PPDL Picture of the Week for
February 11, 2013

Edema and Intumescence

Tom Creswell, Plant & Pest Diagnostic Lab Director, Purdue University

During the winter we occasionally get samples showing strange growths on the leaves. These areas of swollen plant tissue can resemble fungal leaf spots, grains of salt, spray residue or mite injury. Symptoms range from slight bumps to crusty eruptions of tissue that turn corky (Fig 1), causing leaves to yellow and drop. The problem is not caused by a disease, insect or mite but by a physiological disorder known as edema (also spelled oedema). It appears when the plant roots take up more water than can be transpired through the leaves.  Leaf cells swell and burst, causing blisters and small lesions; most often on the underside of the leaf (Fig 2, 3).  Cool, cloudy conditions, high humidity and excess water make for favorable conditions for the development of edema.  Many plants are susceptible to this problem but we see it most often on ivy geranium (Fig 4, 5). Greenhouse growers can reduce problems by using resistant varieties, spacing plants to insure good light penetration, adequate calcium fertilization and by keeping plants slightly drier during cool, cloudy weather. For houseplants try to keep the soil evenly moist and avoid extremes of drying out followed by too much water.

Edema is sometimes referred to as intumescence but recent research suggests the two may be distinct problems based on how intumescence on tomato (Fig 6) develops in response to low UVB light, which had no effect on edema of ivy geranium.

Click image to enlarge

Figure 1: Close view of cork like tissue as a result of edema
Photo: Gail Ruhl

Figure 2: Sweetpotato with edema.
Photo: Charles Averre, NCSU

Figure 3: Collard leaf showing heavy edema. Photo: Joe LaForest, UGA

Figure 4: Ivy geranium showing severe edema
Photo: NCSU collections.

Figure 5: Close up view of ivy geranium showing severe edema.
Photo: NCSU collections.

Figure 6: Intumescence on tomato.
Photo: Gail Ruhl

Purdue Plant & Pest Diagnostic Lab Purdue Cooperative Extension Service