Integrated Pest Management
Oedema is caused by a physiological upset in the water balance of affected parts which occurs when the roots take in water faster than it can be transpired through the leaves. This water builds up pressure in the mesophyll cells (interior cells) of the leaf causing them to enlarge and burst producing water soaked spots on the affected leaves. These spots enlarge forming swollen blister-like areas which break through the leaf epidermis to form tan or brown wart-like, corky protuberances predominantly on the lower leaf surface. These areas harden and darken with age. In severe cases, these corky growths also form on the petioles and stems. If injury continues, the leaves will turn yellow, droop and fall from the plants. Severely affected plants become spindly and cease growth.
Oedema usually is associated with a high level of soil moisture and reduced transpiration of moisture from the leaf surfaces. Cool nighttime temperatures along with high relative humidity surrounding the foliage tend to suppress transpiration. While plant roots continue to absorb water, the plant does not lose enough water through transpiration.
Oedema occurs frequently in greenhouses in late winter and during cloudy weather. Cloudy weather may cause oedema by reducing the rate of transpiration, whereas a reduction in atmospheric pressure increases the rate of transpiration, thus relieving the tendency for oedema to occur.
Overwatering, high humidity and low light intensities are factors which favor the development of oedema. Therefore, maintaining an environment unsuited for the development of oedema is essential for control. Avoid overwatering susceptible plants especially during cool temperatures when they should be kept slightly on the dry side. Keep the relative humidity below 70% in the winter. Improving the flow of air over the leaves by spacing the plants farther apart and increasing ventilation will help reduce humidity. Increasing the exposure to sunlight is also beneficial. Affected plants often recover from oedema with the return of more favorable growing conditions in the spring and early summer, especially if they are placed outside as soon as weather conditions permit in the spring.