Participants in the Diagnostic Walkabouts held as part of the Ohio Chapter ISA Field Day last Friday in Heritage Park, Westerville, OH, observed woody stem galls on hickory (Carya sp.). The galls were observed on the main stem and branches of two mature hickories growing just inside a forest next to a paved trail.
According to the literature, the woody stem galls on hickory are produced under the direction of a fungus belonging to the genus Phomopsis (teleomorph Diaporthe). Sinclair and Lyon (2005) provide an overview of the galls. However, the authors do not provide an identify for the causal fungus beyond Phomopsis sp.
Indeed, I’ve never found a reference that nails down the identity of the fungal gall-maker on hickory beyond the genus. Not knowing the exact species presents a challenge in learning more about gall development and the disease cycle.
On a side note, the woody Phomopsis stem galls on hickory superficially resemble Horned Oak Galls produced on their namesake hosts under the direction of the gall-wasp, Callirhytis cornigera (Family Cynipidae) [See BYGL Alert, “Horn Rise,” https://bygl.osu.edu/node/2616 ]. However, the surface of the Phomopsis galls have a more roughened appearance as shown in the comparison below.
Also, I’m not aware of any cynipid wasps that target hickory in Ohio. The dominant arthropod gall-makers on hickory are gall-midges (Family Cecidomyiidae) and aphid-like phylloxera (Phylloxeridae). None produce woody stem galls.
Brown (1938) published observations he made of woody galls on both oak and hickory. He noted the galls resembled Crown Galls produced by the bacterium Agrobacterium tumefaciens, but he was unable to confirm the presence of the bacterium.
However, Brown observed the pycnidia of a Phomopsis associated with the galls. He extracted spores and induced woody gall formation on hickory, oak (Quercus spp.), European cranberrybush (Viburnum opulus), privet (Ligustrum vulgare), and winter jasmine (Jasminum nudiflorum). He successfully reisolated Phomopsis in each case and noted that the oak strain also produced galls on highbush blueberry (Vaccinium corymbosum).
Brown’s observations are intriguing, but I could find no reports in the literature that confirm his investigations. Indeed, I’ve never observed woody galls on oaks that were not associated with a cynipid wasp. Of course, perhaps we aren’t looking past wasps.
The Phomopsis stem galls are reported to occur on Bitternut (C. cordifirmis), Pignut (C. glabra), and Shagbark (C. ovata) hickories as well as Pecan (C. illinoensis) (Burns and Honkala, 1990). According to the literature, the impacts vary (Juzwik et al., 2008; Juzwik et al., 2014). Although the galls are most often found on branches, they may arise anywhere on the tree. This includes the main stem, where they commonly grow around branch junctions.
As with the horned oak galls, the Phomopsis galls on hickory are woody through and through. Although I’ve never performed a microscopic examination, it appears that Phomopsis galls arise from meristematic cambial tissue. This is also true of cynipid wasp stem galls.
Although there is no information on Phomopsis gall development, it appears the galls follow a predictive developmental arc. Active galls have a rough, fissured surface that’s colored various shades of brown. The inactive “spent” galls turn from brown to black, and the fissures deepen. It’s common for galls to fracture.
Juzwik et al. (2014) divided Phomopsis galls into two types. “Branch galls” simply grow from the surface and may partially surround the stem as shown in the images below.
The image below shows a cross-section of the gall. It’s apparent that some xylem sapwood extends into the gall; however, a high percentage continues to support the stem and leaves beyond the gall. Likewise, a percentage of the phloem continues to function as a conduit for dissolved carbohydrate from the leaves beyond the gall to the rest of the tree.
While the gall disrupts some vascular flow in both the phloem and the xylem, it is not a complete roadblock. Of course, one could visualize that several galls growing along the stem could collectively disrupt phloem flow if they are positioned in a radial configuration around the stem’s axis.
The authors called that second type of Phomopsis galls “stem-encircling galls,” with the galls surrounding the stem. These have a more serious impact by presumably incorporating all of the phloem into the gall structure, thus girdling the stem.
Phomopsis stem galls were investigated for a possible connection to so-called “Hickory Decline” in the Midwest and Northeastern Regions of the U.S. (Juzwik et al., 2008; Juzwik et al., 2014). While studies showed that heavy galling alone can occasionally produce canopy thinning, particularly if dominated by stem-encircling galls, hickory decline was most often associated with stress-inducing environmental issues, such as drought, coupled with other pathogens as well as insects.
I could find no research-based recommendations for treatments that suppress Phomopsis galls on hickory. Again, without knowing anything about gall development and the disease cycle, it would be difficult to develop effective management options beyond simply pruning out the galls. Of course, this is only practical on small trees.
References Cited
Brown, N. A. (1938). The tumor disease of Oak and Hickory trees. Phytopathology, 28(6), 401-411.
Burns, R. M., & Honkala, B. H.; [Technical coordinators] (1990). Silvics of North America: Volume 2. Hardwoods. United States Department of Agriculture (USDA), Forest Service, Agriculture Handbook 654
https://research.fs.usda.gov/treesearch/1548
Juzwik, J., Haugen, L., Park, J. H., & Moore, M. (2008). Fungi associated with stem cankers and coincidental scolytid beetles on declining hickory in the Upper Midwest. In In: Jacobs, Douglass F.; Michler, Charles H., eds. 2008. Proceedings, 16th Central Hardwood Forest Conference; 2008 April 8-9; West Lafayette, IN. Gen. Tech. Rep. NRS-P-24. Newtown Square, PA: US Department of Agriculture, Forest Service, Northern Research Station: 476-482. (Vol. 24).
https://research.fs.usda.gov/download/treesearch/14086.pdf
Juzwik, J., Park, J. H., & Haugen, L. (2014). Assessment and etiology of hickory (Carya spp.) decline in the midwest and northeastern regions (Project NC-EM-07-01). General Technical Report SRS-198, 198, 129-137.
https://www.srs.fs.usda.gov/pubs/gtr/gtr_srs198/gtr_srs198_129.pdf
Sinclair, W. A., & Lyon, H. H. (2005). Phomopsis Galls in Diseases of Trees and Shrubs (2nd ed.). Cornell University Press, 148-149.
