Plant Parasites of Europe

leafminers, galls and fungi

gallers on genus Fraxinus

Dichotomous table for gallers on Fraxinus

by Hans Roskam

1a On buds, leaves, inflorescences or fruits => 6

1b On axial parts of shoot => 2

1c Root with rounded or spindle-shaped swelling, 2–5 mm across; containing eelworms, some may be glistening white pear-shaped females with eggs: Meloidogyne ardenensis

2a On thicker stems and branches => 4

2b On younger twigs => 3

3a Shoot axis with irregular rimmed depressions, containing the causer under a pear-shaped scale, about 2.5–3 mm long. F. excelsior: Chionaspis salicis

3b Younger shoots with cauliflower-like proliferations, about 5–20 mm across. F. americana, angustifolia, excelsior, ornus: Aceria fraxinivora

4a Malformations caused by inconspicuous parasites invisible from the outside or on younger stems or thicker branches => 5

4b Thicker axial parts with spindle-, club-shaped, or nodular swellings bearing: Viscum album

5a Proliferations develop, mainly on nodes, usually closed, rotund, at first glabrous, later on cracked and ruptured, up to about 30 mm long, later on discoloured brown to black. “Bacterial cancer”. F. excelsior: Pseudomonas savastanoi pv. fraxini

= In many areas similar proliferations due to Agrobacterium tumefaciens have been reported.

= So-called bark roses on ash, rather similar proliferations, should not be confused with previous malformations. They occur mainly on thicker axial parts and differ from basally usually distinctly narrowed bacterial cancer by being broadly attached to the host. These proliferations, often cracked, ± rosette-like are responses to wounds caused by feeding of the scolytid beetle Hylesinus fraxini.

5b Open cancer, mainly on branches, usually arranged according to the bearing organ, provided with conspicuously glabrous to warty rims surrounded to varied extent; perennial. “Nectria-cancer”. F. excelsior: cf. Neonectria ditissima

= Apparent “cancers” on ash are often caused by freezing and not by parasites

6a On buds or leaves => 8

6b On inflorescences or fruits => 7

7a Fruit irregularly slightly swollen; gall surface usually strongly undulate, becoming brown and cracked. Containing several to many lemon-yellow larvae. F. excelsior, ornus: Contarinia marchali

7b Inflorescences transformed into at first green, soon browned, tuberculate, cauliflower-like masses. Flower peduncles disfigured, ± cauliflower-like distorted. “Klunkern” in German. F. excelsior, ornus, etc. Aceria fraxinivora

8a On leaves => 9

8b Buds disfigured, scales of buds soon dropping. F. excelsior: Unidentified gall mite

9a Galls confined to single leaves or parts of leaves => 12

9b Malformations extend to several leaves => 10

10a Nest-like clustering of disfigured leaves on shoot tip, caused by aphids => 11

10b Some, usually laterally inserted leaves ± converging, nest-like. Leaf blades curled. Deeper green at site of infestation. Containing a froth-covered nymph. F. excelsior: Philaenus spumarius

11a Petioles, as well as -midrib, severely stunted and distorted, densely clustered. Aphid mainly on midrib and leaf underside, also in much lower numbers on shoot axis. F. angustifolia incl. subsp. oxycarpa; excelsior: Prociphilus fraxini

11b More loosely arranged clustered leaves; aphids mainly on shoot axis and petioles. F. angustifolia, excelsior, mandshurica, pennsylvanica: Prociphilus bumeliae

12a Galls on leaf midrib or -blade => 15

12b Roll of leaf margin => 13

13a Expanded, loose, greenish or pale, partially reddish discoloured, soft to brittle downward rolls => 14

13b Very narrow, cartilaginous downward roll of leaf margin; the rolls are abnormally haired inside. F. angustifolia, excelsior: Aculus fraxini

14a Loose, usually normal, rarely ± pale-green discoloured, soft rolls of leaf margin; containing several midge larvae. F. excelsior: Unidentified gall midge

14b Bladder-like swollen, loose ± thickened roll of leaf margin; brittle, largely discoloured, often with reddish or violet marbling; caused by white-woolly psyllids. F. angustifolia incl. subsp. oxycarpa; excelsior: Psyllopsis fraxini

14c In similar galls also lives Psyllopsis discrepans

14d Leaf roll gall with shallow depression, discoloured bulge above. F. angustifolia incl. subsp. oxycarpa, excelsior, ornus: Psyllopsis fraxinicola

15a Galls on leaf blades, without particular involvement of venation => 17

15b Galls on leaf midrib or leaf veins => 16

16a Leaflets along midrib folded upwards, pod-like, strongly thickened and hardened. Leaf blade not developed or only as a narrow rim. Often all leaflets infected on young leaves. Containing several white larvae with bluish-green gut. F. angustifolia incl. subsp. oxycarpa, americana, excelsior, ornus, pallisiae: Dasineura acrophila

= The yellowish larvae of Contarinia invocata occur as inquilines of D. acrophila as well as the whitish, never greenish translucent larvae of Macrolabis pavida. Reddish-yellow larvae of Clinodiplosis botularia also occur in these galls

16b From similar galls on F. ornus: Trigonodiplosis fraxini

16c On leaf underside bulging, sometimes reddish discoloured pouches on midrib of leaflets, also on midrib and sometimes on petioles, up to 30 (50) mm long. Later with distinctly protruding longitudinal slit on upperside, surrounded by a stronger pubescent rim; often many galls per leaf blade. Each containing a single orange-coloured larva. F. angustifolia incl. subsp. oxycarpa, excelsior, ornus: Dasineura fraxini

17a Variously shaped small grooves, protrusions or pubescence of leaf blade => 20

17b Warts, nodules or parenchyma galls, mostly of constant shape => 18

18a Wart-like or nodular galls => 19

18b Pustule-shaped flat parenchyma galls, hardly 1 mm high, especially protruding on underside with margin, at first red-violet or pale-green, later on brown, up to 8 mm wide. Containing a single yellowish-white larva with green translucent gut. F. angustifolia incl. subsp. oxycarpa, excelsior: Dasineura fraxinea

19a Golden-yellow translucent warts on underside of basal leaves of seedlings, 1 mm across. F. excelsior: Synchytrium aureum

19b Corniculate, or ± hemispherical nodules, up to 1.5 mm high, glabrous, yellowish or reddish, protruding more strongly on one side of leaf. The irregular glabrous tunnels inside the galls usually open on leaf underside. Galls occasionally on leaf midrib and -stalks. F angustifolia, excelsior, ornus: Aceria fraxinicola

20a Flat, irregular, 1–2 mm long, sometimes coalescing yellowish protrusions on upperside of leaf blade, often soon ± browned. F. angustifolia subsp. oxycarpa, excelsior, ornus, pennsylvanica: Aculus epiphyllus

20b Abnormal, at first reddish, then ± brownish pubescence on underside of leaflets, especially along midrib. Hairs multi-cellular, with acuminate or obtuse tip. F. excelsior, ornus, angustifolia subsp. oxycarpa: Aculus epiphyllus

Last modified 18.xi.2023