Edema is a plant’s reaction to an excessive amount of water in its cells. It is a physiological disorder that can affect a wide variety of indoor and greenhouse ornamental plants, particularly geraniums. 
The problem comes about when roots absorb more water than is released, or transpired, through the leaves. As a result, cells in the epidermis of the leaf become gorged with water. Raised blisters appear, cells burst and water-soaked and corky spots develop. Corky spots or ridges also can develop on petals and along stems and leaf petioles.
Be especially cautious of edema in late winter and during cloudy weather. Corrective measures involve changing a plant’s environmental conditions to increase transpiration. Raising the temperature, and increasing ventilation and light all will relieve edema. However, it isn’t advisable to expose house plants to direct sunlight or cold drafts.
