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What are those bumps?

Q: I’ve noticed some small, unsightly brown bumps on the undersides of the leaves on my indoor plants. What could be causing this?

Alfred Whiting, Rockville, MD

Moist conditions after a dry spell can cause oedema. Moist conditions after a dry spell can cause oedema. Photo/Illustration: provided by M.F. Heimann (reprinted from Compendium of Flowering Potted Plants, 1995, American Phytopathological Society, St. Paul, Minn.)

A: B. Rosie Lerner, a consumer horticulture specialist with the Purdue University Extension, replies: It sounds like your plants may have oedema, a condition caused by extreme variations in moisture and humidity levels. Oedema usually becomes visible on plant tissue when conditions are highly moist. It appears as a raised lesion that resembles a corky scab.

The plant tissue blisters, then gradually develops a tan or rust-colored, bumpy appearance. The bumps are commonly found on the undersides of leaves but can also show up on stems and flower petals. Nearly any plant can develop oedema, although thick-leaved plants, such as pansies, jade plants, and ivy-leaf geraniums, are more susceptible.

Oedema is mostly seen in plants in greenhouses and other indoor environments in late winter, but it can occur in outdoor plants as well. It’s likely to develop during cool, cloudy, muggy weather and when moist soil conditions follow a dry spell. The blisters form when the plant’s cells swell and burst from taking on water more rapidly than they can release it. The process of releasing water as vapor, called transpiration, is inhibited in highly humid environments.

Oedema will not spread to other plant tissue, but the lesions will remain unsightly. Affected tissue will often dry and fall away, especially on thin-leaved plants, leaving a hole in its place. In most cases, the plants will develop healthy new leaves and flowers as more favorable conditions prevail. Removing affected leaves and petals can improve the plants’ overall appearance.

To prevent oedema, avoid extremes in watering practices and check the soil for moisture needs before you water. Increase air circulation, reduce humidity, and move the plant or prune nearby plants to increase the exposure to light. Using fans or dehumidifiers indoors will also help.

From Fine Gardening 107, pp. 78