Cause A physiological response by tomato plants that associated with low light conditions, high humidity, or possibly air pollutants. Low transpiration rates along with a rise in water absorption increase cell pressure, erupting epidermal cells so that inner cells enlarge and protrude. That causes the inner cells to potential die and discolor (yellow, brown, or rust), giving the impression that a parasitic organism is involved. The severity of symptoms depends on the variety of tomato. Oedema will not spread from one plant to another, but can appear across a planting due to the similar environmental conditions that promote this abiotic problem. Certain varieties are genetically predisposed and may develop symptoms earlier or to a greater degree compared to other lines when multiple varieties are sown alongside of each other.
Symptoms Symptoms appear primarily on lower leaf surfaces. The disorder shows up first as small, clear, watery blisters (0.06 inch in diameter), which can be very numerous. As blisters dry, they can remain green or turn opaque and take on a reddish-brown coloration. Oedema is unsightly but not a serious threat to the health of affected plants.
Cultural control
- Maintain a well-timed irrigation schedule that prevents overly-wet soils.
- Fans to circulate air and increase transpiration may help to reduce oedema.
- Move transplants into brighter light conditions.
Reference Horst, R.K. 1990. Westcott's Plant Disease Handbook, 5th ed. Reinhold, NY: Van Nostrand.