Cause Physiological problem that is more prominent when air is cooler than soil, soil moisture is high, and relative humidity is high. Low transpiration rates with an increase in water absorption increase cell pressure, erupting epidermal cells and allowing inner cells to enlarge and protrude. This protrusion causes cells to die and discolor (yellow, brown, or rust), giving the impression that a parasitic organism is involved.
Any mechanical injury to epidermal cells has a similar effect. The most common causes of mechanically caused oedema in the Pacific Northwest are insect feeding injury (particularly aphids and thrips), injury from windblown soil particles, and damage from chemical applications.
Symptoms This disease may occur on any plant part but usually is on lower leaf surfaces. First symptoms are watery blisters or galls, many or just a few. The more succulent tissue appears to be more susceptible. Blisters or galls turn dark brown, yellow, or rust and give the appearance of a rust or bacterial infection.
- Control insects, particularly aphids and thrips. See the PNW Insect Management Handbook for details.
- When cabbages are small, control windblown soil particles by sprinkler irrigation when winds are high.
- Harvest storage varieties before the onset of heavy fall rains, which waterlog the soil.
- Varieties differ in susceptibility, so it is good insurance to grow several suitable varieties rather than just one or two.
- Damaged heads often can be salvaged by stripping off outer leaves, which are most severely affected. This makes economic sense only if the market is particularly strong.