It is thought to be due to adverse environmental conditions. Therefore, it does not spread from plant to plant. Ivy geraniums with a large root system as compared to shoot size are particularly sensitive to this disorder.
Small yellow spots in areas between the veins are often the first symptom observed on the top of the leaf. Small, translucent, watery pustules are seen on the underside of the leaf below the yellowed areas. These "blisters" and yellowed areas usually occur first on older leaf margins. The blisters enlarge and become brown in color and corky or scabby in texture. The entire leaf may yellow, die, and drop off. This leaf death and defoliation occurs in a pattern somewhat similar to that caused by bacterial blight.
Edema is caused by an imbalance of the water uptake and loss by the plant. A high level of water absorption by the roots is favored by warm moist soil conditions while a low level of transpiration (water loss) is favored by cool air, high relative humidity, low light levels (cloudy weather), and poor ventilation. Water retention in the cells causes some cells to burst and form watery blisters. As these broken cell areas heal, they become dry and corky.
Mite (two-spotted, Tetranychus urticae Koch) feeding may also play a role in edema development. Potter and Anderson screened ivy geranium cultivars for resistance to two-spotted spider mite attack and evaluated edema severity on the mite-infested plants. There seems to be a relationship in which mite resistant cultivars develop less severe edema than mite susceptible cultivars. The authors theorized that the plant's physiology or nutritional status that results in edema development may also favor mite survival. Edema and mite resistant cultivars were "Double Lilac White," "Sunset," "Madame Margot," "Amethyst," and "Salmon Queen". The most mite and edema susceptible cultivars included "Sybil Holmes," "Yale," and "Pascal."
Edema Can Be Lessened By The Following
- Improve drainage and water less frequently.
- Heat and ventilate the greenhouse to reduce humidity.
- Space plants to provide good air circulation.
- Water in the morning so that the soil is not too wet overnight.
- Avoid wetting the leaves since wet leaves lose less water.
- During cool cloudy weather, follow a watering schedule that maintains an even moisture level.
- Maintain good mite control by regularly applying a miticide.
The above information was summarized from the following sources:
- Balge, R. J., Struckmeyer, B. E., and Beck, G. E. 1969. Occurrence, severity, and nature of oedema in Pelargonium hortorum Ait. Jour. Am. Soc.Hort. Sci. 94:181-183.
- Digat, B. and J. Albouy. 1976. Donnees actuelles sur le probleme de l'oedeme du pelargonium. Pepinieristes Horticulteurs Maraichers 168:51-55.
- Mastalerz, J. W. 1971. Geraniums. A Penn State Manual. Pennsylvania Flower Growers. 103 Tyson Bldg., University Park, PA 16802. 350 pp.
- Potter, D. A. and R. G. Anderson. 1982. Resistance of ivy geraniums to the two-spotted spider mite. J. Am. Soc. Hort. Sci. 107:1089-1092.
Prepared by Gary W. Moorman, Professor of Plant Pathology