Erineum Patches may be found on the upper or lower leaf surfaces of several species of deciduous trees in Ohio. They are considered a type of plant gall and their general appearance accounts for common names such as velvet galls or felt galls.
The fuzzy patches appear in many colors from silver to subtle shades of green to obvious shades of yellow or red. In fact, some patches may pass through this entire color range as they develop. Most will eventually turn brown to black. Their colors and general appearance sometimes causes them to be mistaken for a leaf spot or rust disease.
A close examination will reveal the erineum patches are comprised of tiny hairs growing from the leaf epidermis. An even closer examination using a microscope may reveal eriophyid mites (family Eriophyidae) cavorting among the hairs; the mites are the culprits behind the patches.
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 clearly 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 has only two pairs of legs at any stage in their development.
The eriophyid mite Acalitus fagerinea produces erineum patches on American beech. Although the patches are located on the upper leaf surface, they cause a dimpling of the lower leaf surface beneath the patch. These erineum patches in southwest Ohio are currently light green to a yellow; they will eventually become golden then rusty red to reddish brown later in the season. The mite A. brevitarsus generates garish pinkish-red erineum patches on the upper leaf surface of birch leaves. These patches in the southwest part of the state are currently silvery-white with slight overtones of pink.
Several different eriophyid mites produce erineum patches on Acer spp. The descriptive Crimson Erineum Mite (Eriophyes elongates) targets sugar maples where it generates patches on the upper leaf surface that are deep crimson red; they are currently greenish-white. E. modestus produces silvery colored patches in the forks in the leaf veins on the lower leaf surface of sugar maple leaves.
So-called Boxelder Velvet Galls are erineum patches produced by E. negundi on its namesake host. These irregularly shaped patches are found on the lower leaf surface and cause corresponding raised blister-like bumps on upper surface. The patches are currently silvery-white but will eventually turn brown; the blisters are currently light green but will turn brown later in the season.
I came across this "Oak Velvet Gall" on newly expanding leaves in Michigan in 2013. The patches are obviously produced by an eriophyid mite; however, I have yet to find identification information. The erineum inducing mite may have not yet been described.
As with the vast majority of plant galls, erineum patches are seldom so severe as to cause significant injury to the overall health of their host trees. Thus, control recommendations are not necessary.