Physiological Disorders Related to Irrigation and Fertilization

Edema

Description
This disorder is often mistaken for bacterial or fungal diseases. In its early stages the blister-like swellings on the leaf resemble an undifferentiated callus-type growth. The granulated appearance of the fresh blisters is caused by the splitting of the epidermis, presumably under pressure from within (Grimbly, 1986). This exposes the turgid parenchyma cells. These swollen parenchyma cells eventually erupt. Rupture of these cells over a period of time causes twisting and distortion of the leaves, and produces a necrotic area when the cells dry out.

Cause
This disorder is seen in a number of crops besides tomatoes, including cabbage and sweet potatoes. In all cases, it is caused by water provided to the leaves exceeding that used in transpiration for a period of several days. Sagi and Rylski (1978) showed that under high humidity and excess water, the symptoms increased as light intensity decreased, presumably representing the diminishing ability of the plant to transpire. They also found much greater susceptibility to the disorder in a Israeli field cultivar than in a cultivar selected for greenhouse production in Northern Europe.

Control
In greenhouses or growth chambers, the disorder can be prevented or remedied. Decreasing watering and promoting transpiration by such measures as increased ventilation, higher temperatures and higher light should be effective. In the field, the disorder only appears when there are long periods of excess water and low transpiration. There is little that can be done in the field, assuming that irrigation has already been stopped, except for trying different cultivars.