Dahlia Gall Disease: Unmasking Crown Gall and Leafy Gall

Copyright © 2025 by Steve K. Lloyd
All Rights Reserved

Hidden in the soil and even within seemingly perfect tubers lurk microscopic threats that can disfigure your dahlias. Crown gall and Leafy gall are two of the most vexing—and misunderstood—bacterial diseases in ornamental horticulture.

These distinctive symptoms are caused by bacteria, microscopic single-celled organisms—not viruses, which are even smaller infectious agents. Both of these bacterial diseases begin with pathogens residing in the soil. These soilborne bacteria then have a remarkable ability to hijack the plant's own cellular machinery, essentially redirecting its normal growth processes into abnormal, tumor-like structures.

Cauliflower, anyone? A dramatic example of leafy gall caused by  Rhodococcus fascians  (Antonio Toledo photo)

While gall diseases aren't exclusive to dahlias, as growers of this cherished flower we need to be aware that the same pathogens affecting many other ornamentals, like roses, chrysanthemums, or petunias, can also find a home in our dahlia gardens.

There is a lot of information online about dahlia diseases. Some of it is accurate and understandable, but much of it is incomplete or downright misleading. As an experienced dahlia grower and hybridizer with a curious mind and a scientific streak, I set out to learn all I could about crown gall and leafy gall, and I’d like to share what I learned with other gardeners.

This article brings together what scientists have learned about these diseases with practical insights for dahlia growers to recognize, prevent, and manage gall in their own gardens.

While popular gardening websites often cover the basics, this guide goes deeper, drawing on published scientific research and practical experience to give you a clearer picture of what's going on beneath the surface.

Although both crown gall and leafy gall present as abnormal growths, they are caused by different bacteria and behave quite differently in the plant. Understanding this distinction is key to effective management.

For instance, Agrobacterium tumefaciens, the cause of crown gall, is described as a hemibiotroph , meaning it initially lives off living plant cells before eventually causing their death.

In contrast, Rhodococcus fascians, which causes leafy gall, is a biotrophic pathogen , deriving nutrients from living host cells without immediately killing them.

Crown gall is caused by Agrobacterium tumefaciens, a soil-dwelling bacterium that enters the plant through wounds, especially during propagation or transplanting.

Once inside, it transfers a segment of its DNA into the plant's cells, where it integrates into the plant genome (the complete set of genetic instructions in a plant's DNA) and permanently alters the growth pattern.

The result: round, tumor-like swellings ("galls") that typically form at the base of the stem or on tubers. These galls disrupt the flow of water and nutrients, potentially weakening or killing the plant.

More troubling for dahlia growers is the fact that infected tubers can carry the disease from one season to the next.

A dahlia clump with Crown gall caused by Agrobacterium tumefaciens (Author's photo)

Leafy gall is caused by Rhodococcus fascians, a gram-positive bacterium that triggers a very different kind of growth disorder. "Gram-positive" refers to a characteristic way certain bacteria stain in a laboratory test due to their cell wall structure, a detail that helps scientists classify and understand them.

Instead of tumor-like swellings, Rhodococcus fascians causes excessive shoot proliferation, often looking like a tight cluster of distorted leaves emerging from the crown or along stems. These leafy galls are easily mistaken for natural bud breaks or basal shoot growth.

What makes R. fascians especially insidious is that it can live in a latent state within plant tissue, sometimes without causing any symptoms at all.

Under certain environmental or hormonal conditions, however, it becomes active and begins to reprogram the plant's growth using a blend of cytokinins (a class of powerful plant hormones that promote cell division) and other growth-modifying molecules.

This dahlia tuber shows clear signs of Leafy gall, caused by  Rhodococcus fascians  (Author's photo)

The names of these gall-causing bacteria often provide clues to their nature and the diseases they cause.

•  Agrobacterium tumefaciens : " Agro- " hints at its agricultural relevance, while " tumefaciens " comes from Latin, meaning "tumor-forming," directly referring to the crown galls it induces.
  
  

•  Rhodococcus fascians : "Rhodo coccus" comes from Greek, meaning "rose-colored berry," referring to the bacterium's reddish colonies. "Fascians" is derived from the Latin "fascia," meaning a band or bundle, directly alluding to the "fasciation," or bundling of shoots, characteristic of the leafy galls it causes.

  
  

Although not all growth caused by Rhodococcus fascians exhibits reddish colonies, this dahlia infected by leafy gall does (Author's photo)

Agrobacterium tumefaciens is sometimes called "nature's genetic engineer" because of its unique ability to insert its own DNA into a plant's genome. In fact, scientists have leveraged this very mechanism in the laboratory to genetically modify crops, highlighting its biotechnological utility.

In the wild, however, this fascinating process results in the plant's cells undergoing unregulated division, leading to the formation of crown galls.

The transferred DNA (T-DNA) contains genes that essentially force the plant to produce specific growth hormones, such as auxins and cytokinins, thereby mimicking and disrupting the plant's own intricate hormonal balance.

This T-DNA also instructs the plant to create unusual compounds known as opines . These unique substances are not typically found in plants and serve as a specialized food source for the bacterium.

It's a highly evolved parasitic relationship, and once the bacterial DNA is integrated into the plant's genetic code, the plant is permanently altered.

Agrobacterium tumefaciens—crown gall—with a distorted lump that has formed on a dahlia tuber clump (Author's photo)

Unlike Agrobacterium, Rhodococcus fascians does not alter the plant genome directly. Instead, it relies on a sophisticated form of chemical warfare, releasing a blend of signaling molecules. These include cytokinins and possibly auxin-like compounds.

Together, these disrupt the plant's own delicate hormone balance. This hormonal imbalance then causes the plant to form distorted tissues, especially around the shoot meristems (the specialized regions of actively dividing cells where new growth, such as stems, leaves, and flowers, originates).

Scientific research also suggests that R. fascians may actively suppress the plant's natural defense responses through bacterial effector molecules.

Rhodococcus fascians—leafy gall—distorts the shoot meristem tissues. This often results in a mass sometimes described as “cauliflower” growth (Antonio Toledo photo)

Both Agrobacterium and Rhodococcus bacteria are primarily found in soil and can persist in the environment for varying periods, sometimes for years under favorable conditions, particularly when associated with plant remnants or undecomposed plant material.

They are most commonly introduced into your garden through:

• Infected propagation materials (cuttings, tubers, and seeds): Both bacterial and viral diseases can spread through propagating material. For instance, Rhodococcus fascians (leafy gall) is known to be seed-borne in some hosts, as confirmed by multiple peer-reviewed studies. Dahlia Mosaic Virus (DMV) , a common viral threat to dahlias, can also be transmitted via seeds.
  
  

• Contaminated soil or growing media: Pathogens can live in the soil and infect healthy plants growing there, making infested soil a common introduction point.
  
  

• Contaminated tools or hands that have contacted infected plants: Routine gardening tasks like pruning, dividing, or handling can mechanically transfer bacteria from diseased to healthy plants.
  
  

• Water sources like rain splash or irrigation runoff from nearby infected beds: Water can passively carry bacterial cells from diseased plants or contaminated soil, facilitating their spread to new areas.

It's worth noting that, unlike many plant viruses that rely on insect vectors like aphids for transmission, these bacterial gall pathogens are generally not known to be spread by insects.

While some laboratory experiments have shown aphids can carry Rhodococcus fascians, there is currently no evidence of this occurring significantly in greenhouse or field settings. Similarly, Agrobacterium tumefaciens is primarily spread through direct contact and wounds, not insects.

For Agrobacterium, infection often favors open wounds (such as tiny cracks from freezing temperatures, pinch sites on greenhouse plants, or the base of cuttings being rooted). Rhodococcus, however, can be more subtle. It can colonize the plant surface, living on the outside of the plant before entering through tiny pores called stomata (microscopic openings, primarily on leaves, that allow for gas exchange) or other microscopic injuries.

Once inside, both pathogens can survive within the plant with varying degrees of latency (meaning they can be present but not actively causing visible symptoms for a period).

Because gall diseases are known to be transferred by mechanical contact, such as by clippers or other tools used on multiple plants, commercially-farmed dahlia tubers have become a recognized source for infection.

Although this assortment offered in February features other bulbing ornamentals, by April store displays are filled with imported, Dutch-grown dahlia tubers (Author's photo)

These pre-packaged tubers are sold by the millions each spring in garden centers and big-box stores. They are grown by large factory farms in Europe, where every step of the growth, harvest, and packaging process is often highly mechanized.

Even with efforts to prevent transmission, current sanitation protocols in such large-scale processes cannot always prevent bacteria from spreading between plants. This means the contagion they carry is readily transmissible to gardens everywhere.

Understanding why dahlias are susceptible to gall diseases empowers us to make smarter choices in our gardens. It's not just about what we can see; it's about the hidden vulnerabilities that make these plants a target.

Dahlias, like many other herbaceous ornamentals (plants with non-woody stems), face susceptibility in part because growers propagate them vegetatively. New plants often begin as cuttings or tubers rather than seeds.

This healthy-looking dahlia tuber clump will be divided into separate tubers, then stored over the winter for propagation the following season (Author's photo)

Each cut, nick, or bruise you make during dividing or planting creates a potential entry point for Agrobacterium, which needs these wounds to initiate infection. However, Rhodococcus, the cause of leafy gall, can infect plants even without obvious wounds, entering through microscopic pores or other subtle points of ingress.

Beyond these fundamental vulnerabilities, the insidious nature of gall diseases lies in their ability to remain hidden. Leafy gall, in particular, is notorious for its latency (the ability of the bacterium to be present in plant tissue without causing visible symptoms).

The Rhodococcus fascians bacterium can persist on or within plant tissues for extended periods, sometimes for as long as six months (even longer in woody plants) without showing any visible signs.

This means growers can unknowingly maintain infected plants. You might share or sell seemingly healthy tubers or cuttings. This hidden presence makes diagnosis challenging, as symptoms might only emerge well into the active growing season, or even the following year.

Dahlia tubers stored over winter, for example, may harbor bacteria in dormant tissues, remaining latent until spring.

Cases of Rhodococcus fascians bacterium like this present with obvious symptoms. However, dahlia tubers can be infected but appear normal (Author’s photo)

Environmental conditions during both the growing season and winter storage can strongly influence whether symptoms appear. While the bacteria might be present, certain factors significantly favor disease development and increase its severity. Moderately warm temperatures and high humidity, for instance, often encourage these pathogens.

The degree to which symptoms appear also depends on a plant's age, its specific variety or cultivar, and even the bacterial strain involved. This sensitivity to environmental cues means a latent infection might 'wake up' and become apparent only when conditions become favorable for the pathogen.

Researchers note that Rhodococcus fascians can survive for considerable periods at low temperatures. Even light-related genes and pathways tied to photomorphogenesis (the process by which plants alter their growth and development in response to light) may influence when symptoms appear.

Rhodococcus fascians—leafy gall—has triggered a proliferation of shoots from the meristem tissue on this dahlia stalk (Author's photo)

Off-season storage of dahlia tubers also presents a critical period for vulnerability. Even dormant tubers can harbor the bacteria that cause gall diseases.

Poor storage conditions, especially when ventilation is insufficient or temperatures are too warm, can lead to tuber stress or microscopic wounds. These can potentially exacerbate the risk of infection taking hold or expressing symptoms once growth resumes.

Finally, infected dahlias can easily transmit gall diseases to other herbaceous ornamentals , not just dahlias. Both Agrobacterium and Rhodococcus have exceptionally broad host ranges. They affect numerous species in various plant families, including many popular garden and nursery plants like chrysanthemums, petunias, lilies, and even certain vegetables.

Asiatic lilies, shown here growing near one of the author’s dahlia breeding gardens, are one of the many susceptible species for both  Agrobacterium  and  Rhodococcus  (Author's photo)

By the same token, dahlias can also become infected from pathogens that originate in other susceptible plants. Some types of plants, or even specific cultivars within a type, may also face higher risk. They act as either more effective "givers" or "receivers" of infection, depending on their inherent susceptibility and environmental factors.

Understanding these interactions helps you manage your entire garden ecosystem.

Spotting gall diseases can be tricky. Infected tubers and cuttings can carry the disease even when they're not yet showing visible signs of infection. This latency means symptoms might not appear until much later in the plant's growth cycle. However, once they do emerge, here’s what to look for:

• Crown gall: These appear as rounded, knobby growths, typically at the base of the stem or on the upper tuber necks. When fresh, they are often light green or cream-colored, but they can darken with age and become hard or corky in texture. These can sometimes be mistaken for harmless scar tissue or callus that forms after an injury.

  
  

Crown gall tumors range from tiny to enormous, and can appear anywhere on the tuber clump (Antonio Toledo photo)

• Leafy gall: This presents as clusters of small, misshapen leaves emerging from the crown or along stems. They may look like a dense rosette or an unusual basal sprouting. These symptoms are easily mistaken for normal, vigorous new shoot growth, or even adventitious shoots (growth from unusual places) caused by environmental factors or plant growth regulators.
  
  Judging from hundreds of questions posted on dahlia enthusiast’s groups on social media, it’s all too easy for growers to mistake leafy gall symptoms for “an over-achieving tuber.” Sometimes, perfectly normal growth emerging from several eyes on the same tuber is mistakenly branded as a sign of leafy gall. Visual diagnosis sometimes isn’t easy.

These dahlia tubers are showing multiple shoots originating from each growth point, because cuttings have been harvested for propagation. This is the normal growth reaction of a healthy tuber, rather than an expression of leafy gall infection (Author's photo)

Visual inspection alone is often not enough for an accurate diagnosis. This is especially true for Rhodococcus fascians, where precise laboratory testing is crucial to distinguish pathogenic (disease-causing) strains from non-pathogenic rhodococci (common, harmless bacteria of the same genus that might live on your plants without causing issues).

Diagnostic labs employ sophisticated molecular techniques. They may use PCR (Polymerase Chain Reaction) , a method that amplifies tiny segments of bacterial DNA to detect the pathogen's unique genetic signature. This allows scientists to detect the presence of the bacteria even when symptoms are not visible.

Labs also use ELISA (Enzyme-Linked Immunosorbent Assay) , which uses antibodies to detect specific bacterial proteins. In some cases, labs also perform bioassays using indicator plants, where suspect plant tissue is inoculated onto a highly susceptible plant (like sweet pea) to see if it develops characteristic symptoms.

There were no visible signs of disease when this dahlia was planted. By the end of the season, Agrobacterium tumefaciens caused this distinctive tumor (Katie Conger photo)

One of the most troubling things about gall-infected dahlia tubers is that they often appear normal while in their dormant state, such as when they're sold in retail stores. It’s very common for a grower to plant these seemingly healthy tubers in their gardens or in containers, covering them with the recommended 4–6 inches of soil, and never see what happens when the tubers "wake up" and begin to generate new growth.

Often, the first obvious sign of trouble comes at the end of the growing season, when the tubers are lifted for division and winter storage. It is only then that the bulbous, cauliflower-like tumors of Agrobacterium tumefaciens and the profusion of partially expanded buds characteristic of Rhodococcus fascians (which can appear as multiple small, often malformed shoots at the plant’s base) finally see the light of day.

For this reason alone, I have long made a practice to pre-sprout my tubers indoors a few weeks before my desired planting date. I bring them into growth in seed trays filled with damp peat moss, so I can observe their first shoots and examine them for signs of infection. Although gall won't always show at this early stage, it often does and catching it now, before the tubers are planted, can be an easy way to decrease my risk of infecting other plants in the garden.

These tubers have been “pre-sprouted” indoors early in the spring. Each shows a normal shoot, ready to grow into a new dahlia plant (Author's photo)

Once a dahlia plant gets infected with gall, there's no cure. The damage is permanent. So, prevention is truly your best, and essentially your only, effective approach.

• Sterilize cutting tools between plants: Tools like pruners, scissors, and even mower blades easily spread bacteria from an infected plant to healthy ones. You must disinfect them frequently.
  
  

• Avoid planting dahlias in beds where gall-infected plants grew before: Both Agrobacterium and Rhodococcus can survive in the soil, especially if plant remnants are still there. While some studies suggest these bacteria persist for about two years, other research speculates they can remain viable in field soil for at least four or five years if conditions are favorable. Crop rotation or leaving the bed fallow for a season can help lower bacterial populations.
  
  

• Discard soil used for dahlias known to have had gall infection: Since these bacteria can linger in soil, particularly with plant roots or debris, it's vital to remove and dispose of contaminated potting mix. For garden beds, rotating to non-susceptible plants or letting the soil rest for a season effectively "rests" the soil from dahlia cultivation.
  
  

• Disinfect storage crates, benches, and propagation areas: Thoroughly clean and sanitize any surfaces that touch potentially infected plants—like pots, flats, benches, and storage crates. Use effective disinfectants such as bleach solutions or quaternary ammonium compounds (a class of chemical disinfectants known for their broad antimicrobial activity against bacteria, fungi, and viruses).
  
  

Tools used for dividing and propagating dahlias are a proven vector for transmission of bacterial and viral diseases that can infect dahlias (Author's photo)

• Source stock from small-scale growers: Many large factory farms use highly mechanized processes for growing, harvesting, and packaging. These methods provide no sanitation protocol to prevent transmission. While hand-processing offers no absolute guarantee of clean stock, a human eye examining the tubers can significantly lower the chances of acquiring infected propagation material.
  
  

• Avoid reusing potting mix: Contaminated potting mix can harbor gall bacteria, becoming a source of infection for new plants. Always use fresh, sterilized potting mix for dahlias. For garden soil, completely replacing it isn't practical. Instead, some experts suggest letting beds lie fallow for a season or planting non-susceptible crops like certain vegetables or other ornamentals not known to host these specific gall bacteria.
  
  

• Discard any tuber showing gall symptoms or unusual basal growth: Immediately remove any tuber with suspicious growths, like knobby crown galls. Also discard tubers showing abnormal basal growth (excessive, distorted shoot proliferation from the base, characteristic of leafy gall).

Abnormal basal growth—excessive, distorted shoot proliferation from the base—is characteristic of leafy gall (Author's photo)

• Reduce overhead irrigation or splashing: Water splash, whether from overhead irrigation or heavy rain, is a known way for gall bacteria to spread from infected plants or contaminated soil to healthy ones. Minimizing splashing and ensuring good drainage on benches helps contain the spread.
  
  While some growers in hot climates use sprinklers or misters to cool dahlia foliage, at least seven scientific papers point to "water splash" as a transmission mechanism for gall-causing bacteria in the garden.
  
  

• Minimize plant stress: Healthy, vigorous plants generally resist diseases better. Provide balanced nutrients through proper fertilization and ensure adequate spacing. This helps plants direct energy towards robust growth and natural defense mechanisms.

Dahlias grown in nutrient-rich soil with minimal pest pressure and proper irrigation are better prepared to resist pathogens from bacterial and viral diseases (Author's photo)

Once you identify infected plants, remove and destroy them and their tubers. Don't compost infected material; the bacteria can survive and persist, potentially reintroducing the pathogen to your garden.

Disposal methods vary, but common practices include burning infected material or double-bagging and discarding it with household trash. This destruction is crucial for preventing latent infections from persisting and creating a new source of inoculum (the infectious material, like bacteria, that can spread disease) in your growing area.

Normal-looking sprouts from these dahlia tubers indicate they are ready to grow into new plants, but they could still be infected with a bacterial or viral disease (Author’s photo)

I’ve already talked about pre-sprouting my tubers in early spring, so I can observe their earliest growth. I also monitor my plants closely throughout the growing season, looking for signs of pest pressure, viruses, and the tell-tale signs of leafy gall, which often appear as a profusion of leggy new shoots at a dahlia's base that may otherwise seem normal.

Another inspection opportunity, and arguably the best way to spot gall infection, comes when you lift tuber clumps for winter storage. Even before rinsing my clumps, I knock away some soil and examine each clump for suspect growths that might signal gall infection.

These freshly-dug dahlia tubers at first appeared to have an abundance of normal growth. They were found to have multiple shoots crowding from single growth points, which can be a leafy gall indicator. (Author's photo)

Since I divide in the fall, which allows me to store individual tubers instead of intact clumps, I always rinse freshly-dug tubers before separating them.

While rinsing tubers, it's common to use a strong stream from a garden hose, a process that inherently involves water splash. This inspection provides my best chance to recognize tubers with crown or leafy gall, letting me diagnose and dispose of them before they receive additional handling.

I balance this practice against the knowledge that irrigation water is one of many things that can spread bacterial diseases between dahlias. I am careful to rinse my clumps away from my growing areas, and direct the water where it is unlikely to come in contact with growing plants.

Every spring, the social media groups that are popular with dahlia growers explode with photos and questions, all asking some version of “Is this gall?”

In fact, there are so many gall posts on the major growers’ groups that a casual reader might conclude gall diseases in dahlias are everywhere.

But just like some people don’t go to the doctor until they feel sick, nobody is posting photos of healthy tubers on Facebook saying “Look! Here’s another tuber without gall!”

“Is this gall?” Yes! Lumpy growths such as these are almost always indicative of  Agrobacterium tumefaciens  infection (Antonio Toledo photo)

Gall diseases in dahlias are uncommon but not rare. And while the prospect of bacterial infection can be intimidating, especially given its permanence, the good news is that both crown gall and leafy gall are manageable through vigilance and hygiene.

Ultimately, the fight against dahlia gall is won not with chemicals, but with knowledge. Understanding how these pathogens behave, recognizing their signs, and applying sound hygiene practices lets you defend your garden—and help protect the wider dahlia-growing community.