Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu
An odd problem was seen in tomato plants being grown for harvest in a greenhouse that showed blister or callus-like growths, usually along veins that first appeared on the underside of older leaves (Fig. 1). Leaves also showed unusual curling and other odd distortions on the top side of the leaf (Fig. 2). In addition, there can be spots of necrotic leaf tissue. Leaves with a great deal of this blistering were brittle with the leaf often cracking with any type of handling. Karen Rane from the diagnostic lab recognized these symptoms as edema (or sometimes spelled oedema). Edema is caused by the buildup of excess water in the root and conditions unfavorable for transpiration, usually caused by high humidity. When the tomato plant cells get too much water the cells will expand faster than they can get rid of the water leading to split and cracked tissue. Extensive edema can severely decrease the leaf’s photosynthetic capability and lead to senescence. Other research has looked at poor or low light sources that affect the plants ability to expel excess water. So basically overwatering, high humidity and low or poor light are the major causes for the development of edema. Therefore, avoid overwatering plants in the greenhouse especially during cool temperatures when they should be kept slightly on the dry side. Keep humidity levels below 70% by enhancing airflow around the plants and by spacing the plants farther apart. Though more complicated, research has shown that increasing light quality by providing a more “full-spectrum” of light output, with significantly more short wavelength energy (i.e., UV light), will also decrease the occurrence of edema.
Figure 1. Underside of tomato leaf showing swollen tissue caused by edema
Figure 2. Top side of distorted tomato leaf with edema