How to Deal with Crown Gall or Leafy Gall on Dahlias
Updated October 15, 2024
The first time I spotted leafy gall on a dahlia, I had no idea what it was. My first foray with leafy gall happened while digging up my dahlias in fall of 2020, where I noticed some freaky abnormal stem growth where the base of the stem meets the tuber. That said, leafy gall on dahlias is pretty obvious. There will be dozens of sprouts coming out of one area all at once, as if the dahlia went through a wild growth spurt (see photos below). Crown gall, on the other hand, causes bulbous, abnormal growths on the root itself.
Both leafy gall and crown gall on a dahlia tuber is caused by a bacteria (Agrobacterium), and in my case, came with the dahlia tubers when I ordered them online. Both crown gall and leafy gall are diseases you can introduce into your farm or garden when you buy imported dahlia tubers. If you purchase imported dahlia tubers from the Dutch market, I would say you are sure to eventually experience gall in your garden.
Related: The Difference Between Organic, No-Till, Sustainable and Regenerative Farming
below are some juicy photos of leafy gall in action:
There is one particular online retail supplier I’ve ordered dahlia tubers from that have been the main culprit of my dahlia gall issues (the company’s name begins with Eden). Not only have several varieties of my tubers been totally wiped out by the gall present from other dahlia tubers ordered from this company; they also frequently mislabel/mix up their tubers. It's a big win when you receive the actual tuber that you ordered, and it doesn’t come with a dose of Agrobacterium.
Let us answer some common questions regarding gall on dahlia tubers.
Related: Favorite Cut Foliages on Our Flower Farm
Do leafy gall or crown gall on a dahlia effect flowering?
They don’t seem to. I’ve not noticed my dahlias have had leafy gall until I’ve dug them up because the abnormal growth is concentrated down by the tuber. Crown gall also does not seem to negatively affect flower production, in my experience.
below are some juicy photos of crown gall on dahlia tubers:
does gall kill dahlias?
No, neither leafy gall nor crown gall actually kill dahlia plants.
Then what’s the problem with dahlia gall?
The Agrobacterium that causes leafy gall and crown gall on dahlias will spread to other plants in your garden and cause abnormal growth — but only if they are planted in the same spot where the gall existed previously. There also has to be an injury to the stem or root for the bacteria to enter into the plant.
I will say I planted hundreds of zinnia plants in an area where I noticed crown gall and leafy gall on some of my dahlia tubers after digging the tubers up. The zinnias grew spendidly — not a single zinnia plant seemed to be affected by the previously-discovered gall and they chugged gorgeous blooms for months. I’m not telling this story to say all is a-okay with having gall in your soil, but rather, to quell any looming fears someone might have about never being able to plant anything in an area with gall again.
What to do if you have gall on your dahlia tuber?
When in doubt, throw the entire tuber out.
The Agrobacterium that causes the gall will still be present in the soil where the dahlia was growing, but this is where we can introduce crop rotation (simply plant your dahlias somewhere else in your garden the following year). If you’re lacking space, I understand this can be hard to just move your plants to another spot. To be honest, in 2021, I went ahead and planted dahlias in the infected area the following spring. I do think it transmitted to a few dahlias (I wasn’t labeling nor keeping good track of this), but I just threw those out at the end of the growing season. I’m working with thousands of dahlia tubers, so throwing out a few here and there isn’t a big deal for me.
Make sure you do not throw the infected tubers into your compost pile where you will be making new soil for your garden. You’ll either want to throw the dahlias in a separate “never compost” pile, throw them in the garbage, or throw them in a burn pile.
Is the chance of buying dahlias with leafy gall or crown gall less if I buy from a small flower farmer?
Maybe, but not necessarily. What you get from a small flower grower (as opposed to a large online retail supplier like Eden Bros), entirely depends on the farmer’s experience and attention to detail. Someone who is unfamiliar with gall, or has employees digging and splitting tubers who are unfamiliar with gall, could very well sell you a tuber with gall. Generally, I think small farmers are aware of these diseases and do a good job of culling them out of their stock, but everything is relative to the grower’s knowledge.
How to prevent leafy gall and crown gall in the future?
If you’re ordering dahlia tubers in from anywhere, you’ll never be able to entirely prevent gall. The best thing you can do is be aware of it, and look for it when you dig up your tubers (but don’t dig them all up expecting to see it… that’s unrealistic and stressful!)
It is also SO SO important that you sanitize your tools while splitting your dahlia tubers! < click that link to learn my preferred, super easy method for sanitation. This will prevent spreading the undetected crown gall or leafy gall to other dahlia tuber clumps.
A thought on biological amendments and beneficial bacteria
Since leafy gall and crown gall are caused by bacteria, I would argue that adding beneficial bacteria to your farm or garden could be a great way to help mitigate the spread of the gall-causing Agrobacterium. I would suggest using a compost tea (created by you, using a healthy source of biodiverse compost), or creating some other biological amendment a la Nigel Palmer. To learn how to create healthy, biological amendments for your garden, read Nigel Palmer’s amazing book: The Regenerative Grower’s Guide to Garden Amendments.
Related: Fertilizing and ‘The Law of Return’
Special considerations for flower farmers growing dahlias
If you’re selling dahlia tubers to other people, it is critically important you cull your tuber stock for gall. I go through my dahlia tubers several times before selling to anyone. Your field and dahlia tuber stock should also be inspected by your state’s agricultural department.
When ordering wholesale Dutch-grown tubers in large quantities, I have experienced some leafy gall issues here and there. The bacteria exist; we just need to be aware of what to look for. I have not seen any signs of gall on the tubers prior to planting. It has only made itself apparent after the growing season has finished and I’ve dug them up.
More help with growing, storing & splitting dahlias
Will Deer Eat My Field of Dahlias?
Why Buy Dahlia Tubers from a Small Farm vs. Big Online Retailer
How to Store Dahlias Over Winter & Get Rid of Mold
Using Wood Pine Shavings to Store Dahlia Tubers Over Winter
How to Plant Dahlia Tubers & Maintain Dahlias Organically
I Skipped Drip Irrigation on the Dahlias; Here’s What Happened
When to Split Dahlia Tubers — Now or Later?
The #1 Mistake You Cannot Make When Splitting Dahlia Tubers
I hope my learning experience with crown gall and leafy gall on dahlia tubers was helpful for someone out there! No need to sound the major alarm, though. If you think you might have leafy gall or crown gall… just toss the tuber out!
peace, love and disease-free dahlias,
Fran Parrish
