Blister and Corky Bumps on Plant Leaves from Oedema
Small blister–like bumps on the leaves of ivy geraniums, sweet potato vines, tomatoes and other plants growing indoors or in greenhouses are symptoms of oedema (edema) and intumesence. Fortunately, these are physiological problems and not caused by disease or insects.
Basically, the plant forms these growths in response to the environment. The two terms are often used interchangeably, and the cause is not well known. Many researchers believe the plant absorbs more moisture faster than it loses it through transpiration. Others are finding it related to light quality.
These conditions cause the blister-like bumps that eventually turn brown, dry and corky. The problem often corrects itself once the plants move into the garden and receive the preferred growing conditions.
In the meantime, continue to water and fertilize as needed.
A bit more information: Oedema can be mistaken for mite and thrip feeding. Hold a piece of white paper under the damaged leaves and look for tiny specs, the mites, moving across the paper. Use insecticidal soap or Summit® Year-Round Spray Oil to control the mites.