Carrie Knutson, a horticulture agent with North Dakota State University Extension in Grand Forks County, said on May 12 that her recent struggles with tomato seedlings led her to identify a common plant disorder known as intumescence.
The topic is relevant for gardeners and growers who may notice similar symptoms in their own tomato plants. Understanding the causes and management of this abiotic disorder can help prevent damage and improve plant health.
Knutson described following recommended practices for growing healthy seedlings, including using fresh potting soil and disinfected trays. Despite these efforts, she observed yellowing and deformed leaves falling off her tomato plants. “I think the culprit is the intumescence, also known as oedema or edema, of my tomatoes,” Knutson said. She explained that an abiotic plant disorder differs from disease because it is caused by environmental or cultural factors rather than living organisms.
Intumescence occurs when water uptake by the plant exceeds its ability to transpire through leaf pores, resulting in swollen or blisterlike areas on leaves. If not addressed, affected leaves may turn yellow and drop off. The condition often appears when soil moisture is high alongside cool nighttime temperatures and high humidity—conditions common in greenhouse-grown tomatoes. Other susceptible plants include ivy geraniums, begonias, ornamental sweet potato vine, broccoli, cabbage, and cauliflower.
To manage intumescence, Knutson recommended environmental adjustments such as avoiding overwatering, increasing air movement with fans or ventilation systems, spacing out plants more effectively, and improving lighting conditions. “I transplanted my tomatoes to improve spacing, added a small fan for air movement and invested in a quality set of grow lights,” she said.
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Knutson concluded that despite these interventions her current batch of tomato seedlings might continue to struggle until they can be moved outdoors but expressed hope for better results next season.
