I’ve long admired Butternut (a.k.a. White Walnut) (Juglans cinerea, family Juglandaceae). Its larger and longer-lived cousin, black walnut (J. nigra), was relatively common in my native West Virginia. Our farm had a “walnut hollow," so named for the large number of mature black walnut trees growing in that narrow, sheltered valley. Butternut trees were rare and their nuts (drupes) highly prized.
Butternut trees do not grow as large or live as long as black walnuts. Trees range in height from 40 to 60 ft., with a canopy spread of about the same, making them more rounded than black walnut trees. Trees seldom achieve a DBH greater than 2 ft. Of course, there can be exceptions where sites are good, such as the tree pictured below. However, their lifespan is normally less than 100 years. They are intolerant of shade and are considered an early successional species, typically among the earliest Juglans species to colonize disturbed sites.
However, what butternut trees lack in size and longevity, they more than make up for in winter hardiness and nut flavor. They are the most low-temperature tolerant of any Juglans species and are found growing from Maine and the Canadian Maritime Provinces to Minnesota. They venture south into South Carolina, but not nearly as far south as black walnut trees.
I love black walnuts for their slightly bitter flavor that pairs well with milk chocolate; but butternuts have a rich, buttery, slightly sweet flavor. The first time I tasted Macadamia nuts, butternut meat came to mind. The downside is that the high oil content means they need to be used quickly or stored properly. Rancid butternut meat tastes like old grease mixed with regret.
Butternut Canker
Unfortunately, butternut trees are in real trouble owing to the fungus, Ophiognomonia clavigignenti-juglandacearum; a pathogen with a long, impossible-to-remember scientific name. It’s easier to remember “butternut canker,” which is the lethal disease produced by the fungus.
The origin of the fungus is unclear, but it’s thought to be non-native. Butternut canker was first documented in 1967 in Wisconsin. The cankers first appear as long fissures in the bark of branches and tree trunks. The affected areas eventually become sunken, black, and surrounded by slightly raised wound wood. Canopy dieback inevitably leads to tree death, and there are currently no treatments available.
Butternut canker has been gradually tearing through native stands of butternut. Losses of 70 – 90% are reminiscent of the impacts of chestnut blight, Dutch elm disease, and emerald ash borer. Indeed, butternut is now listed as Endangered on the International Union for Conservation of Nature (IUCN) Red List.
Although the canker fungus can infect black walnut trees, infections remain sublethal. Japanese walnut (J. ailantifolia) appears to be even more resistant. Consequently, research is being conducted to assess hybrids between butternut and Japanese walnut, similar to an approach used for Dutch elm disease and chestnut blight.
The ravages of butternut canker in our forests place greater weight on the value of butternuts growing in our landscapes, particularly in locations far removed from regions where the cankering disease is prevalent. Research has shown that certain insects can transport fungal spores, but the fungus tends to spread slowly from tree to tree in forests. Spores are splash-disseminated during rainfall or blown short distances on the wind.
Consequently, butternut trees planted in parks, arboreta, and other landscapes may serve as “last stands” for the species. While there’s no long-term guarantee that the cankering fungus won’t eventually find outlander trees, it may be worth the risk for the sake of maintaining the species genome.
An Eriophyid Mite Can Spoil the Party
Eriophyid mites (family Eriophyidae) are tiny arachnids unique among mites both in their anatomy and in the range of symptoms they produce on plants. Eriophyids are very small and appear with the naked eye as specks of dust requiring 40x magnification to see details.
Unlike spider mites, which look like tiny spiders, eriophyid mites are cigar-shaped. They have two pairs of legs that extend from the front of their bodies. No other mite has only two pairs of legs at any stage in its development.
Eriophyids have a two-punch impact. They use their needle-like chelicerae to rupture individual plant cells so they can suck-up the contents. The damage can lead to brown or blackened necrotic tissue. Research has shown that their salivary secretions contain chemicals that induce plant growth regulatory effects, resulting in bizarre plant growth and unusual structures.
The eriophyid, Aceria cinereae, is specific to butternut. The mite has no approved common name. However, “butternut eriophyid mite” seems appropriate given that it is confined to a single host and produces characteristic symptoms that are hard to mistake for anything else found on butternut trees.
The butternut eriophyid mite’s feeding activity induces a proliferation of leaf cells to produce raised, wart-like growths on the upper leaf surface that are commonly bound by leaf veins. The growths are at first green but eventually turn brown to black. Heavily affected leaves can become twisted and deformed.
The warty bumps correspond to pouch-like pockets on the underside of the leaves. The pockets are lined with tiny velvet-like hairs called erinea (singular erineum) from the Greek erineon for woolen.
I’ve recently observed some substantial butternut eriophyid mite infestations on its namesake host in southwest Ohio. As with most eriophyids on hardwoods, the impact is aesthetic rather than a threat to the overall health of established butternuts.
However, the damage may be more impactful on small trees, as shown in the images below. This is entirely anecdotal, but I’m speculating that the damage may reduce stem elongation and overall growth.
I’m not aware of any effective treatments to suppress butternut eriophyid mites. However, the possible negative impact on tree establishment means that seedlings and small trees should be closely inspected and, if possible, only “clean” trees planted.
Selected References
Brennan, A. N., McKenna, J. R., Hoban, S. M., & Jacobs, D. F. (2020). Hybrid breeding for restoration of threatened forest trees: evidence for incorporating disease tolerance in Juglans cinerea. Frontiers in Plant Science, 11, 580693.
https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2020.580693/full
Broders, K., Boraks, A., Barbison, L., Brown, J., & Boland, G. J. (2015). Recent insights into the pandemic disease butternut canker caused by the invasive pathogen O phiognomonia clavigignenti‐juglandacearum. Forest Pathology, 45(1), 1-8.
https://doi.org/10.1111/efp.12161
Glasgow, B. (2020). Gall inducing and gall associated arthropods and plant hosts in Great Smoky Mountains National Park. Journal of the North Carolina Academy of Science, (1).
Guzman-Torres, C. R., Trybulec, E., LeVasseur, H., Akella, H., Amee, M., Strickland, E., ... & Wegrzyn, J. L. (2024). Conserving a threatened North American walnut: a chromosome-scale reference genome for butternut (Juglans cinerea). G3: Genes, Genomes, Genetics, 14(2), jkad189.
https://academic.oup.com/g3journal/article/14/2/jkad189/7271757
Joll, E., Ebrahimi, A., Conrad, A., Jacobs, D., & Couture, J. (2023, November). Foliar hyperspectral identification of butternut canker infection in pure and hybridized butternut (Juglans cinerea). In 2023 IEEE International Workshop on Metrology for Agriculture and Forestry (MetroAgriFor) (pp. 847-850). IEEE.
https://ieeexplore.ieee.org/abstract/document/10424124
Keifer, H. H. (1982). An illustrated guide to plant abnormalities caused by eriophyid mites in North America (No. 573). US Department of Agriculture, Agricultural Research Service.
Moore, M. J., Ostry, M. E., Hegeman, A. D., & Martin, A. C. (2015). Inhibition of Ophiognomonia clavigignenti-juglandacearum by Juglans species bark extracts. Plant disease, 99(3), 401-408.
https://apsjournals.apsnet.org/doi/abs/10.1094/PDIS-06-14-0642-RE
van Der Meer, B. M., Solomon, J. P., Kostanowicz, C. A., & Williams, M. (2026). The urgency of genetic decline in New Brunswick butternut, Juglans cinerea L. Plants, People, Planet, 8(1), 290-300.
https://nph.onlinelibrary.wiley.com/doi/full/10.1002/ppp3.70079
