Plant Parasites of Europe

leafminers, galls and fungi

gallers on Alnus

Dichotomous table for gallers on Alnus

by Hans Roskam

1a On parts above ground => 2

1b Lateral roots initially with small, nodule-like, later on with rotund, hazelnut- to fist-size gall masses (rhizothamnia), composed of thick, coral-like branches, usually densely clustered. Alnus spp.: Frankia alni

1c The disease usually starts at the roots and root collar, and extends from there in a few years up into the crown; it manifests itself by large brown bleeding bark lesions, later in a reduction of the foliage and branch die-back. A. cordata, glutinosa, incana: Phytophthora alni

2a On shoots or leaves => 4

2b On flowers or inflorescences => 3

3a Cone scales broadly band-like proliferating, succulent, often temporarily ± reddened, eventually browned. Alnus spp.: Taphrina alni

3b Male catkins distorted, sometimes knobby or appearing thickened over their whole length. Containing a single larva. Alnus spp.: Unidentified lepidopteran

3c s occasional inhabitants of gall-like malformations the following species have been recorded: Epinotia immundana and/ or E. tenerana

4a On leaves => 11

4b On shoots => 5

5a Malformations witches’ broom-like or occupied by mistletoe => 10

5b Malformations different => 6

6a The inducers develop inside the malformations; or have their fruiting bodies on the surface => 7

6b The inducers are to be found in ± lenticular rimmed depressions, up to about 2 mm long, on the bark of young, already woody twigs. A. glutinosa, incana: Chionaspis salicis

= The “alder spittlebug” Aphrophora alni causes distinctive calloused rings on shoots

7a Localised malformations on young woody twigs with tunnels bored inside => 9

7b Malformations caused by fungi, which at least temporarily have their fruiting bodies developing on the surface => 8

8a Bark of older shoots with ± fissured, initially closed, later on centrally open, ridge-like bordered cancer wounds. “Nectria cancer”. A. incana: Neonectria major

= The polyphagous Agrobacterium tumefaciens is also reported from alder as inducer of closed cancers

8b Axial parts of young shoots conspicuously swollen usually over their whole length. Leaves often remarkably swollen, bladder-like and curled. A. glutinosa, glutinosa x incana, rubra (cult.): Taphrina tosquinetii

9a One year old shoots with swelling, up to 10 mm long and about 5 mm broad, downwardly gradually narrowed. The pith contains a yellowish or pale green caterpillar with brown-yellowish head and often a darker neck shield. Alnus spp.: Epinotia tetraquetrana

9b Elongated, weak, eventually usually dehiscing swelling in terminal part of twig. Contains a single larva. A. glutinosa, incana, viridis, etc.: Heliozela resplendella

= The larvae of the snout beetle Cryptorhynchus lapathi tunnel in older stems, as well as in many other woody hosts, which results in conspicuous deformations. However, these results of regeneration are not true galls.

10a Heavier stems with ± distinct spindle- or barrel-like swellings, from which sprout Viscum album

10b Loose to densely bushy, often conspicuous witches’ broom. Shoots elongated, rigidly erect, considerably thickened, especially at base, with many lenticels. Brooms 2–3 (–4) years overlaying, often many per tree. Leaves often slightly thickened, paler, more curled than healthy ones. A. glutinosa x incana, incana: Taphrina epiphylla

10c A fungus with similar symptoms has been described as Taphrina tosquinetii

11a Malformations only on petioles or main veins, not or only secondarily extending to the leaf blade, without accessory pubescence => 27

11b Malformations on leaf blades close to the major veins or including them => 12

12a Galls mostly on expanded, fully developed leaves => 15

12b Malformations on young leaves or leaf blades over the midrib ± folded upwards => 13

13a Leaf blade, according to the extent of development at infection time, ± completely upwardly folded. Midrib and basal parts of the side veins ± spongy, thickened. Gall midge larvae present between the folds => 14

13b Young leaves curled together. Veins undulate. A. glutinosa: Aceria longirostris

14a Larvae gregarious, initially white, later red. A. glutinosa, incana, glutinosa x incana; also on cultivated alders: Dasineura tortilis

14b In similar galls live gregarious yellow larvae with terminal red parts: Jaapiella clethrophila

= Furthermore reddish-yellow larvae of the gall midge Macrolabis alnicola, also an inquiline of D. tortilis

15a Malformations with conspicuous abnormal pubescence which is visible from the outside => 22

15b Pubescence of gall surface normal => 16

16a Leaf blades with ± expanded bladder-shaped swellings => 18

16b Galls globular, 1–2 mm across, constricted at base, often many on the upper side of the leaves, yellowish-green or sometimes reddened. Opening on underside, surrounded by a glabrous rim; only the inside pubescent => 17

17a On A. glutinosa: Eriophyes laevis

17b On A. incana: Eriophyes alniincanae

18a Galls caused by fungi, whose fruiting bodies develop at maturity as loose, ash-grey coverings of upwardly bent leaf undersides => 19

18b Leaf blade with flat, ± extended, not acutely delimited, initially pale, later on brownish swellings with many mites on the underside. Alnus spp.: Tegonotus heptacanthus

18c Similar malformations of mites which are known as inquilines in other mite galls on alder have occasionally been reported: Tegonotus trouessarti and/ or Acaricalus trinotus

19a Swellings only flat, ± pale green, slightly extended; usually with several present on the slightly enlarged leaf blades => 20

19b Leaf blade considerably enlarged, completely or the major part heavily bladder-like swollen, pale green. Alnus spp.: Taphrina tosquinetii

20a On A. incana, viridis => 21

20b On A. glutinosa, glutinosa x incana, incana and cultivated species: Taphrina sadebeckii

21a On A. viridis: Taphrina viridis

21b On A. incana: Taphrina epiphylla

22a Erinea not noticeably encroaching on the venation, or mostly restricted to the vein axils => 23

22b The often expanded areas with abnormal pubescence predominantly along the midrib and lateral veins, secondarily also on the leaf blade and petiole. Pubescence dense, hair longer than normal. Erinea initially white, brown later on. A. glutinosa, incana: Unidentified gall mite

23a Erinea distributed over the leaf blade without particular pattern => 24

23b Galls mostly in the vein axils along the midrib, often on both leaf halves; over a length of 2–3 mm, protruding upwards, initially yellowish, brownish later. In the cavity on the underside initially white, later on ± browned, hairs. On the central part of the erineum slim club-shaped hairs, with acuminate rigid hairs on the margin. Alnus spp.: Eriophyes inangulis

24a Hairs in erinea club-shaped or cylindrical, only slightly different from the normal ones => 25

24b Hairs on top distinctly broadened, irregular bumpy or lobed, head- or toadstool-shaped. Erinea on the underside, more rarely also on the upper side, forming initially yellowish-white, later on rust-brown, oval spots or more extensive coverings. Alnus spp.: Acalitus brevitarsus

25a Hairs cylindrical or slightly club-shaped, irregularly bent and intricate => 26

25b Hairs slightly different from normal ones. Erinea on the underside in weak archings of the leaf blade. A. glutinosa, incana: Unidentified gall mite

26a On A. glutinosa, incana, glutinosa x incana. Erinea on underside, more rarely upper side, mostly rotund-ovate, initially white, reddened later on: Acalitus phyllereus and/ or Aceria bistriata

26b On A. viridis. Erinea usually on upper side and peach red, later like the previous species: Eriophyes euryporus and/ or Aceria bistriata

= The eriophyoid mite Aceria alniviridis has been recorded as inquiline in galls of Eriophyes euryporus

27a The malformations result from oviposition and are already vacated by the departed larvae. “Procecidia”: oviposition scars, not true galls => 28

27b Petioles ± distinctly swollen and sometimes appearing shortened, usually bursting open later. Caterpillar often only occurring in the ± swollen petioles, eventually appearing with a short, full depth corridor into the leaf blade, cutting an oblong-ovate case, and dropping to the ground in that housing. Alnus spp.: Heliozela resplendella

28a Procecidia (oviposition scars) on petiole, sometimes extending to midrib => 30

28b Procecidia on the veins only => 29

29a Midrib with up to 8 punctures made on the leaf upper side; procecidia arranged longitudinally, contacting one another. Batches bursting downwards, recognizable for a longer time. Alnus spp.: Eriocampa ovata and/ or E. umbratica

29b Midrib on the leaf underside usually with up to 15 procecidia, inconspicuous and hardly permanent, which are arranged in one row. Alnus spp.: Nematus septentrionalis

30a Egg capsules usually on both-sides of petiole or in the midrib; now left, now right, alternating or opposite. Alnus spp.: Hemichroa crocea

30b Egg batches on petiole solitary. Alnus spp.: Hemichroa australis

Last modified 26.xii.2020