Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu
Spinach crown mites Rhizoglyphus sp. have been found in spinach fields over the last few days. These mites feed within the folds of new leaves in the crown of spinach plants. This feeding causes the new leaves to become deformed as they grow (Fig. 1). Crown mite adults are extremely small, bulbous, nearly transparent mites that also may have a yellow-beige body color with reddish-brown legs (Fig. 2). A good characteristic to look for to identify these mites is the sparse long hairs mostly found on the back end of the mite (Fig. 2). Crown mite eggs are spherical and clear and laid on the creased leaf surfaces in the crown area. I have talked about this genus of mite several times over the years, but always in regard to bulbs of garlic or onion where they feed on and open the bulbs up to infection from soil diseases. Some reports state that crown mites can act as vectors for plant pathogens such as Pythium and Rhizoctonia, but this is not definitive. Although crown mites are in the same genus as bulb mites they may or may not be the same species. As you can see there is still much that is not understood about these pests.
Figure 1. Crown leaves fed on by spinach crown mites are distorted and ragged with necrotic margins (arrows) developing as leaves expand.
Figure 2. Spinach crown mite adult with sparse long hairs over its body
The spinach crown mite is most damaging in soils high in organic matter and when there are cool moist conditions – plants grow a little more slowly and the mites proliferate in this type of environment. Because these mites can consume organic matter they can survive in soils after the crop has been removed. This is one reason they are difficult to control as they can survive for fairly long periods of time with no crop being present. The other reason they are difficult to ‘control’ is we do not realize they are causing the problem until it is too late.
Most control recommendations include sanitation and crop rotations as being important as are fallow periods. Pyrethroids are a possible chemical control, as is Neem; any chemical control has to get down into the crown of the plant to have any chance of working. There has been little research conducted on the most efficacious material for these mites.