Edema is a physiological problem occurring mainly on ivy geraniums. Edema, simply put, is a bursting of the cell walls on the underside of the leaf. It results in leaves with corky spots which sometimes resembles spider mite damage. At worst it causes leaves to yellow and die. Edema is not a disease, bacterium, or a virus; it is not transmittable from one plant to the next. Edema is caused by maintaining a greenhouse environment that's not necessarily ideal for growing ivy geraniums, often combined with over watering of the crop. In addition, some ivy varieties seem to be more susceptible to developing edema than others.
Why Edema Occurs
To begin with, most growers are cropping their ivies in baskets hung above the benches where the air is most humid. Second, many are growing their ivies in a saucerless hanging basket; this type of pot retains a good bit of water after each irrigation--under certain conditions, too much water. Third, hanging baskets are often on automatic watering systems. Not all the baskets dry out at the same rate yet they're all watered at the same time, whether they need it or not. Fourth, often there is poor air circulation where baskets are hung, reducing the transpiration rate. Lastly, ivies are grown during the time of year when light levels are lowest--January and February--and in response plant transpiration rates are also low. The winter months are also the coldest, which generally means lower temperatures in the greenhouse. These factors combine to create the perfect conditions for edema to occur. High levels of moisture, both in the media and in the air, results in an imbalance between the water in the plant and the water in the air--plants don't transpire normally. The ivy geraniums, in a manner of speaking, are drowning. The plant's natural defense mechanism to avoid "drowning" causes cell walls to rupture so that the moisture can be purged from the plant.
Avoiding the Problem
What can growers do to avoid edema? The main method is to carefully manage the greenhouse environment. Reduce the air humidity by venting your greenhouse first thing in the morning, even if that means turning up the heat (a decondensation program). Second, make sure there is adequate air flow, whether from heat jets or horizontal air flow fans. Air movement is important 24 hours a day. Third, do not use saucerless hanging baskets. Instead, use baskets that have snap on saucers, but do not put the saucer on until the crop is nearly finished, or better yet, until point of sale. This will ensure maximum drainage of each basket. Fourth, if you are using an automatic watering system, place varieties with similar vigor on each line or section, again to eliminate over watering. Lastly, manage your pH properly. Make sure your ivy pH is 5.5. When you maintain pH at 5.5, you can feed once every three feedings with calcium and potassium nitrate. Calcium will thicken up your cell walls, making the ivies more resistant to edema.One more suggestion. If your ivy crop is prone to edema we recommend that you monitor the environmental variables as best you can, and evaluate a broad range of ivy cultivars to determine which varieties best resist edema under your specific conditions.
By thoroughly understanding and controlling the conditions which promote the development of edema, growers can produce beautiful ivy baskets which consumers will be eager to purchase!