formation.

Crown Gall Identification

The galls can form on both the roots and the trunk of a tree, and they can also form at graft unions or at the base of the trunk at the root crown, just below soil level. The bacteria can spread into branches through pruning cuts and other damage.

Identification of crown gall is made more difficult by the fact that the appearance if the galls changes over time. Galls start off light-coloured, soft, and spongy, then become rough and corky with an irregular surface and a hard woody interior, and as they mature, they turn dark brown and woody. Some galls will disintegrate over time and others may remain for the life of the plant.

Galls grow in size over time, affecting the growth, health and vigour of the plant. Galls impede the flow of water and nutrient movement in the plant, as they effectively cut off the bark in the site of infection. Reducing the flow of water and nutrients in a plant causes slow growth, stunting of the size of the plant, deficiency symptoms such as chlorosis and a decline in the overall health of the plant. If galls grows large enough to extend right around a branch or trunk, they can girdle (ring-bark) the tree, causing decline and possible death of the tree, depending on the location of the damage.

Crown gall may kill a very young apple tree by girdling the stem, while a mature apple tree may be able to tolerate it and remain productive.


Crown gall infection on apple tree

Crown Gall Life Cycle

The crown gall bacterium Agrobacterium tumefaciens can persist in the soil in decomposing plant debris for long periods of time, it may live saprophytically (living on dead organic matter) in soil for up to two years. It can only proceed in its life cycle if it finds fresh wound to infect and form galls. Without wounds on trees and plants, no galls can form.

These bacteria mainly live in the outer portions of primary galls, which can break off and fall to the soil, spreading the bacteria. The centre part of the gall is dead and contains no bacteria. Secondary galls may also form without any bacteria present on other parts of the plant or around other wounds. Crown gall bacteria can overwinter both in the galls and in the soil, and reinfect any plant wounds they come in contact with, completing the disease cycle.

Crown Gall Control and Treatment

There is no known cure for crown gall disease, and the best control is prevention.

If there are not too many galls, the branches and parts of the trunk where the tumours occur can be pruned off and destroyed – do not compost infected plant material!

When pruning, disinfect pruning tools between cuts. Use either a solution of 70% alcohol and 30% water, or a 10% bleach solution (1 part bleach with 9 parts water).

If the tree is too heavily infected, the recommendation is usually to dig up, removing as much of the roots as possible, and to destroy all infected plant material.

For prevention of crown gall disease:

  • Avoid wounding trees in any way, especially during transplanting.
  • Don’t replant susceptible fruit trees trees in an area where crown gall infections have occurred.
  • Cover young trees during extreme frosts with hessian or geotextile fabric as a frost cover to prevent frost damage.

Biological control agents exist for preventing crown gall in apples and many other fruit trees, as well as berries and ornamentals (but not grapes). A non-pathogenic (non-disease causing) strain of the same bacterium (Agrobacterium radiobacter strain 84) has been used to protect plants against infection by crown gall bacteria in the soil. Plants are dipped into a suspension of the protective Agrobacterium radiobacter strain 84 at planting time to keep plants free from disease. This biological control cannot cure plants which ae already infected .

Some suggest removing existing galls with a sharp pruning knife, destroying the infected plant tissue and treat the wound with a chemical control.

Researching chemical eradicants of crown gall, most research literature mentions a variation of this statement:

“Eradication of crown gall using creosote-based compounds, copper-based solutions, and strong oxidants such as sodium hypochlorite are transiently effective. The chemical eradicant application procedure is labor intensive and therefore costly both monetarily and to the environment. The superficial treatments are ineffective against systemically infected plants. Generally, chemicals are rarely used for control of crown gall.”

The references are vague in respect to the chemicals mentioned, so to provide further detail, I had to do a lot of time consuming and fairly unproductive literature searches.

  • The creosote based chemical compounds are a mix of 2,4-xylenol (0.463%) and meta-cresol (0.466%), sold in a commercial product known as Gallex, it’s pretty toxic and not something you would want contaminating the soil.
  • The copper-based solutions refer to copper fungicides such as copper oxychloride, which seems to show the best results of the copper compounds from my searches of the literature.
  • The strong oxidants such as sodium hypochlorite refers to common household bleach, read the ingredients on the label and it should be listed there.

What they’re saying is that cutting away the gall and spraying with these chemicals may or may not be effective. What is clear from research is that applying copper and bleach-based bactericides reduce many A. tumefaciens populations on the surface of plant cells.

Want to try a safe chemical control? You can cut away the gall, mix up some copper oxychloride fungicide or a 10% bleach solution and spray the cleaned up wound area, it may not eliminate the crown gall disease but may stop it spreading.

  1. New Mexico State University, Extension Plant Sciences – Featured Diagnosis: Crown Gall (Agrobacterium tumefaciens)
  2. North Dakota State University, NDSU Extension – Crown Gall, Agrobacterium tumefaciens by Esther McGinnis
  3. The Microbial World: Biology and Control of Crown Gall (Agrobacterium tumefaciens) by Jim Deacon, Institute of Cell and Molecular Biology, The University of Edinburgh
  4. Mikiciński, Artur & Sobiczewski, Piotr & Berczyński, Stanisław. (2012). Efficacy of fungicides and essential oils against bacterial diseases of fruit trees. Journal of Plant Protection Research. 52. 10.2478/v10045-012-0075-7.

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Apple Tree Burr Knots: What Causes Galls On Apple Tree Limbs

I grew up in an area near an old apple orchard and the old gnarled trees were something to see, like great arthritic old ladies anchored into the earth. I always wondered about the knobby growths on the apple trees and since then have discovered that there are a couple of things that can cause them. Read on to learn more these apple tree growths.

Apple Tree Burr Knots

Burr knots on apple trees are particularly common on some apple varieties, especially early “June” cultivars. Apple tree burr knots (also spelled burrknots) are clumps of twisted or knobby growths on apple trees branches, usually when they are three years old or older. This occurrence increases on dwarf rootstocks. The outgrowths can produce both shoots and roots, so if you want to start another tree, you need only prune the affected branch from the mother and plant it.

The down side of burr knots on apple trees is that they can be an entry point for disease and pests. Also, a tree bearing a large yield of apples combined with many burr knots may become weak and break if the wind picks up.

As mentioned, some cultivars are more prone than others, and conditions such as low light, high humidity and temps between 68-96 degrees F. (20-35 C.) may facilitate production of burr knots. Also, there is some indication that wooly aphid infestation is causing an injury that results in forming knots. Burrknot borers may also be a cause.

Choose rootstock that is less prone to burr production. You may also paint Gallex on the knots, which can aid in callus formation or healing. If the tree is severely afflicted, you may want to take it out altogether since numerous burr knots can weaken the tree, opening it up for infection or infestation that will eventually kill it.

Apple Tree Gall

Another possible cause for a gnarly prominence might be crown galls on apple tree limbs. Apple tree crown gall causes tumor-like galls to form predominately on the roots and trunks but, on occasion, the branches of not just apples but many other shrubs and trees can be affected as well. Galls interrupt the flow of water and nutrients in the tree. Young seedlings with multiple galls or one encompassing the entire girth of the tree will often die. Mature trees are not as susceptible.

The Webster’s definition for the word ‘gall’ is “a skin sore caused by chronic irritation.” That is indeed what is happening to the “skin” of the tree. It has become infected with the bacterium Agrobacterium tumefaciens, which is found in over 600 species of plants worldwide.

Galls on apple tree limbs are the result of the bacteria entering the root system via injury caused from planting, grafting, soil insects, excavation, or another form of physical wound. The bacteria senses chemicals emitted by the wounded roots and moves in. Once the bacteria have invaded, they induce the cells to create overly large amounts of plant hormones which lead to gall formation. In other words, the infected cells divide exponentially and increase to unusually large sizes much like cancer cells do.

The infection can be spread to other susceptible plants through contaminated pruning implements, and it also will survive in the soil for many years potentially infecting future plantings. The bacteria are also commonly moved to new locations on the roots of infected plants that are getting transplanted. These galls break down over time and the bacteria is returned to the soil to be dispersed by water movement or equipment.

Really, the only control method for apple tree gall is prevention. Once the bacterium is there, it is difficult to eradicate. Choose new plants carefully and inspect them for signs of injury or infection. If you do identify a young tree with gall, it is best to dig it up along with the soil surrounding it and dispose of it; don’t add it to the compost pile! Burn the infected tree. More mature trees often tolerate the infection and can be left alone.

If you have identified gall in the landscape, be wary about introducing susceptible plants such as roses, fruit trees, poplar or willow. Always sterilize pruning implements to avoid cross contamination.

Lastly, trees can be protected from apple crown gall prior to transplantation. Dip the roots with a solution of water and the biological control bacteria Agrobacterium radiobacter K84. This bacterium produces a natural antibiotic that sits in wound sites preventing infestation of A. tumefaciens.

Crown Gall Disease of Nursery Crops

L.W. Moore (deceased), Bacteriologist and Plant Pathologist, OSU

Updated by M. L. Putnam, Diagnostician and Plant Pathologist, OSU

Crown gall continues to be a major problem for the nursery industry, both in woody and herbaceous plants. The pathogen causing crown gall in the most plants is Agrobacterium tumefaciens (Rhizobium radiobacter). A. rubi is much less common, and causes disease only in Rubus spp. (although A. tumefaciens does as well), and A. vitis is specific to grapes. A fourth species A. larrymoorei, causes galling of Ficus benjamina. All of these species have a similar biology. This discussion covers the biology, host range, symptoms, and management of the disease.

Biology

Crown gall is a tumor-forming disease of plants caused by pathogenic species of Agrobacterium. Agrobacterium is a bacterial genus thought to be present in most agricultural soils. The pathogens, in soil or on infested plants, are disseminated by splashing rain, irrigation water, heeling-in galled plants with healthy plants, farm machinery, pruning tools, wind, and plant parts used for propagation. Wounds are required for the pathogen to infect a plant. Wounds are made by pruning and cultivation, emergence of lateral roots, frost injury, and insect and nematode feeding. The pathogen colonizes the wound, attaches firmly to injured plant cells, and transfers part of its DNA into the DNA of the plant. Galls appear in a matter of weeks at temperatures above 70°F. Latent infections typically develop into galls in a later growing season. The gall provides a nutrient-rich environment for further bacterial growth. Pathogenic bacteria escape from the gall into the surrounding soil or water where they colonize or infect new plant tissues.

Host Range

Although commonly reported to have a host range of hundreds, this information is based on artificial inoculations, and on a practical basis, far fewer plants are naturally susceptible. (Examples of host plants infected by Agrobacterium are listed in Table 3.) However, the root systems of non-host plants such as weeds, grasses, and cereals can harbor the pathogen and serve as a reservoir of inoculum in natural settings.

Symptoms and Damage

The disease is called crown gall, but galling may be found at the base of cuttings, on roots, crowns, or on stems, canes, vines, or leaves. Leaf galls are usually found on herbaceous plants that have a systemic infection. (Herbaceous ornamental plants susceptible to crown gall are shown in Table 1.) Galls can be on pruning wounds. Galls are usually rounded and are often textured like a cauliflower head. On woody perennial plants, galls become more woody and fissured with age, sometimes reaching a diameter of 4 inches, and girdling the stem. Galls on grapevines, blueberry, and bramble fruit are usually elongate, erumpent ridges of tissue bursting through the outer stem tissues.

Woody plants infected the first year they are planted out are more severely damaged. (Woody plants susceptible to crown gall are shown in Table 2.) Severely galled young plants are weakened, stunted, and unproductive and occasionally die due to an inferior root system. Literature reports of crown gall damage are contradictory; they range from benign to debilitating to deadly.

Symptoms become evident 2 to 4 weeks after infection if temperatures are at or above 68°F, usually coinciding with warmer soil temperatures in May or June. Initially, the galls look like callus outgrowths but then increase rapidly in size and number. Symptom development slows greatly below 58°F and stops below 50°F. Infection is inhibited above 92°F to 95°F. Latent infections are symptomless and usually occur when soils are cool. Gall symptoms typically develop at the infected wound the following season; on rare occasions galls don’t appear until the third growing season.

Some problems can look like crown gall but are not pathogenic. Aerial burrknot on apple tree trunks and branches is a cushion-like assemblage of adventitious roots; its cause is thought to be genetic rather than an infectious agent. Tissue proliferation is another tumor-like problem, on Rhododendron, that is caused by factors other than Agrobacterium. Vascular tissue differentiation in tissue proliferation distinguishes it from wound callus and crown gall.

Small galls require careful diagnosis because they may be confused with excessive wound callus. Detection using molecular methods specific to plasmid gene regions involved with virulence, or isolation of bacteria later identified as pathogenic is necessary to confirm a crown gall diagnosis. Nonpathogenic Agrobacterium cells are often prevalent in these same tissues and can reach high populations. That makes diagnosis difficult, especially in galls on apple, blueberry, and grapevines where non-pathogens can constitute over 99% of the Agrobacterium population.

Disease Management – Woody Nursery Stock

Pathogen-free plants grown in uninfested soil will not develop crown gall. This emphasizes the importance of planting clean propagating material in clean soil. Good sanitation and cultural practices are important deterrents to crown gall. Discard all nursery stock showing symptoms to avoid contaminating healthy plants and storage facilities. At harvest, leave noticeably galled plants in the field for later pickup and destruction. If possible, choose a rootstock that is less susceptible, avoid planting sites heavily infested by root-attacking insects and nematodes, disinfect pruning equipment between trees, and adopt management practices that minimize wounding. Avoid planting into heavy, wet soil. Don’t plant trees deeper than they grew in the nursery. If possible, incubate dormant seedling roots at 73°F to 76°F for 10 to 14 days to heal wounds and reduce susceptibility to A. tumefaciens before planting them in wet soil. Use irrigation water from wells, if possible. Avoid planting where galled plants grew in the last 4 to 5 years; choose fields that were planted recently to vegetables or grain. In summary, think prevention—avoid exposing plants to pathogenic Agrobacterium at any stage of plant production.

Planting Site

Crown gall is generally much more prevalent in heavy soils or in soil where water stands for a day or so. In New York, crown gall incidence was highest on a heavy clay knoll (15 ft elevation) from which water drained toward flat, loamy portions of the field. In Oregon, gall incidence on an Old Home x Farmingdale pear rootstock selection was severe (495 of 500 trees infected) in a heavy, wet soil, but in the same field only 1 of 500 trees was galled outside the wet area.

Cropping history can influence crown gall incidence. Budded apple trees became badly galled in fields where a previous nursery crop such as grape, peach, raspberry, and rose had been heavily infected. This situation isn’t repeated at every site, but we still recommend avoiding fields with a recent history of crown gall.

Natural Resistance

Reports of resistance in plants normally susceptible to crown gall are limited and depend on the strains of bacteria present in a given location. There are no reliable lists of cultivars with resistance that hold up in all geographic locations. It is better to select plants that are not susceptible in the first place if crown gall is a chronic problem in a particular field.

Biological Control

Using A. radiobacter K84, a biological control agent, has been very effective against crown gall on a number of hosts, but exceptions exist. This biological control is solely preventive, not curative; application timing is critical to properly protect plant wounds caused at harvest or by pruning. Htay and Kerr recommend seed and root treatment with K84 for best results. Not all pathogenic strains of Agrobacterium are sensitive to K84. For example, most pathogenic agrobacteria isolated from grape tumors are A. vitis, which are insensitive to K84. If K84 has been used properly and galling persists, its use should be discontinued since it is likely the bacteria present are not sensitive to the product.

A new, genetically engineered strain of K84 called K1026 had been patented in the United States.

Biological control is compatible with a few pesticides such as metalaxy (Ridomil), thiram and thiophanate-methyl (Topsin) but not with captan, etridiazole alone (Truban), etridiazole plus thiophanate-methyl (Banrot or Zyban), mancozeb, PCNB or streptomycin. It is also not compatible with chlorinated water.

Chemical Treatments

No registered chemicals that effectively control crown gall are currently available in the United States. In general, chemical preplant dips or soil drenches have been ineffective.

Soil Fumigation

Fumigation to rid soil of Agrobacterium generally has been ineffective, and in some cases growers reported more disease after fumigation.

Heat Therapy

Heat therapy has been tried in cherry and plum seedlings, and in dormant grape cuttings. Although these measures can reduce the incidence of disease, there will still be a small percentage of plants that remain infected. Time and temperatures needed for effective heat therapy has not been determined for many plants, and injury to the plant material can occur when temperatures are too high. Although promising, heat therapy is not commonly used due to these difficulties.

Soil Solarization

In solarization, a thin plastic film is stretched over moist soil to capture energy from the sun and heat the soil to temperatures that kill pathogenic microbes. A. tumefaciens populations could not be detected in a solarized sandy loam soil, but solarization did not work in the heavier silty-loam. Mazzard cherry seedlings planted later in solarized and in nonsolarized control plots developed crown gall only in the nonsolarized plots.

Following is a summary of the best practices for managing crown gall. They include experimental results and grower observations. Understandably, physical and economic constraints occasionally may impede applying all these practices. But for best results, follow or adapt the procedures as closely as possible to fit your management plan.

Best Practices for Managing Crown Gall

Cultural Practices

  • Discard diseased plants as soon as noticed to avoid cross-contaminating other plants, equipment, or storage facilities.
  • Don’t heel-in galled plants with healthy plants.
  • Use good sanitation in handling planting stock.
  • Minimize wounding; disinfect pruning tools between plants.
  • Plant only disease-free stock.

Planting Site

  • Plant in clean soil.
  • Avoid fields with a recent history of high crown gall infestation.
  • Avoid fields with heavy infestations of root-attacking insects and nematodes.
  • Select well-drained soils; tile heavy soils.
  • Field-fallowing is helpful but may be impractical west of the Cascade Range.
  • Rotate susceptible crops with small grains.
  • Plant when soil is below 50°F.
  • Solarize lighter soils.

Cultural Conditions

  • Avoid mechanical injury from tillage, hoeing.
  • Irrigate with deep-well water or sanitized pond water.
  • Keep grafts and buds above soil line.
  • Avoid high nitrogen and irrigation late in the growing season.

The following are specific procedures for commonly grown plants that can be used in addition to the above general procedures.

Stone Fruit, Nut Crops, Roses: Dip or spray with the biocontrol agent K84. Apply to seed, bare roots, and aboveground grafts.

Anderson, A.R., and Moore, L.W. 1979. Host specificity in the genus Agrobacterium. Phytopathology 69:320-323.

Bazzi, C. 1983. Biological control of crown gall in Italy. In: Proceedings of the International Workshop on Crown Gall, Wadensville, Switzerland, R. Grimm (ed.) Wadensville, Switzerland: Swiss Federal Research Station for Fruit-growing, Viticulture, and Horticulture.

Bradbury, J.F. 1986. Guide to Plant Pathogenic Bacteria. Slough, United Kingdom: C.A.B International Press.

Cazelles, O., and Epard, S. et al. 1991. The effect of disinfection with oxyquinoline sulfate of the Berl. x Rip. 5C rootstock on the expression of crown gall in grape propagation. Revue Suisse de Viticulture, d’Arboriculture et d’Horticulture 23:285-288.

Deep, I.W., and McNeilan, R.A. et al. 1968. Soil fumigants tested for control of crown gall. Plant Disease Reporter 52:102-105.

Dhanvantari, B.N., and Johnson, P.W. et al. 1975. The role of nematodes (Pratylenchus penetrans, Meloidogyne hapla, Meloidogyne incognita) in crown gall infection (Agrobacterium tumefaciens) of peach in southwestern Ontario. Plant Disease Reporter 59:109-112.

Durgapal, J.C. 1977. Evaluation of rootstocks of pome and stone fruits and related wild species for resistance to crown gall. Current Science 46:389-390.

Garrett, C.M.E. 1987. The effect of crown gall on growth of cherry trees. Plant Pathology 36:339-345.

Gloyer, W.O. 1934. Crown gall and hairy root of apples in nursery and orchard. Geneva, NY: New York Agricultural Experiment Station Bulletin 638.

Goodman, R.N., and Grimm, R. et al. 1993. The influence of grape rootstocks on the crown gall infection process and on tumor development. American Journal of Enology and Viticulture 44:22-26.

Grimm, R. 1987. Control of crown gall in Swiss apple nurseries. Bulletin OEPP/EPPO Bulletin 17(2):269-272.

Heimann, M., and Beicht, W. 1980. Crown gall on (Erica) gracilis: what is it really? Gb + Gw, Gñrtnerbîrse und Gartenwelt. 80(32):712-716.

Ishizawa, Y., and Kyotani, H. et al. 1992. Methods for evaluating the degree of crown gall resistance and the varietal differences in peach. Bulletin of the Fruit Tree Research Station 23:37-46.

Lemoine, J., and Michelesi, J.C. 1993. Agronomic behaviour of pears: incidence of crown gall. Arboriculture Fruitiere 465:23-27.

Lu, S., and Canfield, M. et al. 1993. Use of sensitive nonradioactive methods to detect Agrobacterium tumefaciens in crown gall tumors of naturally infected woody plants. 6th International Congress on Plant Pathology, Montreal, Canada. Ottawa, Canada: National Research Council.

Mirow, H. 1985. Experiments on the control of crown gall on woody plants in the nursery. Deutsche Baumschule 37:300-301.

Moore, L.W. 1976. Latent infections and seasonal variability of crown gall development in seedlings of three Prunus species. Phytopathology 66:1097-1101.

Moore, L.W. 1976. Research findings of crown gall and its control. American Nurseryman 144:8-9.

Moore, L.W. 1980. Controlling crown gall with biological antagonists. American Nurseryman 151:44.

Moore, L.W., and Allen, J. 1986. Controlled heating of root-pruned dormant Prunus seedlings before transplanting to prevent crown gall. Plant Disease 70:532-536.

Ophel, K., and Nicholas, P.R. et al. 1990. Hot water treatment of dormant grape cuttings reduces crown gall incidence in a field nursery. American Journal of Enology and Viticulture 41:325-329

Pierronnet, A., and Eyquard, J.P. 1993. Prunus rootstocks and crown gall. Arboriculture Fruitiere 466:37-41.

Pu, X.A., and Goodman, R.N. 1993. Effects of fumigation and biological control on infection of indexed crown-gall-free grape plants. American Journal of Enology and Viticulture 44:241-248.

Rautenberg, E. 1973. Studies on Exobasidium galls of Rhododendron simsii Planch. II: conditions of gall development and the interaction between host plant and cecidia. Phytopathologische Zeitschrift 78(2):121-133.

Rebandel, Z. 1979. Effect of crown gall, Agrobacterium tume-faciens, and apple mosaic on bud take and tree growth in the nursery. Roczniki Akademii Rolniczej 114:137-145.

Ryder, M.H., and Jones, D.A. 1991. Biological control of crown gall using Agrobacterium strains K84 and K1026. Australian Journal of Plant Physiology 18:571-579.

Siegler, E.A., and Piper, R.B. 1929. Aerial crown gall of the apple. Journal of Agricultural Research 39:249-262.

Toumey, J.W. 1894. Preliminary report of observations on the “crown-knot”. Arizona Agricultural Experiment Station Bulletin 33. Tucson, AZ.

Utkhede, R.S., and Smith, E.M. 1990. Effect of fumigants and Agrobacterium radiobacter strain 84 in controlling crown gall of apple seedlings. Journal of Phytopathology 128:265-270.

Vicedo, B., and Penalver, R. et al. 1993. Biological control of Agrobacterium tumefaciens, colonization, and pAgK84 transfer with Agrobacterium radiobacter K84 and the Tra-mutant strain K1026. Applied Environmental Microbiology 59:309-315.

Vogelsanger, J., and Grimm, R. 1983. Collecting bacterial strains from crown galls of apple rootstocks and identification of Agrobacterium tumefaciens. In: Proceedings of the International Workshop on Crown Gall, Wadensville, Switzerland, R. Grimm (ed.). Wadensville, Switzerland: Swiss Federal Research Station for Fruit-growing, Viticulture, and Horticulture.

Vrain, T.C., and Copeman, R.J. 1987. Interactions between Agrobacterium tumefaciens and Pratylenchus penetrans in the roots of two red raspberry cultivars. Canadian Journal of Plant Pathology 9:236-240.

Table 1. Herbaceous ornamental plants susceptible to crown gall

Achillea millefolium

Yarrow

Argyranthemum frutescens

Marguerite daisy

Aster spp.

Aster

Begonia spp.

Begonia

Chrysanthemum x morifolium

Chrysanthemum

Coreopsis sp.

Tickseed

Cosmos sp.

Cosmos

Dahlia variabilis and hybrids

Dahlia

Dianthus caryophyllus

Carnation

Gaillardia x grandiflora

Blanket-flower

Helianthus annuus

Sunflower

Heuchera sanguinea

Coralbells

Hosta spp.

Hosta

Lamium maculatum

Dead-nettle

Leucanthemum x superbum

Shasta daisy

Lobelia sp.

Lobelia

Osteospermum sp.

Osteospermum

Pentas sp.

Pentas

Perovskia atriplicifolia

Russian sage

Stachys monieri

Betony

Verbena spp.

Verbena

Note: this is not a complete list, but represents the most commonly infected species.

Table 2. Woody plants susceptible to crown gall.

Euonymus spp.

Euonymus

Juglans regia

English walnut

Malus spp. and hybrids

Apple, crabapple

Photinia sp.

Photinia

Populus angustifolia

Willow-leaved poplar

Populus spp.

Cottonwood

Populus tremula

Aspen

Populus tremuloides

Quaking aspen

Prunus amygdalus

Almond

Prunus armeniaca

Apricot

Prunus avium

Cherry

Prunus cerasifolia

Cherry plum

Prunus domestica

Garden plum

Prunus x

Flowering cherry

Pyrus communis

Pear

Pyrus ussuriensis

Manchurian Pear

Rosa spp. and hybrids

Rose

Rubus spp.

Caneberries

Salix spp.

Willow

Vaccinium corymbosum

Blueberry

Vitis spp.

Grape

Note: this is not a complete list, but represents commonly infected species.

Table 3. Frequency of Agrobacterium Infection in Certain Host Plants

High

Low

Occasional/Rare

almond

Chrysanthemum spp.

aster

apricot

Citrus sp.

birch (Betula)

cherry

Clematis spp.

blueberry (Vaccinium)

peach

Ficus sp.

cactus

plum (Prunus)

Gypsophila sp.

Dahlia spp.

apple (Malus)

lilac (Syringae)

gossypium

Euonymous spp.

Macadamia

hydrangea

grape (Vitis)

marigold (Tagetes)

impatiens

poplar (Populus)

olive (Olea)

incense-cedar (Calocedrus)

raspberry (Rubus)

pear (Pyrus)

maple (Acer)

rose (Rosa)

quince (Chaenomeles)

Rhododendron spp.

Rubus spp.

Ribes spp.

sequoia

walnut (Juglans)

wild blackberry (Rubus)

spruce (Picea)

willow (Salix)

wisteria

Crown Gall

The soil residing bacterium Agrobacterium tumefaciens causes crown gall. Agrobacterium tumefaciens modifies the genome of host plants by introducing some of its bacterial genes into the plant’s chromosomes. However, plant breeders also use the crown gall bacterium as a tool in the process of genetic engineering.

Host Plants

Only a few plant species sustain tumors of a noticeable size, but there are over 650 species susceptible to infection. It is most common on brambles, woody vines (such as Euonymus), fruit and nut trees as well as willow and other hardwood shade tree. Conifers are generally resistant.

Description

Crown galls are tumors, which often arise on the stem where it meets the soil (root crown), but they also develop on roots and branches. Infected plant cells contain bacterial genes (actually a plasmid) that replace some of the normal plant cell’s genes. At first swellings on roots and stems are small, light-colored and round. They often enlarge and become more numerous during succeeding periods of plant development.

Crown galls encircle or extend along infected stems and roots due to excessive division and development of infected cells. When crown galls are numerous, plant leaves are yellowed and smaller than normal. Moreover, the impact of environmental stresses such as heat, water shortage and cold damage may be more severe.

Disease Cycle

Agrobacterium tumefaciens manages to survive in the rhizosphere on materials that leak from roots. It infects wounded host plant stems and roots. Actually, sugars and phenolic compounds emitted after wounding attract the motile crown gall bacteria. The bacterium infects stems and roots wounded by insect and nematode feeding as well as cultivation damage, root pruning and transplanting activities. The bacterium primarily affects the plant by attaching to the cell walls and inserting a plasmid with genes that code for production of plant growth-regulating hormones. The bacterial plasmid genes induce the production of higher than normal concentrations of plant hormones (auxins and cytokinins) that favor bacterial growth at the expense of the plant. Thus, the crown gall genes induce the plant cells to grow more profusely and to a larger size than they would normally grow, thereby creating galls. Agrobacterium tumefaciens returns to the soil as the galls decompose.

Management Strategies

Avoid introducing Agrobacterium tumefaciens to a site by inspecting the roots and stems of new plants for galls. The bacterium induces a genetic change distributed throughout infected plants. Thus, removal of galls from these plants is of little benefit other than to improve plant appearance. In soil infested with Agrobacterium tumefaciens, grow crown gall resistant plants. Some of the resistant trees include: beech, ginkgo, golden-rain tree, holly, hornbeam, little-leaf linden, magnolia, serviceberry, tuliptree, yellowwood, and zelkova as well as the conifers. Biologically control crown gall in nursery settings by soaking germinated seeds or dipping seedlings and rootstocks in a suspension of Agrobacterium radiobacter strain K-1026. The antagonistic bacterium inhibits the colonization of the roots by the crown gall bacterium.

Written by: Dan Gillman
Revised: 09/2011

Photos: A. K. Hagan and L. W. Moore, Diseases of Woody Ornamentals and Trees. APS Press.

Crown gall disease: A tumor home to a varied bacterial community

Disease reduces yield

Crown gall disease is triggered by the bacterium Agrobacterium vitis which causes tumour-like growths or galls at the vines. The yield decreases as a result and the vines die in the worst case. So far, no treatment for crown gall disease has been known.

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“Therefore, nurseries and wine growers are extremely careful to produce and use only vines that are free from agrobacteria,” Deeken further. This goal, however, is difficult to reach as the agrobacteria are capable of living hidden in the plant for several years before the disease breaks out. And without a method of early diagnosis, infected plants cannot be singled out in time.

Microbial community in tumour studied

The JMU biologists and doctoral student Hanna Faist also looked into the fundamentals of the vine disease. “We know from experience with humans that certain bacteria are capable of promoting the genesis and destruction of tumours,” Deeken explains. The team therefore focused on the microbial community living in the grown gall tumours.

The doctoral student analysed the bacterial colonization of diseased and healthy vines during one growth period and compared the results. Keller’s established method of high-throughput amplicon sequencing was employed for this purpose; the vines under examination were provided by the Bavarian State Institute for Viticulture and Horticulture in Veitshöchheim nearby Würzburg.

The results have been published in the journal of the American Society for Microbiology (Applied and Environmental Microbiology). They demonstrate that the microbial composition especially in the tumour tissue remains relatively stable throughout the seasons. However, the variety of bacteria in crown gall tumours is greater than in healthy vines.

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Some bacteria inhibit tumour growth

In addition to the bacteria found in the healthy tissue, there are others that are exclusively detected in the gall tissue. These include types that migrate into the tumour as “opportunists,” because it provides protection and nutrients. Others in contrast inhibit tumour growth.

The pathogen itself, however, does not need either the one or the other bacteria to generate the tumour. The scientists proved this on vines that had been cultivated sterilely on agar growth media provided by the nursery Rebschule Steinmann from Sommerhausen. An infection of the sterile vines with only Agrobacterium vitis was sufficient to trigger gall growth.

Deeken’s conclusion: “Our study backs the assumption that crown gall tumours accommodate a very special bacterial community. On the one hand, the bacteria benefit from the tumour environment, and on the other hand, they keep the disease in check to a certain degree: “The tumour does not grow so large that most infected vines would die.” Further investigations are aimed at studying the interactions between the different bacteria in the tumour.

Reduce the incidence of crown gall by planting noninfected “clean” trees. For seedling rootstocks, nurseries should collect the seed so it never contacts the soil where the Agrobacterium resides. Before planting, make sure trees stay moist and the roots do not dry out. It is also important to carefully handle trees to avoid injury as much as possible, both at planting and during the life of the tree in the orchard. Although preplant preventive dips or sprays with a biological control agent are available, their effectiveness can be variable on walnut trees. Strains of Agrobacterium tumefaciens (formerly A. radiobacter) strain K-84 are available as commercial products. However, it is effective only as a preventive treatment and does not eradicate galls. Use as a root dip or spray before heeling in (covering with moist soil until trees are planted) or planting.

Look for and manage crown gall during the growing season when the orchard is not wet because moisture favors the bacterium. When established orchard trees are infected with crown gall, you can use a combination of surgery, flaming, or a bactericide to treat the tumors. The best time to treat is in the spring or early summer because with rapid tree growth occurring, new callous tissue is formed relatively quickly.

Management is most effective for small galls on young trees. The procedure, however, can be expensive and difficult to carry out, depending on the size and location of the galls.

  • It is always best to remove galls when they are small. Usually by the time they’re seen at ground level, there may be extensive galling on crown area beneath soil.Only treat trees that are vigorous. Stunted trees should be removed.
  • If trees less than 4 years old are severely affected with galls, it is more economical to remove the trees and replant.
  • Treatment may be effective on older trees. The decision whether to treat galls or remove trees depends on tree vigor, the severity of galling, and the cost of treatment relative to the cost of replacing trees.

To treat crown galls:

  1. First remove soil away from the crown and roots to completely expose the gall. Soil can be safely removed using pneumatic equipment such as air compressors. Because no water is used, treatment can be done immediately after removal.
  2. To flame the gall, use a propane cylinder or bottle and slowly move the torch tip around the margin of the gall, creating a red-hot zone that is about 1 inch wide. It is advantageous to surgically remove the main part of gall in order to gain access to all parts of the gall margin. If surgery is used, be sure to sterilize the tools with heat before advancing to the next tree. Flaming should never be used on very young trees.
  3. As an alternative to flaming, galls can be treated with a bactericide such as Gallex, but treatment success is dependent upon complete removal of the gall first and then applying the treatment.
  4. Leave the treated areas uncovered for the summer and re-treat if galls begin to regrow. Treatment success is about 80%.

When replanting a previously affected site:

  • Remove as many of the old tree roots as possible.
  • Grow a grass rotation crop to help degrade leftover host material and reduce pathogen levels.
  • Fumigate with Telone C35.
  • Consider rootstock’s resistance (PDF). Clonal ParadoxRX1 has moderate resistance to crown gall. Clonal Paradox Vlach and VX211 have low resistance to crown gall. Although seedling black rootstock is not as susceptible to crown gall as seedling Paradox, walnut varieties on black rootstock generally aren’t as vigorous so should be planted on loamy soils.
  • Offset the new trees from the previous tree spacing to minimize contact of healthy new roots with any infested roots and soil that may remain.
  • Keep the crown area dry to help reduce disease severity.

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