Felt-like eriophyid mite erineum patches are now very obvious on American beech (Fagus grandifolia) in southwest Ohio. I've long identified the culprit behind the patches as Acalitus fagerinea (family Eriophyidae). However, I'm now convinced there may be two species of eriophyids that produce erineum patches on American beech.
The 1982 USDA publication, An Illustrated Guide to Plant Abnormalities Caused by Eriophyid Mites in North America, remains one of the most comprehensive sources of information on symptoms produced by eriophyids [see More Information below]. The authors describe the symptoms produced by Acalitus fagerinea as: "The yellow, common, attractive erineum patches on the leaves of American beech occur in the angles between the midrib and lateral veins on the upper surface of the leaf."
Indeed, felt-like erineum patches on upper leaf surfaces of American beech are now common in southwest Ohio. They will slowly progress through several color stages throughout the season from light green in the spring to brilliant yellow to yellowish-gold then rusty red to reddish-brown and finally dark brown. Although the patches are located on the upper leaf surface, they cause dimpling of the lower leaf surface beneath each patch.
However, I'm increasingly finding erineum patches on American beech that present a complete reversal of the upper – lower leaf symptoms. The felt-like patches occur on the lower leaf surface and the dimpling appears on the upper surface. Additionally, the patches progress through a different range of colors from light green to deep red.
Whether erineum patches occur on the upper or lower leaf surfaces may seem trivial. However, the upper versus lower leaf surface locations are sometimes used to separate eriophyid species found on other trees.
Regardless, a close examination of the erineum patches in both locations will reveal they are comprised of tiny hairs growing from the leaf epidermis. An even closer examination using a microscope may reveal the eriophyid mite culprits cavorting among the erineum hairs.
Eriophyid mites are unique among other mites both in their size and anatomy. While most mites can be clearly seen with a 10x hand-lens, you need to use 40x magnification to see eriophyid mites. Most mites are round to oblong in their body shape and they have four pairs of legs that extend laterally from the sides of their body. Eriophyid mites are cigar-shaped and they only have two pairs of legs that extend from the front of their body. No other mite only has two pairs of legs at any stage in their development.
Erineum patches produced by eriophyid mites on beech are generally viewed as oddities rather than a significant tree problem. The patches seldom cover enough of the leaf surfaces to cause harm to the overall health of an affected tree.
However, it's important to correctly identify erineum patches and not mistake them for other tree problems, as well as vice versa. For example, our cool, wet spring resulted in some heavy infections by the host-specific fungus responsible for beech anthracnose. It's common to find erineum patches on leaves with necrotic tissue from anthracnose.
Eriophyid erineum patches may also occur on trees showing symptoms of beech leaf disease (BLD). As far as anyone knows, there is no connection. Thus far, BLD has only been found in northeast Ohio. The eriophyid(s) behind the erineum patches occur throughout the state. Finding the two unrelated problems on the same tree, or sometimes the same leaf could present a diagnostic challenge.