Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu
Figure 1. Watermelon plant that is yellow with some dark spotting
Starting to see some watermelon, cucumber and other cucurbit crops that are looking a bit pale green or even yellow (Fig. 1). Sometimes the plants have brown speckles or spots on them that may look like a foliar disease has started (Fig. 2), but it has not. Most of the time if you look at the underside of the leaf with spots you do not see the same browning of the tissue or it is much reduced (Fig. 3). Also, as in the field of watermelon that these photos came from, much of the field looked like the plant in Figure 1. When the damage appears on one side of the leaf or is in much of the field it is usually an abiotic problem– one that is not caused by an insect, fungus bacteria, or other living pest, but is caused by a non-living factor such as weather. This is what seems to be popping up in some vegetable fields now, but especially in cucurbits in the last few days. The unusually cloudy, cool weather we had most of last week and this past holiday weekend along with some heavy rains have stressed the cucurbit crop and slowed its overall growth (foliar and root). Plants should recover and grow out of it with warmer weather.
Figure 2. Watermelon leaf that is yellow with dark marks caused by abiotic factors
Figure 3. Underside of a watermelon leaf that had dark spots on top but not underneath