Leafy Gall, Crown Gall, or a Healthy Dahlia Crown?

Two diseases, two bacteria, two different shapes

Crown gall and leafy gall get lumped together because both make a dahlia grow something lumpy and wrong. But they come from different bacteria, and once you know what you are seeing, they look different.

Crown gall is caused by Agrobacterium tumefaciens, recently reclassified as Rhizobium radiobacter. It makes tumors: swollen, disorganized masses of tissue on the crown, neck, or roots. These tumors never turn into buds or shoots. They only enlarge.

Leafy gall is caused by Rhodococcus fascians. It makes the opposite kind of error. Instead of a smooth tumor, it forces out a dense mass of short, stunted shoots fused together at the base. The tissue is organized into recognizable plant parts, tiny leaves and buds, but they are deformed and they do not grow on into normal stems.

Plant pathologists draw the line cleanly: crown gall tissue does not differentiate into buds or stems, while a leafy gall is differentiated into easily recognized plant parts. One is a blank lump. The other is a crowd of failed shoots.

Worth knowing if this is new to you: crown gall was a rare problem on dahlias until the early 2020s, when it became more common in the trade. It is reasonable that you may never have seen it before.

What crown gall looks like

Crown gall starts small. The first sign is usually a cluster of pale, cream-colored warts at or just below the soil line, on the crown, neck, or roots.

As the gall ages it enlarges into a rough, irregular mass, and the surface darkens from cream to brown. Old galls turn woody and corky, and they can grow larger than the tuber itself.

The bacteria enter through wounds: a cut from dividing, a snapped neck, frost-heaved soil, insect chewing, even the natural wound where a new root pushes out. Once inside, the bacterium splices a small piece of its own DNA into the plant's cells, which reprograms them to multiply into the gall.

The harm is to plumbing and growth. A gall on the crown disrupts the flow of water and food, and it can stop new shoots from forming in the crown zone, the exact place you need them. A tuber with true crown gall is not one to plant.

What leafy gall looks like

Leafy gall shows up at the base of the plant, at or just below the soil line, as a dense cluster of growing points fused into one mass. Growers reach for the same two comparisons: a tiny head of cauliflower, or a witch's broom.

The shoots in the cluster are short, thick, and bunched, and they stay that way. They rarely grow on into full, normal stems. That failure to develop is the tell. A leafy gall is a mass of shoots, fused at the base, that do not develop into normal growth.

It can also appear higher up, in the leaf axils along a stem, though the crown is where dahlia growers usually meet it.

The bacterium mostly lives on the surface of the plant rather than running all through it. That detail matters later, for how it spreads and for why you cannot simply trim it off and move on.

Telling leafy gall from a crown that is just waking up

This is the call that trips up careful growers, because a healthy dahlia crown in spring also sends up a cluster of shoots. That is what it is supposed to do.

Look at where the shoots begin. On a healthy crown, each eye has its own distinct starting point, spaced around the crown, and each shoot goes on to grow into a normal stem. On a leafy gall, the shoots are fused at the base into a solid lump of tissue with many tiny points sticking out, and they stay stunted. Distinct and developing is healthy. Fused and stalled is the warning.

Be honest about the hard cases, because they are real. Leafy gall can look like normal spring growth, and like the bushy flush you get from rooting hormones. The bacteria can also ride along on a plant for weeks or months with no symptoms, and a leafy gall can take up to two years to show up. A clean look this spring is not a lifetime guarantee.

So when you are not sure, do not destroy on a hunch, and do not plant the tuber in among healthy stock. Pot it up on its own, away from the others, and watch. Healthy shoots lengthen into normal stems within a couple of weeks. A gall stays a stunted, fused knot. Either way, do not take cuttings or divisions from a crown you are unsure about, because propagation is how this disease travels.

How gall spreads, and why one plant is a garden problem

Both bacteria move in the same handful of ways, and all of them are ordinary garden actions. That is why a single infected plant is worth taking seriously.

Tools are the most common route in a home garden. A blade that cuts through a gall, or even through a symptomless infected plant, carries bacteria to the next plant it touches. Splashing water from rain or irrigation moves them short distances. Insects and any fresh wound open a door.

For leafy gall, the biggest route is propagation itself. Take a cutting or a division from an infected crown and every plant you make from it is infected too, often before it ever shows a symptom.

And the soil remembers. Rhodococcus fascians can persist in ground where diseased plants grew for one to two years. Crown gall bacteria can survive even longer, as long as three years. Pulling the plant does not reset the ground underneath it.

What to do, for each answer

If it is healthy, plant it. A crown with distinct, spaced eyes that grow into normal stems is doing exactly what it should, and a busy spring crown is a good sign, not a bad one.

If it is gall, or you become sure that it is, accept that there is no cure for either disease and no spray that rescues an infected plant. Dig out the whole plant, not just the visible gall, and put it in the trash or on a burn pile. Do not compost it, because a home pile does not get hot enough to kill the bacteria. Remove obviously affected neighbors too, and clear away loose debris.

Rest the ground. Avoid replanting dahlias, or other susceptible plants such as roses and geraniums, in that spot for a few seasons.

Going forward, sanitation is the whole game. Start with the cleanest stock you can find, and do not take cuttings from a symptomatic plant or its close neighbors. Disinfect blades between plants, for example for at least 30 seconds in a 10 percent bleach solution, or a dip in 70 percent alcohol. Use clean trays and fresh media, and keep tubers up off any ground that has grown galled plants.

One caution on products. The biocontrol dips sold against crown gall, such as Galltrol or NoGall, are preventive only, and in testing they have not reliably controlled gall on soft, herbaceous plants like dahlias. They are not a rescue for a plant that is already infected. Clean stock, clean tools, and clean ground do more than any bottle.