Jerry Brust, IPM Vegetable Specialist, University of Maryland; firstname.lastname@example.org
There are reports of spider mites in multiple crops, with injury in tomatoes and a variety of cucurbits, mostly watermelon. Spider mites love hot, dry weather and that is what we have been having the last two weeks or so. They tend to start out at field edges or by drive rows. Anywhere dust settles on the crop is a likely spot for the earliest infestations. In watermelon, infestations usually start in the crown and spread from there. The oldest leaves will take on a yellow color along the midrib with necrotic spots (Fig. 1). This damage can be misjudged as a disease. Check for the mites on the underside of leaves to verify their presence (Fig. 2).
Figure 1. Mite damage to watermelon crown leaves
Figure 2. Two spotted spider mites on underside of watermelon leaf
For control there are several good miticides out there, but you need high gallonages of water 70-100 gallons/A and thorough coverage of the top and underside of the foliage or you will not reach all the well-hidden mites. Agri-Mek has translaminar movement, so if it is sprayed on the upper leaf surface it will penetrate into the leaf and reside there. Portal works on all stages of mites while Acramite is primarily active on the motile stages of mites (not eggs) and has a long residual. Oberon can take longer to work (check back in 5-7 days) but will give excellent control. Zeal is a growth regulator and will not kill adults but will kill immature mites, it works especially well if you catch the infestation early on. As always be sure to check the label before spraying any pesticides.