There are blister-like growths on the undersides of ivy geranium leaves. What is the problem?

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Question: 

There are blister-like growths on the undersides of ivy geranium leaves. What is the problem?

Answer: 

Oedema is probably responsible for the lesions on the undersides of the ivy geranium leaves.  Oedema is not caused by an insect or disease pathogen.  It is a physiological problem.  Oedema is most often encountered in greenhouses in late winter. 

While oedema can occur on many different plants, it is most commonly found on ivy geraniums.  Affected leaves develop small blisters on their undersides.  The blisters rupture and turn tan or brown and become corky.  Seriously affected leaves may turn yellow and fall from the plant. 

Oedema typically occurs when plants absorb more water through their roots than they use or transpire through their leaves.  This usually happens during prolonged periods of cool, cloudy weather in late winter.  Oedema develops when the soil is warm and moist, while the air is cool and moist.  Water pressure builds up in the cells of the leaf causing them to enlarge and form swollen, blister-like growths. 

While oedema is unsightly, it does not cause serious harm to affected plants.  Problems can be minimized by not over-watering during cloudy weather and reducing humidity levels in the greenhouse. 

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