Some spinach cultivars are expected to have pretty bumpy, puckered leaves. If your plants are not that type, but the leaves look like that or have small holes in them when they expand, crown mites (Rhizoglyphus sp.) are a possible cause. Conditions that favor mite activity and damage are a soil high in organic matter and a cool, moist environment. Mites can be found in both soil and within the smallest, just-opening leaves in the crown of the plant.
Magnification is required to see these tiny, nearly transparent mites. Transparent, spherical eggs are laid in the inner folded leaves and mites of all stages of development can be found together. They feed on germinating seeds, seedlings and young plants. Damage is reported to be less significant as the plants increase in size. The mites are characterized by having long hair-like setae attached to their posteriors as seen in the magnified image.
This pest was first described in the U.S. in California in 1949, so it’s familiar to spinach growers there, but it’s pretty new to us in New England. There was a report of this pest from Rutgers University in New Jersey in 2013. Definitely something to be aware of if you grow spinach. The occurrence reported here was from New London County, Connecticut. The spinach was being grown in a high tunnel and about 75% of the young plants were affected with symptoms as pictured above.
Recommendations for this pest include good sanitation (removal of infested plants) and crop rotation. Efforts to promote warm temperatures and reduced humidity may also be helpful. For additional information, this is a good fact sheet from University of California.
By J. Allen