Several ivy geranium samples have arrived in the Plant Disease Clinic showing symptoms of edema. Early symptoms of edema include tiny blisters on the undersides of leaves. Petioles and stems may also be affected. These blister-like areas can enlarge and will turn tan or brown in color and appear corky. Affected leaves eventually yellow and drop.
This condition is caused by environmental factors, not an infectious disease. It tends to occur when the soil is moist and warm and when the air is moist and cool. Under such conditions water uptake by the roots is faster than water loss from the leaves. This results in an over-turgid condition that can lead to bursting of cells or blistering. The corky appearance develops as this tissue begins to heal.
Control measures for edema include adequately spacing plants to allow for good air circulation and avoiding overwatering during periods of cloudy, cool weather.
This article originally appeared in the March 31, 1995 issue, p. 33.