Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
With recent wet, cloudy weather we are seeing some edema (or oedema) on bell pepper and banana pepper fruits. Edema is also called water blistering. The most common cause of edema is warm wet soils, high humidity in the air, low wind, and overcast (cloudy) skies. Under these conditions the roots of the pepper plant absorb water at a rate faster than is lost through transpiration. Excess water accumulates in the developing fruit, some fruit cells enlarge, stomatal openings become blocked by these enlarging cells, and water continues to accumulate in the fruit. The enlargement of these fruit cells then causes a rupture of the epidermis leaving raised bumps and scarring. To reduce edema, limit irrigation during cloudy, humid weather.
With fall cole crop season underway we also commonly see edema in crops such as cabbage. Edema is water blistering on cole crop leaves. The most common cause of edema is the presence of abundant, warm soil water and a cool, moist atmosphere. Under these conditions the roots absorb water at a rate faster than is lost through transpiration. Excess water accumulates in the leaf, some parenchyma cells enlarge and block the stomatal openings through which water vapor is normally released from the plant; thereby contributing to further water retention in the leaf. If this condition persists, the enlarged cells divide, differentiate a cork cambium, and develop elongate cork cells externally to form a periderm. The rupture of the epidermis by the enlarged inner cells and the periderm account for the raised, crusty appearance of older edema spots.
Edema in Banana Pepper Fruits. Note the raised brown bumps and scarring.
Magnified picture of ruptured epidermis on edema affected banana pepper fruit.
Edema on a cole crop leaf.