The Gall Of The Dahlias!

We finally had our first killing frost earlier this month. Like everything else this past growing season, it was late. For the madman and his dahlias, a late frost is a big deal.

The Frost Signal

Frost tells our plants to stop growing. It is a signal to send energy to the tubers that need to prepare for winter by storing starches and sugars. After a killing frost, we leave our tubers in the ground for a week or two. During this time, the skin of the tubers hardens making them more resistant to damage during storage. The madman has also read that a waiting period encourages the dahlia tubers to produce more eyes for future growth. In truth, I think the madman actually leaves them in the ground until he gets around to digging them out.

Of course, there is never just one way to do things, especially in gardening. In recent years we have heard of other gardeners who dig their tubers before frost claiming that it doesn’t make a difference and they would rather keep their fingers warm.  And, finally, thanks to climate change, some gardeners leave their tubers in the ground all winter, even here in Connecticut. The madman says that if whatever you do works, you’re not doing anything wrong.

Unpleasant Surprises

As we mentioned in a previous post, the madman likes his Dahlias and likes to think he is very careful with them. We are glad we dug our tubers up this year because we found a couple of surprises. All summer the madman has been reading posts from dahlia groups that talked about gall. He said, “That can’t happen to us.” But, guess what? As we began digging up our tubers, we found a couple of funny-looking ones that had separated from the main clumps. We just didn’t know which clumps. Research suggested we had crown gall on those funny-looking single tubers.

Crown Gall is a bacterial infection caused by the organism Rhizobium radiobacter (formerly known as Agrobacterium tumefaciens). Interestingly, until the early 2020s, it was considered a rare problem but has become increasingly more common. Just check any dahlia growers’ Facebook page. The madman does this all the time because he likes to look at pictures. Anyway, each of our funny-looking tubers had a growth that was rounded with a whiteish color. After the discovery, we began researching the gall.

Appropriate Research

During our Master Gardener training, we learned to begin research on “.edu” websites as opposed to social media. We all know how accurate social media can be. We also learned that bacterias are difficult to diagnose without proper laboratory testing. For a disease that is a relatively new problem, a lot has been written about the science behind it. All very interesting, but in the end, without testing our tubers for Crown Gall, we can’t be sure we have it. We simply assume the worst. Before you fall into a rabbit hole like we did, the madman suggests using the following sites for starters.

Oregon State University Extension Service has a nice article on dahlias in general – here is the link. We also found a three-part discussion on dahlia diseases by biologist Nichole Warwick. The section on galls contains a detailed description along with great photos of gall diseases. If you are concerned at all, click here to check out Nichole’s articles. But, back to our problem. We may have Crown Gall. So, what do we do? We stop the spread of the disease.

Out With The Bad

We have to toss the infected tubers and the clumps they came from. Unfortunately, we don’t know which clump each “infected” one came from, but we do know what bed they came from. We’ll start there.

Most likely if we have gall the bed itself is now contaminated because bacteria can spread through the soil. We won’t plant dahlias in that bed for a couple of years. Bummer. But we will plant onions or another crop that is not affected by crown gall. We have some planning and negotiation to do over the winter. Of course, we could solarize the area or replace the soil, but the madman says that rotation sounds like the best option. I concur.

Since we are not sure which clumps may be infected (the disease quickly spreads through the plant), we will plant dahlias from that bed in pots. But where to put the pots? You are correct in guessing that we need more negotiation.

More Gall

A quick note – we also found suspected Leafy Gall on a dahlia. As usual, I succumbed to the lure of late-season bargains and purchased some inexpensive last-minute tubers. One didn’t do well all summer, and we wrote it off to our poor growing season. When we dug it up, we found what we thought was lots of new growth underground. Unfortunately, it may be Leafy Gall, not new growth. Again, testing will tell, but Leafy Gall is not that big an issue.

With Leafy Gall, we manage it by tossing the tubers. Already done. This disease is caused by the bacterium, Rhodococcus fascians. It doesn’t stop growth but produces weak growth like collapsed stems. Fortunately, it doesn’t linger in the soil without the host (dahlia) and the same control methods work for both suspected infections.

Moving Forward

As mentioned before, we will rotate the crops – not planting another dahlia in the area for a couple of years. The “Leafy Gall” tubers are already history. Next year we will plant the tuber clumps from the “Crown Gall” infected bed in a separate area. We’ll watch them closely.

In the future, we will plant any new dahlia tubers in a pot for 1 year. Because bacteria, like viruses, are easy to transmit we plan to step-up our sterilization process from once a day to once a cut. Currently, we mostly use 70% alcohol, but the madman is considering a partial switch to bleach. Bleach is cheaper but the disinfection process takes longer. Looks like we have some research, not just negotiation to take us through long winter nights. But we can’t leave you with just problems. We did make a great discovery this season.

Now Some Good News

If you grow dahlias, you know that staking them is a challenge. A while ago, we saw a nice system in use at Longwood Gardens – a horizontal grid of twine netting. We couldn’t find a supplier and began making our own grids out of heavy twine. It worked for the most part, but it was cumbersome and a pain-in-the-neck to weave the individual strands into the grid.

By accident, the madman found biodegradable netting in a catalog. “Will this work in the dahlia beds?” he asked. It was perfect. The Coconut Trellis Netting at A.M. Leonard’s Gardener’s Edge came in packages of 6 ½ ft. X 15 ft. We ordered two as a trial. With 5 ft. steel fence posts for support, we put two layers of grids in the dahlia bed. The flowers grew straight and tall.

But, as we put them in, the madman wondered if they would also work for our cherry tomato plants. Wow! Did they ever work. We made sure the plants grew within the grid. Upkeep and harvesting of the tomato plants was so much easier. The only issue is that we now need taller steel posts in the tomato bed. We never knew tomatoes could grow so tall. Depending upon the weather, that task may wait for the spring thaw.

Right now, the madman wants to negotiate garden beds over a cup of hot cocoa.