PART 1 OF A 3 PART SERIES Plant Galls: Myths and Misconceptions

An unusual plant growth can be disconcerting, especially if you’re not sure what it is—or what kind of damage it can cause. In Part I of a series, Ohio State Univer- sity entomologists and plant pathologists help to identify galls.

By Joe Boggs and Jim Chatfield

lant galls have a rich histo- of images; some accurate, some inaccu- the death of plant tissue. A brief review ry—and a dedicated branch rate. Various fungal fruiting structures are of tree anatomy will be helpful in under- of science—so it could be occasionally mistaken for galls, particu- standing the cankering process as well as assumed that plant galls larly the shelflike growths sprouting out stem gall formation that will be described are well understood. In fact, of trees that are produced by bracket fun- later. Tree stems are composed of a series despite plant galls being gi. These structures are the macroscopic of rings within rings, with the outermost one of the most obvious form of the microscopic wood decay fungi ring being the protective bark. Beneath and unusual of plant structures, there re- that are digesting the tree from within. the bark is a thin ring of phloem, and be- Pmains much that we do not know. Scien- Their activity may be dramatically dis- neath the phloem is an even thinner ring tists continue to debate exactly how galls closed when trees fall; galls do not cause of cambium; it’s only about three cell lay- are formed; so far, no scientist has ever such damage. The brackets are solely ers thick. grown a plant gall without help from a composed of fungal tissue with the pur- Beneath the cambium are multiple natural gall-maker. The identification of pose of producing fungal spores, thus the rings of xylem, which is the “wood” of the the gall-makers behind many of our most name “fruiting structures.” tree and the target of the aforementioned noteworthy plant galls remains a mystery. Fungal cankers on trees are some- wood decay fungi. Cambial cells are “un- This dearth of gall-knowledge does little times mistaken for galls, and vice versa. differentiated” (= meristematic) meaning to remedy many longstanding myths and The big difference is that cankers involve they can become something else. The misconceptions about plant galls. So in this first part of our explora- tion into galls, we’ll introduce you to the basics: namely, how to tell whether an unusual plant structure is a gall or some other oddity. Is it a gall? Occasionally, abnormal gall-like growths that are caused by something other than a living gall-maker can occur on trees. These are not true plant galls. The growths may be stimulated by exposure to chemi- cals such as herbicides or even misdirected plant hormones circulating within the tree. If no organism is found to be directly asso- ciated with the formation of the abnormal gall-like structure, we have eliminated the path to a “gall diagnosis” and opened other paths that may lead to a correct diagnosis, such as a physiological problem or expo- sure to an herbicide. Tree trunk basics The name “gall” may conjure a range All photos: Courtesy of Joe Boggs

10 | May 2015 | American Nurseryman Although this growth may appear to be a gall, it’s These shelflike structures are not galls, but dryad Although galls are sometimes incorrectly called actually a reaction to exposure to a herbicide. saddle bracket fungi (Polyporus squamosus). “plant tumors,” bacterial crown galls—such as this found on euonymous—resemble tumors because of their lack of organized structure. cells may become phloem to the outside broken bark aside. This two-step process Is it a canker? Is it a gall? Reading Shake- or xylem to the inside, which is how trees sometimes generates confusing descrip- speare doesn’t help. increase girth. However, if exposed to ox- tions of cankers: They are sunken areas of Galls involve the growth of plant tis- ygen, the cambial cells form an entirely the bark; they are raised areas of the bark. sue. However, one size does not fit all. different type of tissue called callus tissue. Of course, they may be both! There are many organisms that can in- This tissue overgrows wounds through the While cankers form in response to an duce gall formation including microor- bark. It is the tree’s method for closing a injury, the term “galled” has also been used ganisms such as viruses, bacteria, fungi, wound; trees do not heal, they seal. to mean “injured” for hundreds of years. nematodes and eriophyid mites; as well Fungal tree cankers develop in a two- When the Earl of Salisbury unsheathed as macroorganisms such as insects. Ex- step process. First, the cankering fungus his sword in Shakespeare’s play, King John actly how the galls grow depends on the infects and consumes the phloem, which (Act IV, Scene III), he bellowed, “Stand by, gall-maker. produces a void beneath the bark. The loss or I shall gall you, Faulconbridge.” Salis- of the supporting phloem tissue may cause bury wasn’t threatening to cause Faulcon- Bacterial crown gall the bark to become sunken and crack, ex- bridge to sprout galls; he was warning him Crown galls produced under the di- posing the surrounding cambial tissue that he may injure him with his sword! rection of the bacterium Agrobacterium to oxygen. The second step occurs when When Shakespeare used “canker,” he was tumefaciens represent a number of unique cambial cells produce callus tissue in re- often referring to an injury caused by a features compared to other gall-makers. sponse to exposure to oxygen. If the fungus cankerworm, a type of moth caterpillar. Plant galls are sometimes incorrectly re- does not immediately infect and destroy “The canker galls the infants of the spring ferred to as “plant tumors”—this is inac- the callus tissue (e.g., diffuse cankers), a too oft before their buttons be disclosed.” curate, because they are often highly or- raised boundary of liplike callus tissue de- (Hamlet, Act II, Scene III). Translation: ganized structures. However, crown galls velops around the void beneath the bark Cankerworms often injure the flower buds are much like tumors in that they lack an and may eventually expand to push the of spring before the buds open in bloom. organized structure and are composed of a chaotic agglomeration of cells. The bacterium transfers genetic material in the form of T-DNA into plant cells, which is then integrated into the cell’s chromo- somes. The new DNA induces a frenzied proliferation of infected plant cells with each cell becoming “bacterial factories” cranking out more bacterial bodies. The second unique feature is the way the bacterium spreads. Because each cell includes infectious material, a very tiny piece of the gall can induce new gall for- mation. Most other plant galls arise as the result of the movement of the gall-maker to new sites as with insect gall-makers, or by the movement of spores as with fungal galls, not by movement of a piece of the gall. Finally, crown galls may be found on an impressive number of hosts, in- cluding members of 93 plant families. Most gall-makers have a very narrow host range, often confined to a single plant Liplike callus tissue—the result of Botryosphaeria canker—is evident on redbud. Continued on page 12

www.amerinursery.com | May 2015 | 11 Elm sackgalls Translucent oak gall Hawthorn pod galls

Continued from page 11 Although black knot is primarily associ- simple structures, while others are com- species. Thankfully, while the bacterium is ated with cherry and plums, fungal galls plex. The galls can be separated into dif- capable of infecting a wide range of plants, have been recorded on 24 species of ferent groups, which helps with identifica- in reality galls tend to occur on only a few Prunus. Young galls are olive-green or red- tion. First, where is the gall located? Is it on landscape plant species, most notably on dish-green and have a velvety texture; old- the leaf or petiole? Is it growing from flower rose (Rosa spp.), euonymus (Euonymus er galls are coal-black and corky. The galls or fruit parts? Has the gall-maker hijacked spp.), forsythia (Forsythia spp.), and wis- may develop a white or pink discoloration a leaf bud, or did it commandeer meri- teria (Wisteria spp.). caused by the fungal parasite, Trichothe- stematic stem tissue? Second, how many cium roseum. chambers are found in the gall and how Fungal galls many immature gall-makers live in the Various fungal galls represent anoth- chambers? Unilocular galls have only one er type of gall-growth. The cedar-apple chamber; plurilocular galls have multiple rust fungus, Gymnosporangium juni- chambers. Unilarval galls only have one peri-virginianae, is so named because one gall-maker per chamber; multilarval galls of its hosts is eastern red cedar (Juniperus have more than one gall-maker per cham- virginiana), which is a type of juniper; the ber. Thankfully for identification purpos- fungus does not infect cedar. It also in- es, most arthropod gall-makers strongly fects apple (Malus spp.). In fact, the fun- adhere to these rules of segregation. Stay gus cannot complete its development on tuned: The next article in this series will either juniper or apple; it must alternate cover insect and mite galls in depth. between the two hosts to complete its life cycle. Galls are only produced on juniper; Leaf/petiole galls rusty-orange lesions are produced on the These galls are further separated leaves and fruit of apple. by where they occur on the leaf. Are the The brainlike galls produced on ju- galls only found on the petiole, on leaf niper are more organized compared to veins, or between the leaf veins? Or are crown gall and are composed of a combi- Figure 6. This cedar-apple rust fungal gall exhibits they found on two or three of these lo- nation of plant tissue, mainly parenchy- telltale bright orange, gelatinous, tentacle-like telial cations? Are they found on the lower leaf ma, and fungal hyphae. The fungal part- horns. surface, upper leaf surface, or both? This nership is dramatically revealed in the is not Gall-Trivial Pursuit: Many gall-mak- spring during wet weather when bright ing arthropods confine their activity to a orange, gelatinous, tentacle-like “telial Like the cedar-apple rust galls on ju- well-defined area of the leaf. horns” are extruded from the galls. The nipers, the knot-like galls are composed The unilocular, multilarval elm sack- horns are composed of fungal teliospores, of fungal tissue mingled with plant tis- galls that are produced by the aphid, Col- which give rise to basidiospores that are sue. The galls disrupt vascular flow, and opha ulmisacculi, only arise on the up- ejected into the air to drift onto apple heavy galling causes stem dieback. The per leaf surface between the leaf veins. where they germinate, marking the begin- fungus infects newly developing twigs in The galls split open to release the aphids. ning of the other half of the fungal life cy- the spring, and production of infectious The colorful, fleshy, unilocular, unilar- cle. Although the infectious spores can be spores occurs about two years after the val translucent oak galls produced by the blown a considerable distance, infections initial infection. Thus, pruning and de- gall wasp, Amphibolips nubilipennis, are on both hosts are certainly enhanced by in stroying newly developing galls is an im- firmly attached to veins on the underside plantings where the two hosts are in close portant step in preventing development of leaves. Among the most dramatic leaf- proximity to one another. of more fungal galls. vein galls are the so-called hawthorn pod Black knot of Prunus is caused by galls produced by the gall midge, Blae- the fungus, Apiosporina morbosa, and is Insect and mite galls sodiplosis (syn. Lobopteromyia) venae. characterized by thick, rough, elongat- Insect and mite (arthropod) galls The half-inch-long galls are at first light ed growths on the twigs and branches. exhibit a wide range of forms; some are green, but turn deep red as they mature.

12 | May 2015 | American Nurseryman Erineum galls on beech Maple spindle galls Acorn plum gall

They arise from veins on the underside of that must involve some chemical direc- innocuous. However, they can have a se- leaves, and their size and weight cause af- tion. Two examples are maple bladder rious impact on reproduction. One of the fected leaves to droop making the galling galls produced under the direction of the most spectacular galls in this group is the more noticeable. eriophyid mite, Vasates quadripedes, on acorn plum gall (a.k.a. acorn gall); these The felt-like “erineum galls” pro- the upper leaf surfaces of some red and sprout from acorn caps under the direc- duced by the eriophyid mite, Acalitus silver maples, and the finger-like spindle tion of the gall wasp, Amphibolips quer- fagerinea, creep across the upper leaf galls produced by V. aceriscrumena on the cusjuglans. The ball-like galls are around surface of American beech (Fagus gran- upper leaf surface of sugar maple. 1 inch in diameter, and their unique col- difolia). It is believed these simple galls oration of yellowish-brown shot through are produced by constant but subtle Flower/fruit galls with purplish-brown “veins” makes the feeding irritation, perhaps coupled with While the vast majority of arthropod galls look like blood-shot eyeballs; a dis- the release of chemical inducers by the gall-makers cause little to no injury to the concerting sight once the galls detach and gall-maker. However, some eriophyid overall health of their plant hosts, those drop to the ground in late summer! The mites induce truly unique plant growths that only affect flowers or fruit are truly Continued on page 14

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www.amerinursery.com | May 2015 | 13 Ash flower gall A yellowjacket is attracted to nectar Scales that envelop this gall resemble a pinecone— produced by a bud gall. thus the name willow pinecone gall.

Continued from page 13 clusters of wedge-shaped “kernels,” which sugary treat exuded from the nectar- deep reddish-purple color of the mature gives them a multilobed appearance, thus ies serves as a “bribe” to entice ants and galls is responsible for the “plum” in their their common name. The kernels of the stinging insects that offer protection to common name. oak lobed gall produced by the gall wasp, the immature gall-maker. A predator or The attention drawn to North Amer- Andricus quercusstrobilanus, range in col- parasitoid intent upon targeting the help- ican ash trees due to emerald ash borer or from light to dark-brown tinged with less wasp larva within the gall would need (Agrilus planipennis) has also focused a red. The galls vaguely resemble pine cones to run a gauntlet of stinging and biting in- spotlight on the handiwork of the erio- and are sometimes called “pine cone oak sects fueled by sugar! The downside is that phyid mite, Eriophyes fraxinivorus. The gall.” The entire gall structure may mea- heavily galled trees may literally buzz with mite targets male flowers in the spring, sure 3 to 4 inches long and involve several stinging insects presenting a serious chal- inducing the flowers to become distorted, buds. However, the origins of the kernels lenge if the tree is located near a home. brushlike, witches’-brooms. The affected can only be revealed by peeling them Most stem galls arise from the sur- flowers are green while the mites are in away to show they arose from oak buds. face and cause no harm because they do residence and become brownish-black One of the most dramatic looking bud not disrupt vascular flow within the stem. once the mites vacate the galls. Spent galls galls are willow pinecone galls formed by The exception is the horned oak gall pro- may cling to trees for several years. Insec- the gall midge, Rhabdophaga strobiloides, duced by the wasp (Callirhytis cornigera). ticide treatments for emerald ash borer on its namesake host. The common name Of the more than 800 types of galls that will have no effect on the eriophyid mites; is very descriptive with the inclusion on may be found on oak, this is one of the treated trees may still become festooned pinecone-like scales on the surface of few that can potentially cause significant with spent galls. these fuzzy, greenish-white galls. The galls damage to its oak host. That’s because the are formed from terminal buds of black galls may completely surround and girdle Bud galls willow (Salix nigra) presenting the bizarre the stem, and gall tissue may invade the Unlike the leaf/petiole gall-mak- display of “pinecones” growing from the xylem to choke off the flow of water and ers that commandeer a limited number tips of the willow branches. nutrients. As the result, the stem beyond of meristematic cells in the leaf buds, the gall often dies. Although horned oak gall-makers that produce bud galls hijack Stem galls galls do not typically kill trees, the stem all of the meristematic cells. In some cas- The ring of stem cambium located dieback can significantly disfigure tree es, the effect is obvious. The descriptive- between the xylem and phloem is mer- canopies and the stress associated with ly named oak bud gall produced by the istematic tissue; the cells are undiffer- loss of leaves can make heavily infested gall wasp, Neuroterus vesicula, is formed entiated. However, unlike bud cells, the trees more susceptible to other problems. when a single bud is directed to become cambium remains meristematic tissue Stay tuned … we’ll cover horned oak a reddish-brown, ball-like gall that is only throughout the growing season; undif- gall in the next installment. slightly larger than a normal bud. The ferentiated cells are continually available. Next up: Insect and mite galls, fol- small size should not be allowed to con- Since cambial cells remain free agents lowed by gall management. travene the complexity of this gall. A close throughout the growing season, galls can examination of the gall’s surface may re- be formed from these cells anytime during Joe Boggs is an assistant professor with the Ohio veal tiny droplets of nectar produced by the growing season, although most stem State University (OSU) Extension and OSU De- nectaries located within the gall; this is galls start growing early on to provide am- partment of Entomology. He works as a com- one of the galls with functional plant or- ple time for the gall-maker to complete its mercial horticulture educator for OSU Exten- gans. The nectar attracts ants and stinging development. sion, Hamilton County (Cincinnati). Boggs can insects, which provide protection for the Oak bullet galls produced by the gall be reached via e-mail at [email protected]. developing gall-maker. wasp, Disholcaspis quercusglobulus, is a In other cases, the gall-growth is so good example of a stem gall arising from Jim Chatfield is an associate professor and dramatic; the gall structure must be bro- cambial cells. They are also another exam- an extension specialist with OSU Extension in ken apart to reveal the source of the “par- ple of a gall that develops functional plant the Departments of Plant Pathology and Hor- ent” tissue. There are several types of “oak organs. Like the aforementioned oak ticulture and Crop Science. Chatfield can be lobed galls.” They are composed of tight bud galls, bullet galls have nectaries. The reached via e-mail at Chatfi[email protected].

14 | May 2015 | American Nurseryman