Dakota Gardener: Mystery solved — Intumescence of tomatoes
By Carrie Knutson, Horticulture agent
NDSU Extension – Grand Forks County
If you live within a few blocks of my house, you may have heard me screaming in frustration recently. My tomato seedlings keep threatening to die.
I followed all the recommended steps to grow healthy seedlings. I used fresh potting soil, cleaned and disinfected my seedling trays, and even purchased new ones specifically for my tomatoes.
Much to my dismay, the bottom leaves on my tomato plants are falling off again! The leaves are turning yellow, and some of them are deformed. I don’t remember this specific symptom from last year, but it gave me a big clue toward solving this mystery.
I think the culprit is the intumescence, also known as oedema or edema, of my tomatoes. Regardless, this is an abiotic plant disorder. An abiotic plant disorder is different than a plant disease. It is not caused by a living thing, such as an insect, fungus or bacteria, but by environmental or cultural factors such as weather, nutrient deficiencies, soil conditions or excess water.
Let’s take a quick look at the plant process involved in this disorder: transpiration. Transpiration is the loss of water through pores (stomata) in the leaves. The process moves water and nutrients up the plant for use in photosynthesis.
Intumescence occurs when plants take up water faster than it can be transpired through the leaves. This water buildup causes swollen, blisterlike areas on the leaves. If the injury continues, leaves will turn yellow and drop. Sound familiar?
The disorder usually occurs when soil moisture is high, combined with cool nighttime temperatures and high humidity. It is a common disorder in greenhouse-grown tomatoes, and varieties differ in their susceptibility. It is common in ivy geraniums, but can occur in begonia, ornamental sweet potato vine, broccoli, cabbage and cauliflower.
Changing the environment is the best way to manage the disorder. Management strategies include avoiding overwatering, increasing air movement and ventilation, spacing plants, and improving lighting. I transplanted my tomatoes to improve spacing, added a small fan for air movement and invested in a quality set of grow lights.
I have done as much as I can to manage the disorder. Unfortunately, I think the tomato plants will continue to struggle until it is nice enough to put them outside in my makeshift greenhouses. I think I will search for another tomato variety to grow next year. Happy gardening!
NDSU Agriculture Communication – May 12, 2026
Source: Carrie Knutson, 701-780-8229, carrie.knutson@ndsu.edu
Editor: Dominic Erickson, 701-231-5546, dominic.erickson@ndsu.edu