By: Bonnie L. Grant, Certified Urban Agriculturist
Got galls? Galls are overgrowths of the stems in plantsthat resemble tumors. In chrysanthemums, they appear on the main stemand peripheral twigs. The fat, ugly tumors are the most obvious of thechrysanthemum crown gall symptoms. What causes this and how do you prevent it?The disease affects plants in over 90 families and is as contagious to plantsas the common cold is to humans.
Crown gall of mum plants disrupts the flow of nutrients andwater to other parts of the specimen. The first observed symptoms are usuallyon the crownof the plant but can also be seen on the stem. The disease also affects theroots, but this is less easy to detect without digging up the plant.
The galls are warty tumors seen upon the basal or crownparts of the chrysanthemum. They are light green to whitish and soft whenyoung, but become brown and woody as they age. Galls can also appear on leaves,generally at the mid-veins. They are smooth, tan and about ¼ inch (.64 cm.)across.
Over time, crown galls will cause stunted growth and limitedvitality in the plant. Crown gall of mum plants can lead to lower production offlowers; yellowed, limp leaves; and overall diminished plant health. Thesesymptoms can mimic many other issues such as lack of water, low nutrients andplant injury.
Agrobacteriumtumefaciens is the culprit when crown galls appear. It is a naturallyoccurring bacterium in the Bacillus groupthat persists in soil where aeration is adequate. It can also survive on theroots of plants. The most common soils in which the bacterium survives aresandy loams.
The disease spreads easily through poor sanitation practicesand plant injury. Any slight nick in the plant surface can invite the bacteriumto enter. Even tissue that has experienced frost damage may allow the diseaseinto the plant’s vascular system. Using unsanitized pruning tools can alsotransfer the disease to the chrysanthemum.
There are a couple of methods of treating mums with crowngall, but inspecting plants before planting can help prevent the spread of thedisease in the garden. Often, nursery stock is already contaminated with thedisease, which can be seen early in the roots of new plants.
Look for nodes and irregular growth on plants beforeplanting. Additionally, disinfect your cutting shears to preventtransfer of the disease.
In greenhouse situations, a creosoteor copper-based product is used to some effect. In the home garden, use of suchproducts is not recommended and it is best to dig out and destroy any affectedplant.
Before planting any susceptible stock in the soil again, solarize it to kill the bacteria and avoidre-infection in your garden. A useful pre-planting chrysanthemum crown galltreatment is to dip the roots of a new plant into Agrobacterium radiobacter, abiological control that essentially inoculates your plant. This can be hard tosource, however, but good sanitation, crop rotation and inspection of newplants is usually sufficient.
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By Alyssa Collins
A Class Project for
PP728 Soilborne Plant Pathogens
North Carolina State University
Department of Plant Pathology
Agrobacterium tumefaciens, the cause of the economically important disease, crown gall, has also been studied for years because of its remarkable biology. The mechanism this bacterium uses to parasitize plant tissue involves the integration of some of its own DNA into the host genome resulting in unsightly tumors and changes in plant metabolism. A. tumefaciens prompted the first successful development of a biological control agent and is now used as a tool for engineering desired genes into plants.
Host Range and Distribution
Agrobacterium tumefaciens is cosmopolitan in distribution, affecting dicotyledonous plants in more than 60 different plant families. Crown gall can be found most often on stone fruit and pome trees as well as brambles and several species of ornamental plants.
Agrobacterium tumefaciens is a member of the family Rhizobiaceae. These bacteria are Gram-negative and grow aerobically, without forming endospores. The cells are rod-shaped and motile, having one to six peritrichous flagella. Cells are 0.6-1.0 m m by 1.5- 3.0 m m and may exist singly or in pairs. In culture on carbohydrate-containing media, cells produce large amounts of extracellular polysaccharides, giving colonies a voluminous, slimy appearance.
Recently, a reclassification of the species of Agrobacterium has been undertaken by use of ribosomal RNA sequencing as a taxonomic tool. The resulting nomenclature places the former species, A. tumefacians biovar 1, A. radiobacter biovar 1, and A. rhizogenes biovar 1, within the new taxon: Agrobacterium tumefaciens.
A. tumefaciens can be effectively isolated for identification from gall tissue, soil or water. Optimal gall tissue for isolation is white or cream-colored from a young, actively growing gall. The gall should be washed or surface sterilized using 20% household bleach, and rinsed several times in sterile water. Cut a few samples from different parts of the white tissue of the gall, and further divide samples into small pieces. Place these pieces into a culture tube containing sterile distilled water or buffer, vortex and allow to stand for at least 30 minutes. Using an inoculating loop, streak this suspension on Medium 1A (Schaad et al., 2001), and incubate at 25-27 ° C. Different strains will grow at different rates. One may also use this selective medium to detect A. tumefaciens in soil dilutions or irrigation water.
It should be noted, however, that the presence of A. tumefaciens cells in a sample does not necessarily dictate the existence of the crown gall-inciting strain in the sample. Only cells containing a specific plasmid (the Tiplasmid) can cause disease. A. tumefaciens strains lacking the plasmid live as rhizosphere-inhabiting bacteria without causing disease.
Crown gall manifests itself initially as small swellings on the root or stem near the soil line, and occasionally on aerial portions of the plant. Young tumors, which often resemble the callus tissue that results from wounding, are soft, somewhat spherical and white to cream colored. As tumors become older, their shape becomes quite irregular, and they turn brown or black. Tumors may be connected to the host surface by only a narrow bit of tissue, or may appear as a swelling of the stem, not distinctly separate. The tissue can be spongy and crumbling throughout the gall or can be woody and knot-like. Several tumors may occur on the same plant and may rot from the surface of the plant completely or partially, possibly developing repeatedly in the same area season after season. Additional symptoms include stunting, chlorotic leaves, and plants may be more susceptible to adverse environmental conditions and secondary infection.
Pathogenic strains of A. tumefaciens may live saprophytically in soil for up to two years. When a nearby host plant is wounded near the soil line by insect feeding, transplant injury or any other means the bacterium chemotactically moves into the wound site and between host cells. These bacteria then stimulate the surrounding host cells to rapidly and irregularly divide. The bacterium accomplishes this by inserting a piece of its own DNA into the host cells' chromosomes, causing overproduction of cytokinins and auxins which are plant growth regulators, and opines which serve as nutrients for the pathogen. The resulting tissue is undifferentiated with a white or cream color, and cells may have one or more nuclei. This tissue continues to enlarge and a tumor is formed on the root or stem of the plant, depending on original wound site. The bacteria occupy the intercellular spaces around the periphery of the gall and are not found in the center of the enlarging tumor. The tumor is not protected by an epidermis, leaving the tissue susceptible to secondary pathogens, insects and saprophytes. Degradation of the tumor by secondary invaders causes brown or black discoloration and releases A. tumefaciens cells back into the soil to be carried away by with soil or water, or remain in the soil until the next growing season. In perennial plants, part of the infected tissue may remain alive and inhabited by A. tumefaciens, which, even if the tumor has sloughed off, can persist to cause a new tumor the following season in the same place.
Introduction of pathogenic A. tumefaciens strains can be avoided by thorough inspection of nursery stock for crown gall symptoms. Susceptible varieties should not be planted in soils known to be infested with the pathogen. These soils should be planted in a monocotyledonous crop like corn or wheat for several years. Nursery stock should be certified crown gall-free and should be budded rather than grafted. If the threat of crown gall exists, all practices that wound tissue should be avoided and chewing insects should be controlled.
Preventative treatment of seeds or transplants with the non-pathogenic biocontrol organism Agrobacterium radiobacter is a relatively inexpensive and effective means of managing the development of crown gall in commercial operations. Application of this antagonist by soaking seeds or dipping transplants can prevent infection by most strains of A. tumefaciens due to the production of the antibiotic agrocin 84 by strain K84 of A. radiobacter. Some curative properties are exhibited by a commercially available mixture of 2,4-xylenol and metacresol in an oil-water emulsion when painted directly on established tumors. But this is rarely used due to labor and time constraints.
Agrios, G.N. 1988. Plant Pathology, 3 rd Ed. Academic Press Inc., London. pp. 558-565.
Horst, R.K. 1983. Compendium of Rose Diseases. APS Press, St. Paul, MN. pp 23-25.
Schaad, N.W., J.B. Jones & W. Chun. 2001. Laboratory Guide for Identification of Plant Pathogenic Bacteria, 3 rd Ed. APS Press, St. Paul, MN. pp. 17-35.
Links to other sites with information about Agrobacterium tumefaciens
Crown gall is a bacterial disease that can impact a broad range of host plants. The disease organism is Agrobacterium tumefaciens causes crown gall disease. Common hosts are fruit trees, grapes, euonymus, rose, willow, and several other broadleaf trees and shrubs. It is often found in our area and gardeners should learn to recognize, remove it and, hopefully, reduce its spread.
Crown gall disease symptoms are wart-like growths or galls principally on the root crown at the soil line or just below the soil surface. Galls first appear as smooth swellings and develop rapidly into large tumors with a rough, warty, or cracked appearance. Galls can also form on roots, limbs, and trunks of many woody plants. On blackberries, raspberries, and grapes, galls may also form on stems or canes. Crown gall usually does not seriously harm woody plants unless the galls occur in the root crown area when plants are young. This interrupts or slows the flow of compounds necessary for plant growth. As a result, the plants become stunted and subject to wind damage and drought stress. If galls are large, young plants can be girdled and killed.
The crown gall bacteria stimulate the plant to produce actively growing, disorganized tissue which originates in the cambium (the layer between the inner bark and wood) and then swells and erupts through the bark. The surface of a crown gall is the same color as healthy bark and gall wood has the same color as normal wood. However, when cut with a knife, crown galls are softer than normal wood and lack the typical pattern of annual growth rings. Galls can be tiny and smooth on young plants, but are commonly rough and sometimes massive on mature trees.
Crown gall bacteria can survive in the soil for at least 2 years in the absence of host tissue or for several years in decomposing crown gall tissue. Bacteria are released into the soil when galls are wet or when older gall tissue disintegrates. Seedlings may also be affected by the bacterium during germination established trees and vines are infected only through fresh wounds such as those caused by growth cracks, pruning, damage by cultivation equipment, or freeze injury.
Gardeners can greatly reduce problems with crown gall by obtaining planting material from a reputable nursery that uses clean propagation materials and growing media. The presence of visible galls on ornamentals in nurseries is considered sufficient for plant destruction. Galls do not appear until after the plant has been growing for one or more seasons.
Whenever possible, you should examine bare-root trees and vines before purchase, and plant only high-quality nursery stock. Avoid susceptible varieties. Gardeners should plant only resistant species where crown gall has previously been a problem. Some resistant or immune species are cedar, magnolia, pine, boxwood, redbud, smoke tree, liquidambar, holly, mahonia, and pyracantha.
Other practices can reduce the incidence and spread of crown gall disease. Avoid injuring trees during transplanting, and avoid injury or pruning wounds that will come into contact with soil. Existing galls may be excised by cutting into healthy wood around galls, then exposing the tissue to drying. Cut out galls only during the dry season and minimize the amount of healthy tissue into which cuts are made. Disinfect pruning and digging tools before using on healthy plants. Remove and destroy severely infected plants. Solarization during the hot dry season before planting may reduce crown gall bacteria in the soil. Do not be too aggressive on larger trees and vines as they can usually tolerate the development and presence of some crown galls.
Discovery of crown gall in your orchard, vineyard, or landscape is not the end of the world, but its presence should increase your vigilance and attention to sanitation practices when working around infected plants. Below, I have included some additional information resources that include photos of crown gall disease.
Prevention is the best method of control because once established in an area, the crown gall bacteria can be very difficult to eliminate.
Check all new plants carefully.
Do not plant any tree or shrub with galls on the roots or stems.
Pay extra attention when planting roses, fruit trees, poplar or willows.
The biological control bacteria Agrobacterium radiobacter K-84 can be used to protect trees and shrubs from crown gall infection during planting.
Dip roots of bare root plants or drench potted plants with a solution of water and biological control bacteria, Agrobacterium radiobacter K-84.
These bacteria protect roots by producing an antibiotic.
They also cover wound sites that the crown gall bacteria would use to start an infection.
Galltrol is a commercial formulation of A. radiobacter K-84 that is registered for use in Minnesota.
On a recently planted tree or shrub:
Dig up the plant and the soil immediately around the roots and dispose of it.
Do not add infected plant material to compost piles.
Burning is the best way to dispose of infected woody plants.
On established trees and shrubs:
Established trees and shrubs can tolerate infection with crown gall and can be left in the landscape.
Make sure to disinfect pruning tools with a 10-percent solution of household bleach after using them to prune crown gall infected trees.
If infected plants exist on your property, avoid planting highly susceptible species like rose, willow, poplar and fruit trees.
Michelle Grabowski, Extension educator and Rebecca Koetter