Dahlia gall: identify and prevent leafy gall and crown gall in dahlias

What is Dahlia Gall?

Dahlia Gall disease is a bacterial disease that affects the stems and roots of Dahlia plants.

Technically, there are two types of gall that affect dahlias: crown gall and leafy gall.

Crown gall is a dahlia tuber disease caused by the bacterium Rhizobium radiobacter (also known as Agrobacterium tumefaciens). Leafy gall is caused by the bacterium Rhodococcus fascians.

In both cases, the bacteria enter the plant through wounds in the roots or stems and stimulates the plant tissues to grow in a disorganised or distorted way.

Dahlia gall: leafy gall

As discussed in this video from Moorfield Farm Flowers, leafy gall affects the genetic make up of plant, disrupting normal hormone activity in infected plants. It shows up in dahlias as dense clusters of distorted, leafy shoots or buds, usually at the base of the stems of the plant.

The location of the distorted shoots means that leafy gall can be hard to spot whilst the dahlia is in growth in your garden beds.

Mostly these are stunted and grow no more than an inch or two. Some shoots may grow longer (this is known as shoot proliferation), although these do not reach the size of uninfected shoots.

Leafy gall can also appear in other plants, including wall flowers, strawberries, nicotiana and pelargoniums.

Dahlia gall: crown gall

With dahlia crown gall disease, cells proliferate in tuber and other tissues infected by the crown gall bacteria. This causes swollen lumps and protuberances on the dahlia tubers, roots or stems. These galls are usually rounded with a rough surface. This is how you can differentiate them from the surface of normal tubers, which are usually smooth.

Galls on Dahlia plant stems often decay and disintegrate, but those on the tubers are likely to persist.

Crown gall affects many other plants including Begonia, Lupins, Phlox and Sweet Peas.

What is the difference between crown gall and leafy gall?

Crown gall and leafy gall are both caused by bacteria, but they affect different parts of the dahlia plant. Crown gall causes knobbly swellings or galls on the stems and roots of the plant, while leafy gall causes the plant tissues to grow in a disorganized way, producing dense clusters of distorted, leafy shoots.

What does dahlia gall do to dahlias?

As indicated above, the symptoms of dahlia gall can vary depending on the type of gall and the severity of the infection. Many varieties of dahlia can be affected, although some varieties are less prone to gall. Some common symptoms include:

• Swellings or galls on the plant stems or roots (crown gall)
• Leafy shoots or clusters of distorted leaves (leafy gall)
• Abnormal growth or bulges on the roots (crown gall)
• Reduced plant growth and vigour (both)
• Stunted or distorted flowers (both, but rare)

Causes of dahlia gall

The bacteria cause the two types of dahlia gall can be present in infected soil or on infected plant material, such as tubers or cuttings. They can also be introduced to the plant through contaminated tools or equipment.

They usually enter the plant through wounds or natural openings, such as stomata or lenticels. Once inside the plant, they stimulate the plant cells to divide unnaturally and grow abnormally, leading to the formation of galls or leafy shoots.

Does dahlia gall affect flowering?

Dahlia gall can affect flowering by reducing plant growth and vigour, and by causing stunted or distorted flowers. However, significant impact on flowering is quite rare and not all infected plants will show symptoms, and most will still produce healthy flowers despite the presence of gall.

Preventing and managing dahlia gall

There is no real treatment for leaf gall and crown gall. In other words, once a plant is infected there is nothing you can do to make the infection go away.

Therefore the focus has to be on preventing infection with dahlia gall before it occurs.

There are several ways you can try prevent and manage dahlia gall as follows:

Avoiding damage to roots and stems: Care should be taken when planting dahlias to avoid damaging the roots or stems, as this can create entry points for bacteria.

Planting in well-draining soil: Dahlia tubers should be planted in soil that drains well, as excess moisture can create an environment that is conducive to bacterial growth. Bacteria present will enter the plant if there are wounds in the tubers or if tubers begin to rot because of wet conditions.

Removing infected plants: Any plants that show signs of dahlia gall should be removed immediately to prevent the spread of the disease. Do not compost diseased plant materials.

Disinfect tools and equipment: Do not take cuttings form infected plants and disinfected knives between cutting from different plants.

Water plants at the roots: Water facilitates bacterial infection, so water your dahlias carefully and try to avoid watering in a way that leave the foliage wet for prolonged periods.

Purchase disease free tubers: Avoid purchasing dahlia tubers which may be infected with the bacteria. Instead, purchase tubers from reputable nurseries or growers that have been inspected for diseases.

Conclusion

Although Dahlia gall is not common, it can be damaging to dahlias in gardens. In many cases, you will notice little impact on your flowers, but it is best to prevent the build up and spread of the disease through good hygiene practices, as outlined above.

More on growing dahlias

You can also get much more guidance on growing dahlias in these posts:

• Taking care of dahlias: the trick with deadheading
• Dahlia pinnata: a dahlia original
• How to grow dahlias: the complete guide to dahlia care
• How to overwinter dahlia plants and tubers
• All you need to know about dahlia tubers and dahlia bulbs
• When to plant out dahlia tubers
• Dahlia varieties: your complete guide to all types of dahlias
• Dahlias in my garden: Six on Saturday
• Dahlia Wizard of Oz – Beautiful pink pompon dahlia
• Can you grow dahlias in raised beds?
• Do dahlias grow in Florida?
• Dahlia Wine Eyed Jill
• Can you grow dahlias in a tropical climate?
• Dahlia Ivanetti: magnificent magent dahlia
• Dahlia Night Silence – dusky pink dahlia beauty
• Visit a dahlia farm near you
• Best mulch for dahlias: a comprehensive guide
• Dahlia Islander: large, pink, loud, showy. What’s not to like?
• Heat tolerant dahlias: beat the heat with these 120 choice varieties