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.