Fragrant Sumac (Rhus aromatica, family Anacardiaceae), which is sometimes called Aromatic Sumac, is a tough plant. It tolerates a wide range of conditions that would snuff the life out of more discriminating plants. This quality has made it a favorite for use in challenging sites such as concrete parking lot planters, where trees go to die.
Of course, staying alive in difficult sites isn’t the only positive attribute of fragrant sumac. Most notable, both the common and specific epithet names reference the pleasing, lemony scent that wafts from its stems and leaves when crushed.
Its flowers attract a range of pollinators, and its berries support wildlife. The dark green glossy leaves contrast with its reddish-hued stems. The multi-toned fall colors of this native deciduous shrub are a show stopper as they range through radiant reds, brilliant oranges, and subtle yellows.
However, fragrant sumac displays can be marred by the appearance of small, wart-like bead galls, appearing on the surfaces of otherwise dark, glossy leaves. The galls are the handiwork of the eriophyid mite, Aculops rhois (= A. toxicophagus).
I first started filing BYGL reports about the eriophyid gall-maker on fragrant sumac in the late 1990s. That was back when the BYGL was a faxed newsletter, and the “L” in BYGL stood for “Line” rather than the current “onLine.” I consistently described the galls as oddities. While noticeable, they did not appear to affect overall plant health.
However, based on recent observations, I may amend my assessment. Early in the growing season, the galling is not so extensive as to cause a significant reduction in photosynthesis. But the limited impact can change as the season progresses.
I’ve observed galling that is so intense by mid to late summer that few leaves remain functional. I suspect the gradual increase in galling may be associated with a continual rise in mite population densities, coupled with the constant supply of new leaves that are susceptible to galling. Of course, I don’t know if the loss of leaf function occurs before or after plants have produced and stored enough carbohydrate to support next season’s new growth.
Unfortunately, as far as I know, there are no effective treatment options based on non-biased, research-based data. However, based on personal observations, I suspect there may be some variability in host susceptibility among fragrant sumac varieties and cultivars. For example, I do not see the same level of galling on the straight species as I have observed on some of the selections. Of course, this is anecdotal.
A Taxonomic Conundrum
This same eriophyid mite that produces bead galls on fragrant sumac is listed in Bugguide as the culprit behind the production of bead galls on the leaves of Eastern Poison Ivy (Toxicodendron radicans, family Anacardiaceae). Indeed, the mite is called the Poison Ivy Leaf Gall Mite in the online publication. However, the Entomological Society of America has yet to approve a common name.
A wider host range for the eriophyid mite is presented in the USDA publication, An Illustrated Guide to Plant Abnormalities Caused by Eriophyid Mites in North America. The authors report that A. toxicophagus also produces bead galls on Poison Sumac (T. vernix) in Ohio, and on Poison Oak (T. pubescens) in Oregon and California.
Fragrant sumac, poison ivy, poison oak, and poison sumac were once grouped in the genus Rhus. So, it made sense that the poison ivy leaf gall mite could infest all of these “closely related” species.
However, plant taxonomists have upset the gall cart by pulling poison ivy, oak, and sumac out of Rhus and placing them in the genus Toxicodendron, which remains in the shared family, Anacardiaceae. The host range of the poison ivy leaf gall makes less sense given that eriophyids typically have narrow host ranges, often involving a single plant genus or one plant species.
I tried to gain a further understanding by untangling the taxonomic wanderings of poison ivy, oak, and sumac but found myself going down a rabbit hole. It seemed fitting since taxonomists once grouped rabbits with rodents (order Rodentia) until the early 20th century.
