Dahlia Disease Biology Part 3 – Crown Gall (Agrobacterium tumefaciens aka Rhizobium radiobacter)

By Nichole Warwick  on 3/30/2023

You’re likely reading this because you have an inherent interest in understanding dahlia disease biology and the diseases that plague dahlias. In Part 1 we discussed differences between bacteria and viruses. Then in Part 2 we looked into Leafy gall (Rhodococcus fascians). All things considered, Leafy gall is benign compared to Crown gall. The lesson for Part 3 will likely be a bit longer as Crown gall is a more complex infection, but it’s fascinating at the same time. 

The Pacific Northwest Pest Management Handbook states in one line that Crown gall is “a rare problem in dahlias”(1). Take a look at all the questions on Dahlia Growers Facebook page, and you may hold a different opinion. Crown gall certainly seems persistent and common, and advising your local extension center of your diseased tubers may help move dahlias and their infections to the forefront. Washington State University has been pairing with the American Dahlia Society to raise awareness of dahlia diseases, but much of the work is in viruses.

Crown gall is caused by the bacterium Agrobacterium tumefaciens which translates roughly into agricultural bacteria causing tumors and ribbon growth. There is debate over the name of the bacteria and it also goes by (and scientists are moving towards calling it ) Rhizobium radiobacter which roughly translates into bacteria around the radius of a plant’s rhizome (root) area. While this bacteria can be aerial and impact upper tissues of the plant, it largely presents on the roots of the plant. The basics of the infection occurs when the bacteria gets beneath the skin of the plant during an injury.

Like Leafy gall, this bacteria can be on the surface of the plant and a wound on the root stock can provide an opportunity for the bacteria to enter the plant. When the plant is wounded, it releases healing chemicals and sugars to feed the cells that are healing, inadvertently attracting the bacteria causing Crown gall to the site(4). Crown gall bacteria have many flagella off one end of the rod shaped cell that the cells use to move towards the wounded tissue(9). The bacteria then disrupts cell signaling similar to the bacteria of Leafy gall, causing profuse growth that forms the gall. Galls can be smooth or rough skinned and sometimes resemble a cauliflower growth. In the early stages, they may be pea size, but can grow to over a golf ball in size. The specifics of the infection are really interesting. 

This bacteria is REALLY COOL, but also a bit terrifying and a fun lesson in biology. We are used to thinking of genes spreading from one generation to the next. Mother and father plants produce sons and daughters of the next generation in a gene transfer known as vertical transmission. Some bacteria have special appendages on their surface known as pili (singular is pilus). These allow bacteria to bind to another bacteria of a related species. When two bacteria of the same generation connect their pili, it becomes known as the sex pilus. Inside the bacteria are little bits of genetic material called plasmids. They contain transfer DNA (T-DNA) which is basically a little gene that gets passed from one bacteria to another. This allows bacteria of the same generation to pass genes between each other in a process known as horizontal gene transfer.

Here is where things get a little science fiction like. Horizontal transfer can happen between bacteria and plant cells. The bacteria that do horizontal gene transfer are looking for specific receptors on the surface of the cell that allows them to recognize each other. Interestingly the bacteria that causes Crown gall sees a similar receptor on the surface of dicot cells that allows the bacteria to bind to a dicot plant cell (essentially plants that are not grasses). The bacteria (evil buggers) use their sex pilus to inject the plasmid carrying T-DNA into the plant cell. The real horror begins as the T-DNA then causes the plant cells to change hormone expression. This causes growth of the plant cells. Each cell that divides makes a copy of itself, including the bacterial plasmid with the T-DNA. Increased cell signaling brings the sugars the bacteria likes to feed off to the area, and mitosis increases at the infection site to create the gall. Once the plasmid is inside the host cells from a horizontal transfer, every new generation of cell passes the infection on in a vertical transfer.

The bacterial plasmid gene in the horizontal transfer means the plant cells are going to pass this gene on to each future generation. So consider how a dahlia from a tuber grows. The crown (infected with the plasmid) starts to grow. It forms leaves and stems. Each one of those cells has come from the infected part of the plant. Each new generation of cells carries the infected plasmid. Systemic infection has been well noted in grapes and roses(5). Just like in human tumors, there is an increase in vascular tissues for the plant to support the growth of the new diseased tissue. Plants have two types of vascular growth called xylem and phloem. Both of these grow into the tumor, connecting the bacteria to the rest of the plant(10).

Disease can present as early as 2-4 weeks after infection if the plant is 68 degree F (20 degrees C) or higher. This means it will be dormant over the offseason and then as growers start to wake their tubers, the bacteria will also be activated. Below on the right is a picture of a dormant tuber, you can see the dried lesions of Crown gall. When tubers are lifted in the fall, there is naturally a bit of trauma from lifting out of the soil.  Loosening the soil around the plant and then lifting the stalk in one hand while using a large shovel or spade to support from underneath can help reduce some of the damage inflicted by lifting purely by the stem. Furthermore, when snips, dahlia knives, loppers, or power tools are used to divide tubers into singles, that inflicts a major wound that allows the bacteria into the tuber. When the tubers are going into dormancy, they may look fine in the fall because the bacteria is on the surface, but it’s introduced during the digging and dividing process, and then in spring when the temperatures become favorable for tubers to wake, the bacteria is activated too.

Lesions may be small initially, or present all over the tubers at various locations. At this stage of infection, it may be difficult to tell if there is a Crown gall lesion because they may appear similar to a callus. Blind tubers often will try and force growth as well. Without a crown, they may create a bulge that can be confused with Crown gall. Tumors can form not just at the crown like seen with Leafy gall, but also along the length of the tuber or at the end. Some try to convince themselves they are seeing lenticels, an adaptation tubers make to deal with excess water.  Stored tubers may have remnants of lenticels (see image to the right of lenticels), but a tuber stored in dry medium will have no need for this adaptation, so growths on the tuber coming out of storage are Crown galls, not lenticels.

Heeling in is a practice used by nurseries and growers to densely plant an area for a temporary time with bare root stock. When a gardener takes their tubers and lays them crown up, row over row in a tray with some soil, this is a version of heeling in. This method  contaminates all the tubers in the tray because of soil and water. The practice of healing in has been linked to the spread of Crown gall. The water in the soil creates the fluid way for the bacteria to use their flagella to move towards the plant and a wound on the plant. Water is a main transmission route for both types of gall. Crown gall bacteria use their flagella to move through the wet soil. When tubers are heeled in to a common tray for cuttings or waking tubers up, there is risk of spread, particularly if water is added.

Insects, especially burrowing insects can disrupt the root area of the plants. Deep and older galls have cracks that allow insects like earwigs to hide. It’s been suggested that by inducing a gall, insects have a place to hide and locate food(13).  Insects may pick up the bacteria on their feet and as a vector can move the bacteria to new plants(9). Other insects like aphids (11, 13), thrips (13) and mites (12) can pass the infection too. There is debate if these insects carry the disease in their mouths or simply create the wound by biting the plant for the bacteria on the surface of the plant to enter. Nematodes are also linked to gall spread, so if nematodes are a problem in the garden, they will spread the bacteria as well. Before planting a suspicious tuber, risks to the rest of the garden should be considered. 

In July of 2017, Oregon State University published a paper and filed for a patent on a test that could allow home gardeners to test for pathogenic strains of Crown gall in their garden(5). The root of this work was to try and identify the presence of the plasmids that carry the bacterial gene that transforms the plant. At this point, I have been unable to find a retailer of this product meaning testing only happens when you send tubers to a university plant clinic or Department of Agriculture office in your state. As such, prevention is the best practice to reduce spread of this disease. 

There are several things you can do to prevent spread. The small wounds of a plant release sugar and nutrients that allow the Crown gall bacteria to flourish. So it is important to be careful around the rhizosphere (area around the roots and tubers) to avoid making cuts or small wounds that would require healing. Tools should be disinfected and transmission precautions used. 

Clay soils and poor draining host the bacteria longer, upwards of 3 years(3). Persistence in well draining loose soils is much shorter. Soil is a potential reservoir of the bacteria and with numerous host plants that are dicots, only grasses (monocots) should be planted in that location for up to three years (if clay based soil) to prevent reinfection of plants. Data supports removing the bacteria by solarizing soils(2). This process involves tilling the soil, then watering heavy 2 days followed with a clear plastic over the contaminated soil (preferably that doesn’t allow condensation) with the edges buried to trap heat. The solar treatment needs a minimum of 4-6 weeks, preferably in July/August during the hottest months of the Northern hemisphere summer. This treatment did not work well in clay soils(3) likely because clay holds water in the soil that the bacteria need to move to infect a plant. There is no data to support dosing the soil with a 10% bleach solution. Pasteurization of soil (this is done with a bag of soil, not directly in the ground) has also shown to reduce bacteria.

Used predominantly for bare root agriculture crops of berries, fruit trees and roses, there is a biological control called Agrobacterium tumefasciens strain 84 which out competes the pathogenic strains. This works as a preventative measure, not a treatment after infection and is available in the form of Galltrol-A or NoGall(2). This has been used to reduce infection in bare root stock, but has not been studied with tubers, a likely side effect of dahlias being on the “rarely infected” list. Curiously in the US, Idaho does not support the use of either of these treatments.

Another possible protectant used for woody herbaceous bare roots is Fertilome Fire Blight Spray (streptomycin)(2). This is a form of antibiotic and the USDA is cracking down hard on the availability of antibiotics for use in agriculture and farming. This is related to the unprecedented increase in antibiotic resistance, including presences of antibiotics in the water supply. Changes go into effect June 2023, so while nurseries and professional growers may be able to access this treatment through their Ag inspector, this will likely not be available to the common gardener for much longer. 

The Pacific Northwest handbook suggests only 90 plant species affected (2) including roses, blueberries, and raspberries. However the University of Massachusetts reports over 650 plants susceptible to infections(4).  Host plants are numerous(3). Root attacking insects and nematodes can cause wounds allowing bacteria to enter and can act as vectors to move the bacteria in the garden, carrying disease from dahlias to roses, blueberries, raspberries and other susceptible plants in the garden. 

There is hope for the future. I recently listened to a podcast by Short Wave on NPR (7, 8). They discussed gene therapies to help plants keep up with rapidly changing climate in which natural adaptations would be too slow to keep up with. Oregon State University has developed an oncogene silencing treatment employed in root stock of apples and like crops that make the plants resistant to Crown gall(6). Perhaps this idea can be used to create dahlia stock resistant to Crown gall in the future. 

Until the OSU developed tests are readily available, any plant suspected of Crown gall should be removed from the garden. Fiercely more contagious than Leafy gall, with so many potential hosts and so many ways for the bacteria to be transferred, a suspect tuber or plant should be immediately trashed before more of the garden is impacted. 

Cheers and Happy Gardening.

Special thanks to:

Thank you to Antonio Toledo and Wendy Vreeken Banham for additional photos to provide this section with several photos of all the ways that Crown gall can present.

Sources

1. – Dahlia-Crown Gall | Pacific Northwest Pest Management Handbooks
2. – Rose (Rosa spp.) and hybrids-Crown Gall | Pacific Northwest Pest Management Handbooks
3. –Crown Gall Disease of Nursery Crops | Pacific Northwest Pest Management Handbooks
4. –Landscape: Crown Gall | Center for Agriculture, Food, and the Environment at UMass Amherst. 
5. – Isothermal Amplification and Lateral-Flow Assay for Detecting Crown-Gall-Causing Agrobacterium spp.
6. – Transformation of Plants by Agrobacterium – OREGON STATE UNIVERSITY
7. – Could de-extinction of dodo, wooly mammoth aid conservation? | Short Wave
8. – From dodos to Neanderthals, it’s boom times in ancient DNA research | Short Wave
9. – Crown gall
10. – Crown Gall – an overview | ScienceDirect Topics
11. –Symbiotic bacteria of the gall-inducing mite Fragariocoptes setiger (Eriophyoidea) and phylogenomic resolution of the eriophyoid position among Acari – PMC
12. – One aphid species induces three gall types on a single plant: Comparative histology of one genotype and multiple extended phenotypes – ScienceDirect
13. – The mechanism of plant gall induction by insects: revealing clues, facts, and consequences in a cross-kingdom complex interaction

More Reading

IS IT CROWN GALL OR LEAFY GALL

Beyond horizontal gene transfer: the role of plasmids in bacterial evolution | Nature Reviews Microbiology