OEDEMA (OR EDEMA)
Oedema is a physiological plant disorder that occurs when water uptake by a plants' roots exceeds transpiration rates by its' leaves. In other words, more water is being absorbed by the plant than is being given off.
The symptoms may start as discolored foliage. Sometimes a whole leaf may turn yellow. But then water soaked blisters appear which develop into corky callused tissue.
The corky callused tissue is the result of damage caused by the plants' cells bursting. The cells burst because of the excess water pressure that builds up inside the cells.
Oedema is often mistaken for a disease, usually rust. But it is not contagious, nor is it fatal. However, it will make a plant look unsightly.
Oedema can be managed by decreasing irrigation during low light conditions, watering in the morning as opposed to latter in the day, and by increasing light or air movement around the plant in order to stimulate transpiration from its' leaves.