Dichotomous table for gallers on Acer

by Hans Roskam

1a On above-ground parts => 2

1b Roots with rotund, single-chambered brownish galls, up to 9 mm across, often gregarious and similarly ± flattened later on and partially coalesced. Wall initially succulent, lignified later on. Each contains a single larva. A. pseudoplatanus: Pediaspis aceris, asexual generation

1c Galls smaller, massive. A. negundo: Meloidogyne sp.

2a Malformations of flowering or fruiting, inflorescences and their parts => 48

2b Malformations of buds, axial parts or leaves => 3

3a On leaves => 13

3b On buds or axial parts => 4

4a Galls on shoots => 6

4b Malformations of buds => 5

5a Buds swollen. A. pseudoplatanus, ? saccharinum: Aceria vermicularis

5b Buds atrophied, widely opened. Inside a crumbly mass. A. pseudoplatanus: Inducer unknown

6a Witches’ broom-like malformations or galls caused by mistletoes => 12

6b More or less distinctly protruding swellings or proliferations on axial parts => 7

7a Malformations mainly on the younger twigs => 8

7b Bark with ± expanded open wounds, with irregularly bulging callus proliferations at their margins. “Nectria-cancer”. Acer spp.: Neonectria sp.

= On various occasions so-called “burrs” have been recorded on the bark of older maple stems. These structures, usually lacking pith, are not true galls, but develop as a response to biotic or abiotic factors.

7c Similar gall-like proliferations are attributed to either Agrobacterium tumefaciens and/ or Phomopsis sp.

8a Inducers inside the galls => 9

8b Bark of younger as well as older side branches with flat, irregularly pock-like swellings. Causer in shallow, rimmed depression. Heavily infected parts additionally thickened and ± stunted. Acer spp.: Chionaspis salicis

9a Galls on bark => 10

9b First- or second-year twigs swollen on all sides below a node. The pith contains a caterpillar; grey-yellow to brownish with dark- to black-brown head. A. campestre, platanoides: Gypsonoma aceriana

10a Galls irregular, up to 2 mm high and 3.5 mm across, tubular-wart-shaped, often on base of one-year twigs. Solitary or gregarious, variously coalesced in the latter case => 11

10b Globular, up to 8 mm across, single-chambered galls on the bark of young twigs. A. pseudoplatanus: Pediaspis aceris, sexual generation

11a On A. platanoides: Aceria heteronyx

11b On A. campestre: Aceria aceriscampestris

= taxonomy of A. heteronyx, aceriscampestris and A. myriadeum is confused. I restrict A. hereronyx and A. aceriscampestris for bark galls on Acer platanoides and campestre, respectively, and A. myriadeum for leaf galls on Acer campestre.

12a Older branches with sometimes giant spindle-, club-, or more barrel-shaped swellings, often arched, bearing the inducer Viscum album

12b Witches’ broom-like malformations. Leaves with a tuft-like cover of dirty white asci during the fruiting of the fungus. A. platanoides: Taphrina acerina

12c Similar witches’ broom on A. campestre: Taphrina acericola

= Other witches’ brooms found on A. platanoides, negundo, pseudoplatanus, saccharinum have unknown causes

13a Malformations of the leaf blade => 15

13b Galls on petiole => 14

14a Galls rotund or thick-spindle-shaped, up to 5 (6) mm thick, one-chambered, thin-walled, yellowish-green, sometimes ± reddened. Containing a single larva. A. pseudoplatanus: Pediaspis aceris sexual generation

14b Slender spindle-shaped, one-sided thickenings, often on upper part, also in the middle of the petiole, rarely on the base of the midrib; 5–10 mm long, up to 2.5 mm thick; usually dark purple to violet coloured. Containing a single white larva. A. campestre: Atrichosema aceris

15a Galls open, sometimes with only a narrow entrance => 20

15b Galls closed => 16

16a Galls flat, protruding about equally on both sides of leaf => 17

16b Galls globular, only on underside, visible on the upper side as a rotund to oblong-oval disc, 6–8 (10) mm across, single-chambered, thin-walled; usually glabrous, yellowish or ± reddened. Galls inhabited by inquilines or parasitoids are often significantly larger, irregular or with blunt tubercles. Containing a single larva. A. monspessulanum, opalus incl. subsp. obtusatum, platanoides, pseudoplatanus: Pediaspis aceris sexual generation

17a Galls larger than 4 mm, rotund in the leaf blade, but acuminate oval on the main lateral veins; caused by gall midge larvae => 19

17b Galls only 1–3 mm across, mite- or midge galls => 18

18a Galls irregular rotund, up to 3 mm across; almost woody, thick-walled; at the upper side protruding more than on the underside; one-, more rarely multi-chambered, opening on underside of leaf. A. tataricum, also A. campestre:
Acericecis szepligetii

18b Leaf blade with pustule-like swellings. A. monspessulanum:
Unidentified gall mite

19a About 4–7 mm wide, round pustule gall. On the underside with an arched, translucent, tender wall; in the middle green, at the margin a little lighter. Often many per leaf blade, sometimes ± coalesced, only exceptionally reaching the main venation. Each pustule contains a single larva. A. opalus, pseudoplatanus:
Acericecis vitrina

19b Galls 4–6 (8) mm wide, browned later on, with small elevation in the middle on the underside. The central larval chamber often situated in a locally swollen side vein. Contains a single larva. A. campestre: Dasineura tympani

19c Similar galls, vacated already mid-July. A. pseudoplatanus: Unidentified gall midge

20a Leaf blade with pouch-like, globular or horn-like galls or with abnormal pubescence. Mite galls => 30

20b Leaf blade otherwise disfigured => 21

21a Leaf blade with slight bulge- to bladder-shaped swellings, with folds, curls or rolled inwards => 23

21b Leaf blade on the underside with small grooves => 22

22a Depression on underside about 1 mm deep, surrounded by a rotund, 5–7 mm wide, lighter area. The depression contains, sometimes enveloped by a droplet, glossy, white, also pale yellow larvae. A. monspessulanum, opalus, platanoides, pseudoplatanus: Drisina glutinosa

22b On Acer campestre. Raised pimple 1–2 mm high on upper side corresponding with depression below; translucent white larva in depression: Acericecis campestre

22c Very flat small grooves on the underside; on the upper side even shallower, initially green, eventually yellowish or brownish bulges, which lack a broad pale surrounding area. A. pseudoplatanus: Unidentified psyllid

23a Leaf blade with bulge- to bladder-like swellings, curls or loose rolls, caused by aphids, cercopids or fungi => 25

23b Folds, curls or leaf rolls caused by gall midge larvae => 24

24a Leaf blade often with blood red weakly thickened folds, which is glabrous on the outside and is situated between two only slightly changed veins. Opening on the underside. Cavity covered with white hairs. Sometimes combined with, or exclusively, a downward roll of the leaf margin. Containing several white larvae. A. campestre, monspessulanum, opalus, pseudoplatanus, saccharinum, etc.: Contarinia acerplicans

24b Leaf blade irregularly folded upwards and ± undulatingly curled; marginally deflected or rolled, often ± reddened. Between the folds on the ± curled, usually ± thickened veins are several white to reddish larvae. Acer spp., also ornamental forms: Dasineura irregularis

24c On A. campestre: Dasineura rubella

= The gall midge Contarinia sp. causing leaf margins bent downward on Acer monspessulanum has been recorded from Greece

25a Malformations caused by aphids or cercopids => 28

25b Leaf blade with weak bladder-shaped swellings, caused by fungi, which develop a tuft-like bloom of asci => 26

26a On A. campestre, pseudoplatanus => 27

26b On A. tataricum, orientale. Leaf blade with several, rotund, yellow to red brown, sometimes a little bladder-like swollen patches up to 1 cm wide. At maturity on the upper side with a bloom of asci: Taphrina polyspora

27a On A. campestre. Leaf blade with several usually distinctly bordered, about 2–3 mm wide, exceptionally slightly arched, sometimes coalescing discoloured patches: Taphrina acericola

27b On A. pseudoplatanus. Sori in several 0.5–3 cm wide, ± bulging discoloured patches on leaf blade: Taphrina pseudoplatani

27c Regarding similar leaf malformations on A. platanoides: Taphrina acerina

= Up to 10 (20) mm long black, inside white, tough patches on A. campestre, platanoides and other Acer species. These are not true galls but consist of overwintering mycelium of Rhytisma acerinum. On several Acer species on living leaves in only small sori in ± yellowish patches, etc. also Rhytisma acerinum

28a Galls caused by aphids => 29

28b Leaf blade distinctly reflexed; curled close to the froth-covered nymph, dark green. A. platanoides: Philaenus spumarius

29a Leaf blade loosely rolled downwards: Drepanosiphum platanoidis

29b Leaves of terminal shoots variously stunted, curled. Acer spp.: Periphyllus testudinaceus

29c Similar malformations are also caused by the next species, only distinguishable by microscopic characters:

= on A. pseudoplatanus: Periphyllus acericola

=on A. campestre, platanoides: Periphyllus aceris

30a Leaf blade with abnormal pubescence, free on the leaf blades or in depressions => 35

= To what extent the next galls, caused by unidentified gall mites, might be induced by several species, needs further study.

30b Galls pouch-shaped, globular or horn-like => 31

31a Galls scattered over the leaf blade => 32

31b In the vein axils on the upper side, more rarely anywhere on the leaf blade 2–4 mm, irregular rotund, glabrous to thick pubescent, ± yellow-brown or reddish, solitary or situated in pairs, often pouch-shaped galls. Opening on underside, like the cavity, clothed with multicellular hairs. A. campestre, less frequent on platanoides, pseudoplatanus: Aceria macrochela

32a Small pustules, usually less than 2 mm and often only up to 0.5 (1) mm wide, mainly on upper side, usually very many per leaf blade and coalescing into dense reddish groups. Opening on the underside, like the cavity, clothed with cylindrical, one- or multicellular hairs => 33

32b Galls horn-like or sometimes pustule-shaped, about 2–3 (4) mm long, yellowish or mostly reddish, mainly on the upper side, several to many. Opening mostly on underside, like the cavity, clothed with 1-celled, cylindrical hairs. On A. pseudoplatanus, more rare on A. campestre, platanoides, opalus, saccharinum: Aceria macrorhyncha

32c Gall round or elongate with a short, slender neck, 1.5–5 mm high, on veins or leaf blade; surface wrinkled, glossy, yellowish-green at first, becoming dark red to black; opening with single-celled hairs; galls may be very many. A. saccharinum more rare on A. monspessulanum. opalus subsp. obtusatum, tataricum: Vasates quadripedes

33a Hairs within galls 1-celled => 34

33b Hairs in the cavities multicellular, galls often horn-like. A. opalus: cf. Aceria opulifolii

34a On A. pseudoplatanus: Aceria cephalonea

34b Similar galls on A. campestre, ? tataricum: Aceria myriadeum

= taxonomy of A. heteronyx, aceriscampestris and A. myriadeum is confused. I restrict A. heteronyx and A. aceriscampestris for bark galls on Acer platanoides and campestre, respectively, and A. myriadeum for leaf galls on Acer campestre.

34c Similar galls developing on A. monspessulanum: Aceria monspessulani

35a Hairs of the erinea hardly changed or cylindrical, to slender club-shaped, or irregularly branched. Erinea exposed or in oblong depressions => 39

35b Hairs ± distinctly stalked, cup-shaped, toadstool-like or terminally globular, erinea freely extending over the leaf blade => 36

36a Erinea on A. monspessulanum => 38

36b Erinea on other Acer species => 37

37a Erinea on various sites or may cover all of the underside, at first white, then red and finally brown felt mass. Leaf blade may be ± bulged on opposite side. Hairs relatively short, toadstool-, cup- or funnel-shaped. ~, especially on A. campestre, obtusatum, pseudoplatanus, rubrum: Aceria macrocheluserinea

37b The mites free-living on the underside of large-leafed maple species have been taxonomically separated:

a On A. pseudoplatanus: Aceria pseudoplatani

b On A. platanoides: Aceria platanoidea

= Erinea in leaf axils on underside sometimes occur and may be caused by these mites. Other mites may also cause erinea on A. pseudoplatanus and A. campestre. Acer mites need further study

37c Erinea on underside; predominantly in the vein axils; extending from these or also on the upper side of the veins; at first white, then red, finally brown. Arching absent or only weak. Hairs toadstool-, cup- or funnel-shaped. A. pseudoplatanus: Unidentified gall mite

38a Erinea thin and forming flat expansions, often on leaf base or on the venation, rarer on other areas of the leaf blade. Hairs short and finely stalked, toadstool-shaped. The infected areas brown prematurely and dry: Unidentified gall mite

38b Erinea on upper side, rarely on underside, yellowish-white, then reddish or rust brown. Hairs long stalked, toadstool-shaped or also simply elongated: Unidentified gall mite

39a Hairs cylindrical, elongated club-shaped or irregularly branched => 40

39b Erinea with only slightly changed, hardly thickened hairs. Leaf blade not disfigured. A. campestre, platanoides, pseudoplatanus: Cecidophyes gymnaspis

40a Erinea on the underside in vein axils or along the veins => 43

40b Felt masses at various places on the leaf blade, especially in oblong depressions or on arched bulges => 41

41a Erinea in rather flat or strongly curved archings => 42

41b Leaf blade with somewhat oblong depressions, up to 15 mm long and 4 mm wide on underside. Exit on upper side, slit-shaped. With cylindrical, repeatedly curved hairs inside, at first white, later on brown. A. campestre: Aceria carinifex

42a Erinea predominantly on the underside, at first white, later on brown, on various only slightly changed or also upwardly arched, ± oval, also coalescing areas. Hairs relatively long, cylindrical or weakly club-shaped, the apical part sometimes curved or also crooked. A. pseudoplatanus, platanoides: Aceria pseudoplatani

42b Leaf blades of A. illyricum, monspessulanum on the underside with, at first white, later on brown-red felt masses in weak, similar upward archings. Hairs cylindrical: Aceria monspessulani

43a On A. campestre, monspessulanum => 45

43b On A. platanoides, pseudoplatanus => 44

44a On A. plataniodes. Felt masses in distinctly upwardly arched vein axils. Hairs club-shaped: Unidentified gall mite

44b On A. pseudoplatanus. Small tufts of brownish, irregular branched hair on the underside in vein axils or in stripes up to 2 mm wide along the main veins; galls not visible on upper side: Unidentified gall mite

45a On A. campestre => 46

45b On A. monspessulanum. Reddish felt masses on the underside of the leaf blade base along the veins as well as in the vein axils. Hairs slender and club-shaped, variously curved: Unidentified gall mite

46a Small tufts of stalked or terminally weak club-like or nodular thickened hairs in the vein axils; infestation extending up to the midrib and the petiole => 47

46b Erinea only in the vein axils; infestations weakly arched on the upper side. Hairs irregular branched and bent: Unidentified gall mite

46c With reservation, also the malformation reported as “yellow tufts” consisting of longer, widened, densely felt-like hairs, reported as “Erineum abnorme” is caused by Aculops acericola

47a Small rotund elevations in the vein axils, 1–2 mm wide, flatly bulging on the upper side, usually yellowish discoloured. Hairs cylindrical, terminally weakly club- or nodule-like thickened, emerging from leaf blade cells: Unidentified gall mite

47b Small, rotund crumbly elevations of ± stalked slender hairs in vein axils which are not bulging. The infestation expands usually in narrow stripes on the midrib as well as in the petiole: Unidentified gall mite

48a Inflorescences distinctly shortened, compact; flowers vestigial or disfigured. A. pseudoplatanus: Periphyllus testudinaceus

48b Solitary or many flowers, and young fruit disfigured to form a one-chambered gall, rotund, up to 8 mm, initially succulent, later on thin- and tough-walled, yellowish or sometimes reddish, containing a single larva. A. pseudoplatanus, platanoides: Pediaspis aceris sexual generation

48a Disfigured fruits. Acer campestre: Acumyia acericola

48b Swollen flower buds contain up to eight small, pink-coloured gall midge larvae among swollen stamens. A. campestre: Dasineura sp.

mod 1.iv.2020