Intumescence is an abiotic disorder which produces small blisters on leaf surfaces. Severe cases can result in defoliation. Certain plants are especially susceptible to intumescence; these include members of the Solanaceae such as Ipomoea, peppers, and tomatoes, and Cuphea in the Lythraceae family. The symptoms resemble and are often mistaken for oedema (or edema), but their development is distinctly different. In addition to the genus of the afflicted plant, the location of the symptoms is also helpful in distinguishing intumescence from edema. Oedema is commonly observed on the undersides of leaves, as on ivy geraniums, whereas intumescence usually appears on upper leaf surfaces.
Recent research from Kansas State University indicates that intumescence is caused by decreased UVB radiation, a condition that may occur as a result of a long stretch of cloudy weather, or the application of shade cloth, or other material that blocks UVB radiation. Symptoms are nearly always seen in greenhouses or growth chambers. Rare cases have occurred outdoors. Recommendations thus far for preventing intumescence are limited to spacing plants to allow full light exposure. Some cultivars of a species may be more susceptible than other, though little information about this currently exists for plants other than Ipomoea.
For more information: Oedema and Intumescences, UMass Extension
Intumescence: A Physiological Disorder of Greenhouse-Grown Crops Kansas State University
Intumescences: Further Investigations into an Elusive Physiological Disorder Kansas State University