Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu
The hot hazy conditions we have had this summer have resulted in some ozone damage in cucurbits and tomatoes. Ozone is the most common air pollutant in the eastern United States. On watermelon, the vegetable that is more susceptible than other crops, the damage starts off as small white spots or tiny asymmetrically shaped flecks that eventually become bleached areas (Fig. 1). Flecks can be dark or light in color. These symptoms usually occur between the veins on the upper leaf surface of older and sometimes middle-aged leaves. Leaves later develop brown or black spots with white patches (Fig. 1). The crown leaves of watermelon can look pretty ragged at this time of year, especially when fruits are maturing, and plants are under stress. In muskmelons the upper surface of leaves turns from a chlorotic yellow to a bleached white (Fig. 2). Due to the tissue collapse produced by ozone, leaves are prone to infection by pathogens such as Alternaria sp. and generally will senesce more readily than non-damaged leaves.
A few tomato fields also have shown ozone symptoms of irregular dry-looking dark or light brown flecks usually starting on lower leaves (Fig. 3A). These flecks at times can look similar to bacterial spot (Fig. 4). But bacterial spot lesions are dark brown to black and initially circular in shape and appear ‘greasy’, while ozone damage appears as dry flecks often with swollen veins on the underside of a leaf (Fig. 3B). Bacterial spot lesions also are often surrounded by a yellow halo while ozone damaged areas will not have this yellow halo. Numerous bacterial spot lesions can coalesce causing a general yellowing of leaves (Fig. 4).
Trying to estimate yield loss due to air pollutants in the field is difficult and only approximations can be made. In a California study, ozone damage to crops caused the greatest yield losses (10-30%) in watermelon, cantaloupe, grape, onion, and beans. Other research has shown that when average daily ozone concentrations are moderate to high, yields of vegetables can be reduced by 5-15%.
Figure 1. Ozone damage to watermelon crown leaves
Figure 2. Ozone damage to cantaloupe leaves
Figure 3. Ozone damage to tomato leaves
Figure 4. Bacterial spot on tomato leaves