Marginal Chlorosis (Yellowing Edges)
Dan Egel, Extension Vegetable Pathologist, SWPAC, Purdue University
You may look at the muskmelon leaf at the right (Figure 1) and say that it is yellow around the leaf edges. In the plant trade, we say that the same leaf has ‘marginal chlorosis’. Whatever your choice of words, this type of symptom is usually not infectious - that is, marginal chlorosis is not usually the result of a disease that can spread from plant to plant. Instead, marginal chlorosis is typically caused by a combination of stress factors such as heat, drought and old leaf age. With time, the chlorotic margins may turn brown or necrotic.
One way in which marginal chlorosis gets started is illustrated in the photographs at right (Figures 2 and 3). The drops of water on the leaf are not dew, but water exuded from pores at the margins of the leaf known as ‘water of guttation’ by botanists. Water of guttation can be observed on many different types of plants. These droplets accumulate overnight and are visible early in the morning before drying. The content of these droplets includes sugars, salts and other plant related compounds. As the droplets dry, the plant related compounds are left on the margin of the leaf where, under the appropriate conditions, the leaf margin becomes chlorotic. The marginal chlorosis often shows up on older leaves since it takes time for the accumulation of plant related compounds to affect the plant. As long as the younger leaves appear healthy, a few leaves with marginal chlorosis should not be cause for worry.
There is no need to apply pesticides to a plant with marginal chlorosis, since no insects, bacteria, fungi or viruses are involved in the symptoms. Learning to recognize non-infectious symptoms such as marginal chlorosis discussed here is part of IPM, Integrated Pest Management. Commercial growers or gardeners who practice IPM use pesticides only when they are needed. Plants with questionable symptoms can be sent to the Plant and Pest Diagnostic Laboratory.
Click image to enlarge
Figure 1: The muskmelon leaf above is an older leaf that has marginal chlorosis (yellow leaf edges). Marginal chlorosis in typical of a non-infectious problem that will not spread from plant to plant and in most cases requires no treatment.
Figure 2: The water droplets on the margins of this muskmelon leaf are not dew, but water of guttation, a normal process on many leaves. Such leaves may eventually display the marginal chlorosis seen above.
Figure 3. Close up of water of guttation.