Dichotomous table for gallers on Ulmus

by Hans Roskam

1a On plant parts above ground => 2

1b Young roots with many, 1–2 mm swellings, variously coalescing and may eventually cover the root as a thick layer. U. minor: Mimeuria ulmiphila

2a In vegetative plant parts => 3

2b Flower buds containing many whitish, rarely yellow, larvae. U. glabra, laevis, minor: Coniophora autumnalis

3a On leaves => 7

3b On axial parts (twigs, branches, stems) => 4

4a On thicker branches => 6

4b On thinner twigs => 5

5a Bark of young shoot axial parts with blunt conical swellings, up to about 3 mm long, at first succulent, later tough-walled. Containing a yolk-yellow larva. U. glabra, laevis, minor.: Janetiella lemeei

5b Thinner stems, particularly of young hosts, with shallow, rimmed bulges, containing an about 3 mm long pear-shaped scale. Ulmus spp.: Chionaspis salicis

6a Bark of stems or thicker branches with conspicuous nodular swellings, at first closed, which ± tear open later on and change into open, irregularly rimmed cancer wounds. Rare on Ulmus spp.: Neonectria ditissima

= Tumors caused by Agrobacterium tumefaciens have also been observed on elm

= Proliferations on elms, so called “bark roses” are caused by the damage of the bark beetle Pteleobius vittatus, which tunnels in the bark anticipating its resting period. The proliferation develops after the feeding of the beetle has been completed, hence it is regenerating tissue, not a true gall

6a Spindle- or barrel-shaped thickenings on branches. Exceptionally on Ulmus spp.: Viscum album

7a Localised or extensive malformations of the leaf blades => 9

7b Galls on leaf veins or petioles => 8

8a Midrib, often at the base of the leaf blade, with a broadly attached swelling up to 15 mm long and 10 (15) mm wide, protruding on the upperside, distorting the leaf; gall wall thick, felt haired, pale sometimes reddened. Opening on underside but dark green aphids leave through a ± star-shaped cleft hole in top of gall. U. glabra, x hollandica, laevis, minor: Kaltenbachiella pallida

8b Midrib, or first order side veins, swollen; mainly on underside, with blunt conical to cylindrical swellings, up to 3 mm long, succulent and single -chambered. Only slightly protruding on opposite side of leaf. Opens at apical part of gall. Also on petioles and young shoots. Containing a single yolk-yellow larva. U. glabra, laevis, minor: Janetiella lemeei

9a Various, small to expanded, localised malformations of unfolded leaves => 12

9b Margins of leaf blade rolled or curled over substantial areas. Aphid galls => 10

10a Aphids pale green to brownish, powdered with wax => 11

10b Aphids black, without wax. Leaves of young shoots curled and loosely deflected, hardly discoloured. Ulmus spp.: Aphis fabae

11a Leaf blade, often only one half, loosely rolled downwards over its length into a bladder-like swollen, slightly thickened, pale green to yellowish tube; often passing over the midrib, especially the terminal part. Surface undulately rugose; veins protruding at the outside, keel-shaped. U. glabra, minor, scabra, according to many authors absent from U americana, glabra, minor, pumila, thomasii: Eriosoma ulmi

11b Open, green to yellow curled-leaf galls containing non-waxy aphids. U. americana, glabra, minor: Eriosoma grossulariae

11c Similar leaf rolls on the lower leaves of young shoots. The developing brood migrates partially to the younger leaves of the same, or neighbouring, shoots, inducing extensive curls and downward rolls but no discolouration. Galled leaves on the ± stunted short shoots clustered later on, nest-like. U. glabra, minor, pumila: Eriosoma patchiae

11d Rolls by inward curling of both edges of leaves, which become blistered, yellow or pale green, or sometimes bright red on younger trees and often considerably distorted. U. minor: Eriosoma anncharlotteae

12a Malformations of various shape, flat or only a few mm high => 15

12b Galls bag- or pouch-shaped, hollow, always higher than 5 mm. Aphid galls => 13

13a Galls pouch- or bag-shaped, about 7–15 mm high => 14

13b Bladder-shaped, 30-80 mm long, irregular enlargements on upperside, usually mostly closed on underside usually developing from a complete leaf. Surface uneven to rugose, densely short-haired, pale green or reddish. Rupturing with irregular slits when mature, then drying out, persisting after leaf fall. Containing waxy aphids. U. glabra, minor incl. subsp. canescens: Eriosoma lanuginosum

13c Leaf blade blistered and raised upwards between veins, green, becoming pinkish brown; containing waxy aphids. U. glabra, minor: Eriosoma flavum

14a Galls laterally flattened, up to 10 mm high; often in the vein axils at the base of the leaf blade; usually slightly grooved, on top cockscomb-like dentate. Often conspicuously red, more rarely yellowish; solitary or with 2–3, more rarely with several broadly attached to the leaf blade. Opening at maturity with a round exit hole on a flattened side. U. laevis, exceptionally U. glabra, laevis, minor: Colopha compressa

14b Galls egg-, pear-, or club-shaped, 7–15 mm long, laterally not flattened, distinctly stalked, mainly on upperside, often ± obliquely protruding, on underside with haired, eventually closed entrance. Surface glabrous, pale green, ± tarnished red. Leaf blade close to the gall discoloured and thickened; often many on a ± distorted leaf. Opens at maturity in the lower part of the swelling, with a lateral crack on the upperside of the leaf. “Fig gall”. U. glabra, laevis, minor; rarely on U. americana, davidiana: Tetraneura ulmi

14c Galls rotund, short stalked, more globular, pea- to walnut-size, often strongly reddened, thin-walled and, in contrast to the former species, short-haired. U. glabra, minor, pumila: Tetraneura caerulescens

14d Galls spindle- or pouch-shaped, hairy, 15‒40 mm high, with pointed apex and usually green and rose-red when mature. U. glabra, minor incl. subsp. canescens, procera, pumila: Tetraneura nigriabdominalis

15a Malformations without conspicuous expanded abnormal pubescence => 16

15b Veins on the leaf underside, especially at the branching points, strongly white haired. Erinea flaky, occasionally restricted to the vein axils. Hairs longer than the normal ones. U. glabra, laevis, minor: Unidentified gall mite

16a Malformations arched, more than 3 mm long, bladder-, groove-, or pustule-like => 20

16b Galls pock-like, wart- or nodule-shaped, about 2 (3) mm => 17

17a Swellings protruding only on one side of leaf => 18

17b Galls nodular, rotund, protruding about equally on both sides, semiglobular on the upperside, more or less conical on the underside, up to 1 mm across, ± short-haired; at first yellowish, eventually brownish. Exit on underside, surrounded by a glabrous rim. Often many per leaf blade. U. glabra, x hollandica, laevis, minor: Aceria campestricola

18a Swellings pock- or wart-like, mainly protruding on underside => 19

18b Pimples, up to 2 mm, rotund, constricted at base, ± rough-haired and pale green pouch galls on the upper side of leaf. On the underside with a small, haired exit hole. U. glabra, x hollandica, laevis, minor: Aceria campestricola

18c Additionally in both previous galls: Aceria multistriata and/ or Anthocoptes galeatus

18d Froth-covered nymphs on underside of leaf produce much honeydew. Leaves wither and become disfigured. U. glabra, laevis, minor: Cacopsylla ulmi

19a Basal leaves on the underside with many, multicellular warts, less than 1 mm across, golden-yellow translucent, distinctly margined. U. minor: Synchytrium aureum

19b Higher situated leaves with many irregularly margined, 1.5‒2.5 (3) mm wide, flat, at first pale-green, eventually brown pocks, predominantly recognisable by their discolouration. Swelling only distinct on the underside. U. glabra, x hollandica, laevis, minor: Aceria ulmi

20a Galls caused by midge larvae, residing in groove-like dimples or in the tissue of flat, rotund malformations, up to about 5 mm wide => 21

20b Leaf blades with few to many, diffuse patches, 2–10 mm long and larger, rotund, sometimes coalescing, usually flat but may be swollen slightly bladder-like to distinctly bulge-like, fungus when mature mainly on underside with ± conspicuous tufts of asci. U. glabra, laevis, minor: Taphrina ulmi

21a Larvae inside the tissue => 22

21b Small grooves, swollen on top, less than 5 mm long, on the leaf underside; surrounded by a discoloured area; often several per leaf. Each groove containing a white larva. U. minor: Dasineura ulmicola

22a Rotund, 3–4 (5) mm wide pustules, situated in the leaf parenchyma, slightly bulging on the upperside, darker or yellowish-green to deep yellow coloured, not protruding on the underside. Containing a single white larva. U. glabra, laevis, minor incl. subsp. canescens: Physemocecis ulmi

22b pperside with very weak, at first yellowish, later on brownish bulges; on the underside a narrow exit hole which is surrounded by a ring of raised cells. Gall chamber is situated below the epidermis and containing a single larva. Ulmus spp.: Unidentified gall midge

mod 15.xii.2019