Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu
Several watermelon fields I have visited in the last week or so have shown characteristic signs of two spotted spider mite damage. This damage usually shows up first in the crown leaves (oldest leaves of the plant). The top of leaves have a yellowing between the veins with an occasional necrotic spot (Fig. 1). If the underside of the leaf is examined, the area looks more yellow to tan and there is often dirt or sand that seems to be stuck to the leaf—and there is (Fig. 2). The sand is ‘stuck’ to the underside of the leaf because two spotted spider mites have webbing on the leaf and soil that is blown around or splashed up onto the leaf is held by the fibers. This results in a protective layer, shielding mites from many natural enemies and insecticide sprays. It was surprising to see these levels of infestation in watermelon and in eggplant as it has not been very hot this summer. However, it has been very dry in most places these last few weeks, and mites tend to thrive in dry weather. Growers should be sure to check that they have mites, as ozone damage can cause symptoms to crown leaves similar to mite feeding, i.e., interveinal yellowing, necrotic spots on the leaf (Fig.3). A 10x hand lens works best to not only see the mites, but also see the webbing and the numbers of mite eggs — round spheres about a quarter the size of an adult mite (Fig.4). At several fields that had mite infestations I found few adult mites, but many eggs. Mite eggs are difficult to kill and after the initial miticide application an additional application may be needed 5-7 days after the first application to catch all the eggs that have hatched. Agri-Mek EC, Oberon 2SC, and Acramite 50WS have all given excellent control of mites in watermelon.
Figure 1. Two spotted spider mite damage on watermelon
Figure 2. Two spotted spider mite damage on the underside of watermelon leaf
Figure 3. Ozone damage on watermelon; notice different degrees of yellowing and necrotic areas as the damage worsens from A through F. Photograph B looks very much like TSSM damage. Photographs courtesy of Gerald Holmes, NCSU Dept of Horticulture.
Figure 4. Two spotted spider mite eggs and an adult on watermelon leaf