By: Jackie Carroll
Before you decide to start crown gall treatment, consider the value of the plant you are treating. The bacteria that causes crown gall disease in plants persists in the soil as long as there are susceptible plants in the area. To eliminate the bacteria and prevent the spread, it’s best to remove and destroy diseased plants.
When learning about crown gall treatment, it helps to know more about what is crown gall in the first place. Plants with crown gall have swollen knots, called galls, near the crown and sometimes on the roots and twigs as well. The galls are tan in color and may be spongy in texture at first, but they eventually harden and turn dark brown or black. As the disease progresses, the galls can totally encircle the trunks and branches, cutting off the flow of sap that nourishes the plant.
The galls are caused by a bacterium (Rhizobium radiobacter formerly Agrobacterium tumefaciens) that lives in the soil and enters the plant through injuries. Once inside the plant, the bacterium injects some of its genetic material into the host’s cells, causing it to produce hormones that stimulate small areas of rapid growth.
Unfortunately, the best course of action for plants affected by crown gall is to remove and destroy the infected plant. The bacteria can persist in the soil for two years after the plant is gone, so avoid planting any other susceptible plants in the area until the bacteria dies out for lack of a host plant.
Prevention is an essential aspect of dealing with crown gall. Inspect plants carefully before you buy them and reject any plants with swollen knots. The disease can enter the plant in the nursery through the graft union, so pay particular attention to this area.
To prevent the bacteria from entering the plant once you get it home, avoid wounds near the ground as much as possible. Use string trimmers with care and mow the lawn so that debris flies away from susceptible plants.
Galltrol is a product that contains a bacterium that competes with Rhizobium radiobacter and prevents it from entering wounds. A chemical eradicant called Gallex may also help prevent the crown gall disease in plants. Although these products are sometimes recommended for crown gall treatment, they are more effective when used as a preventative before the bacteria infects the plant.
Over 600 different plants are affected by crown gall, including these common landscape plants:
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Read more about Plant Diseases
A gall in a plant is somewhat similar to a tumor in an animal. In winter creeper (Euonymus fortunei) these galls often form near ground level or on lower limbs and branches. They are caused by a soil borne bacterium called Agrobacterium tumefaciens. It enters the garden through infected nursery stock or contaminated soil.
The bacteria may enter the plant through injuries during transplanting, on pruning equipment, in the soil, or through feeding by chewing insects. Once inside the plant, it stimulates the abnormal growth of cells that result in the ugly tumors. The bacterium has been known to survive up to two years in the soil after the removal of infected plants.
The major symptom is, of course, the dark, corky textured growths called galls that are borne on the plants down near ground level. Plant tissue above the tumor may exhibit signs of dieback when the gall girdles an individual stem or the main trunk of the euonymus.
Note that the plant does not automatically die from crown gall. Generally a branch here and a branch there will die but the entire plant may survive in a weakened state for years.
Hardy chrysanthemums that develop yellow-brown spots starting on the lower leaves and gradually moving up the stems may be infested with foliar nematodes. Nematodes are slender, unsegmented roundworms that are barely visible to the unaided eye. Foliar nematodes overwinter in the soil, in infested plant material. They swim up the film of water on the plants, created by spring rains, and enter leaves through the stomata. Nematodes can become dormant and survive for over a year in fallen leaves. Yellow-brown spots on the leaves eventually run together and cover the entire leaf, which dies, turns brittle, and falls. Severe infestations can kill entire plants. Foliar nematodes are easily confused with leaf spot (see above), but fungal leaf spots are most often black, not brown. They additionally infest hosta and ferns.
Prevention & Treatment: Remove infested plant material, along with the surrounding soil. Mulch plants in the spring to discourage nematodes from climbing up from the soil, and avoid spraying water on the leaves when watering. Foliar sprays with insecticidal soap may help reduce nematode populations. See Table 1 for examples of brands and products.
This disease is caused by the fungus Puccinia chrysanthemi. Rust infection causes pale areas to appear on upper leaf surfaces, with powdery orange pustules or spots directly beneath on the undersides of the leaves. Severely infected plants are much weakened and fail to bloom properly.
Prevention & Treatment: Remove infected leaves as soon as possible. Set new plants farther apart and provide better ventilation. Water the soil without wetting the plants. If disease is severe enough to warrant chemical control, use a fungicide with mancozeb as active ingredient. See Table 1 for examples of brands and products. Follow all directions on the label. Some chrysanthemum varieties, which are resistant to rust, are ‘Achievement’, ‘Copper Bowl’, ‘Escapade’, ‘Helen Castle’, ‘Mandalay’, ‘Matador’, ‘Miss Atlanta’, ‘Orange Bowl’, and ‘Powder Puff’.
This disease is caused by the fungi Verticillium albo-atrum and/or Fusarium oxysporum). The first symptoms of wilt are yellowing and browning of the leaves, which die from the base of the plant upwards. Infected plants are stunted and often fail to produce flowers. The entire plant may wilt and die. The fungus is soil-borne, and enters the plant through the roots, later invading the vessels of the stem and cutting off the water supply.
Prevention & Treatment: Control of this wilt on plants grown in infested soil is difficult. Remove and destroy all infected plant material. Ultimate control lies with purchasing certified disease-free plants and strict sanitation locally. It is best to use resistant varieties. If Fusarium has been a problem, increase the pH of the soil to 6.5 – 7.0.
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:
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.
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.
Rose rosette disease is an untreatable rose disease caused by the Rose rosette virus (RRV), and is spread and introduced into the rose during feeding by the rose leaf curl mite (Phyllocoptes fructiplilus). This extremely small eriophyid mite feeds on cell sap of the tender stems and leaf petioles. The rose leaf curl mite alone causes little damage while feeding, but if it is a carrier of RRV, symptoms begin to appear in the rose typically within one to three months.
Roses exhibit reddened terminal growth on infected branches, and the stems become thicker and more succulent than those on unaffected parts of the plant. These stems exhibit an abnormally high number of pliable thorns, which may be either green or red. Rose leaves that develop on infected branches are smaller than normal and may be deformed similarly to herbicide injury by 2,4-D. Lateral branches may grow excessively from main stems and create a witch’s broom symptom quite like glyphosate (Roundup™) injury on roses. Flowering is reduced, and the petals may be distorted and fewer in number.
Rose with Rose Rosette Disease showing symptoms of reddened new growth, thicker stem, excessive thorns, and smaller leaves.
Meg Williamson, Plant and Pest Diagnostic Clinic, Clemson University
These symptoms generally become evident in the late spring to early summer and progress during the growing season. Once the rose becomes infected, RRV moves throughout the plant and the entire plant is infectious. By the time symptoms are evident in a rose, it already may have spread to adjacent plants by the movement of the eriophyid mites. Infected plants typically die within a couple of years.
Prevention & Treatment:The wild multiflora rose is very susceptible to the rose rosette disease, so any nearby wild plants should be removed and promptly disposed. Any infected, cultivated roses should be immediately removed, then burned or bagged. Also remove any roots, which might re-sprout later. Do not leave an uprooted infected plant in the garden, as the mites may leave this rose for other nearby plants. Always space rose plants so they do not touch.
Because RRV is systemic within the infected rose plants, grafting asymptomatic stems onto other rose plants will transmit the virus. Pruners used on diseased plants must be disinfested with rubbing alcohol or a dilute bleach solution before being used on uninfected plants, as sap on the pruners is contaminated with the virus.
To reduce the spread of the eriophyid mites from the site of an infected rose, nearby roses can be treated with a bifenthrin spray every two weeks between April and September. This may prevent additional plants from becoming diseased. See Table 1 for examples of brands and products containing bifenthrin. Always check product labels for the correct active ingredient. Follow label directions for use.