If you garden long enough, eventually, you’ll find yourself familiar with every pest and disease in the book. So, this spring, I was completely shocked to find myself dealing with something I had never seen before. And worse, it was taking down my tomato seedlings, one by one.
Off to a Great Start, But Wait, What’s That?
This is the first year I’ve grown my seedlings in my basement using only my Barrina grow lights. Having moved into a location where I had the space to set up a permanent grow tent, I decided to go big. I wanted a space for more than seedlings, something that could handle all of my plant endeavors.
I wanted a grow tent where I could propagate new African violet cuttings for my collection, boost new growth on my recently pruned Schlumbergera, and propagate my blueberry bush cuttings.
So, I splurged and bought an industrial stainless steel shelving unit and a vinyl cover to go over it. My old Vivosun heated mats were too small; luckily, they made longer ones that fit the shelves perfectly.
In February, I got out my soil-blocker and started sowing seeds, including tomatoes. This year, I decided on five different tomato varieties and started twenty-two seedlings. They all germinated quickly, and within weeks, I had beautiful, healthy tomato seedlings that were quickly outgrowing their little soil blocks.
Time to pot up!
Everything was going well until I noticed a couple of my smaller seedlings had weird spots on the stems near the base. The spots were lighter green and seemed translucent. They were raised, almost like a blister on the stem.
Because they appeared to be coming from the base of the plant upward, my first thought was damping off.
But it couldn’t be. There were no other symptoms. The stems weren’t brown. The plant was fine otherwise. As I potted up each seedling, the roots looked perfectly healthy.
I knew they weren’t adventitious roots developing either. The plants were watered well, and I’m quite familiar with how those bumps look.
This was something altogether different.
So, I did what any self-respecting gardener would do. I finished potting up my seedlings and hoped the problem would correct itself.
It didn’t.
Not only did it not go away, it got worse.
And it spread.
More and more of my seedlings were developing these weird blisters.
Upon closer inspection, the first seedling I noticed with the blisters now had blisters on the leaves, too. When I touched the leaf to get a closer look, the entire stem fell off. The plant had actually sealed that leaflet off from the main stem, and it dropped off with the slightest touch.
The internet was no help. Everyone chalked it up to adventitious roots or had no clue. Finally, a deep dive with Google Lens netted results – Intumescence.
The last time I heard anything like that was in seventh-grade health class while discussing… Never mind. Ahem.
Intumescence or Edema in Tomato Seedlings
The reason the internet was so unhelpful is that this weird issue is rather uncommon unless you have the right growing conditions for it. And I had the perfect conditions for it.
But we’ll get to that later. First, let’s discuss what this is, how it develops, and what it means for your (and my) seedlings.
Intumescence in plants is a physiological disorder.
This means it’s not caused by a vector (virus, pest, bacteria, etc.). It’s simply the plant’s physical attributes changing as a consequence of what’s happening in its environment. It’s also not contagious. (Phew!)
I thought I had some rampant plant disease on my hands. It turns out I had the perfect conditions for my tomatoes to tumesce. Yay, me.
It presents just as I described above, as small blister-like bumps along the stem of the plant. They appear light green to clear on closer inspection. The blisters will also begin to appear on the undersides of leaves. If the conditions that cause intumescence aren’t corrected, the plant will eventually drop leaves. In severe cases, you can lose the plant altogether.
What Causes Plant Intumescence?
As I stated earlier, most home gardeners will likely be puzzled if this shows up, as it requires specific conditions more often found in a greenhouse setup, so it’s not seen in your home seedling setup too often.
There is little information out there about the exact cause because it’s not precisely known, which is why I had such a hard time finding information about the disorder. But here’s what scientists do know: intumescence shows up when plants have a combination of damp soil, high humidity conditions and not enough UV-B light.
You know, like in a grow tent with no ventilation in a basement where the only light comes from grow lights.
Or in a greenhouse where the clear plastic exterior has been treated to filter out certain types of UV light to prevent the plastic from becoming brittle or yellowing prematurely.
Your plant can’t transpire (move water through its vascular system to evaporate on the surface of the leaves and stem) because there is too much water in the plant already (too moist soil) and in the air (high humidity). So, the cells swell up with water and burst, forming those wild-looking blisters on the stem and leaves.
Temperature comes into play as well, but mostly at night. If the plants are kept in a high-humidity environment and the temperature drops, as is the case at night, intumescence is more likely to occur.
The University of Connecticut extension has put out a good fact sheet with more information about the causes of intumescence in tomatoes. You can read that here.
What Do I Do with Tomato Seedlings Affected by Intumescence?
Fix your growing conditions, STAT!
- Let the growing media dry out a bit, and only water when the top inch of soil is dry.
- If you can, move your plants to where they will receive natural light.
- (My seedlings are taking turns on a sunny windowsill.)
- Reduce the humidity.
- For me, this has been the biggest issue and the easiest to address. I leave the tent flap up during the day and roll it down, but I don’t zip it at night. I’ll be installing an exhaust fan on my grow tent once all the seedlings get moved outdoors.
Be Patient! Your Plants Should Survive
Once you address the growing conditions, you should notice that your seedlings will begin to heal. The blisters will scab over and turn brown. The spots where leaves dropped will also scab over and will likely produce a new stem.
The absolute best thing you can do for tomato seedlings with intumescence is to get them outdoors and in the ground. Natural sunlight and the relative humidity outdoors are just what the plant doctor ordered.
Granted, you might still be waiting for that all-important last frost date, but in the meantime, so long as you can improve your seedlings’ growing conditions, they will slowly bounce back.
How Did My Seedlings Fare?
Out of twenty-two seedlings, I only lost one. A Napa Chardonnay cherry tomato, but it’s early enough that I simply started a new seed.
Oddly enough, I had two seedlings that were completely unaffected.
Last year, I decided to grow Norfolk Healthy Produce’s infamous The Purple Tomato. This is the first GMO seed available to home gardeners. It was the only one of my tomatoes that didn’t succumb to late blight later that summer. And this spring, the two seedlings are the only ones who have not developed intumescence. Interesting, to say the least.
While intumescence showing up in your tomato seedlings can certainly be alarming. Once you’ve identified what it is, it turns out that it’s hardly the end of the world. I’m hoping that by getting this out there, anyone who finds themselves in my boat will have an easier time diagnosing and treating their plants than I did.
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