Dichotomous table for gallers on Malus

by Hans Roskam

1a On above-ground parts => 3

1b On roots => 2

2a Rotund, cancer-like proliferations on roots and root collar, several cm long, at first glabrous on surface then ± bumpy, soon lignifying, laterally inserted, often many and variously coalescing. Especially on young plants. Malus spp.: Agrobacterium tumefaciens

2b Roots with nodular, rotund or oval swellings, up to about 7 mm long ± cracked or bulging, eventually bursting; close to ground level, even on older roots groups often coalescing, stimulated new tissue developing into voluminous cancer-like growths. Occupied by white woolly aphids. Malus spp.: Eriosoma lanigerum

2c Nodular, usually smaller, glabrous swellings. Malus spp.: Meloidogyne hapla

= The red bud borer Resseliella oculiperda damages rootstocks of cultivated roses and apples when these are grafted with scion buds by “shield budding”.

3a On leaves, buds, flowers or fruits => 7

3b On stems, branches and twigs => 4

4a Galls on or in the bark => 5

4b Expanded spindle- or club-shaped swellings on branches or stems. Malus spp.: Viscum album

= Witches’ broom-like malformations at the end of shoots are partially caused by virus diseases. The early developing malformations are distinguished by their discoloured leaves, which are longer stalked and have larger stipules than the normal ones.

= Cancer-like structures usually develop as excessive callus proliferations in over-sensitive responses. Also many wound-calluses may develop into smaller, similar, but locally depressed nodules.

5a Younger plant parts with nodular, or older ones with cancer-like proliferations of irregular nature => 6

5b Up to 2 mm long, lenticular, bulging-rimmed depressions on bark of young shoots, in case of heavy infestation slightly swollen. Each depression containing a red insect, covered by a ± rotund scale, 1.3–1.6 mm wide: Epidiaspis leperii

5c In similar, but deeper depressions on the swollen bark: Chionaspis salicis

6a Nodular swellings of bark, up to walnut-size, later on erupting and cracking; often many coalescing and developing expanded cancer-like swellings. Especially on young trees. Containing white powdered aphids in cracks. Spring to autumn. Malus spp.: Eriosoma lanigerum

6b Buckled margined cancer wounds, increasing on older branches and of very varied shape. At first, on younger parts, because of the strong regenerative ability of host, usually transformed into variously delimited, bumpy, ± closed, nodular tumours; later on, the central tissue decays on older branches resulting in fissured cancer wounds, open, into the wood: Neonectria ditissima

= Caterpillars of clear-winged moth Synanthedon myopaeformis or Enarmonia formosana are occasional inhabitants of cancers, but do not cause them.

= Also the sac fungus Neofabraea perennans has occasionally been reported as inducer of Nectria-like cancers on apple and pear.

7a On buds, flowers or fruits => 29

7b On leaves => 8

8a Malformations associated with various curls, swellings, folds or loose rolls, which often involve the complete leaf blades sometimes of many terminal leaves => 15

8b Narrow, compact marginal rolls or leaf blades with locally defined galls => 9

9a Narrow roll of leaf margin => 13

9b Pock-like or slightly bladder- to bulge-shaped galls, with erinea => 10

10a Pocks, bulges, or minor bladder-like swellings => 11

10b Irregular, initially reddish white, then brownish erinea, mainly on leaf underside, usually along venation. Hairs long, cylindrical, blunt, spiral. Mainly on M. sylvestris, less so on M. pumila, etc.: Phyllocoptes malinus

11a Malformations caused by fungi => 12

11b Leaf blades with many ± oval pocks, usually 2–4 mm across, protruding from both sides. M. communis, sylvestris, etc.: Eriophyes mali

11c Vagrant mites on the underside of the leaves. At high densities rusting may occur. M. domestica, pumila, sylvestris: Calepitrimerus baileyi

11d Translucent pustules, visible on both sides, soon collapsing, in 5, 6, usually on the midrib, are procecidia of the black pear sawfly, occurring mainly on pear, occasionally on apple. Pristiphora abbreviata

12a Yellowish pads rotund on leaf blade, ± spindle-shaped on midrib, protruding on underside, later on with oblong-cylindrical, often curved aecia. Peridium thread-like rupturing onto base. Malus spp.: Gymnosporangium tremelloides

12b Rarer on apple are the cylindrical, tuft-like aecia of Gymnosporangium cornutum

12c Minor, usually several per leaf blade, covered with ± radiating, blackish mycelium, rotund, up to 7 mm long upward archings on young leaves of hawthorn shoots. Malus spp.: Venturia inaequalis

13a Rolls of leaf margin lacking conspicuous pubescence => 14

13b Expanded, border-like upward rolls with abnormal pubescence, occasionally encroaching well into the leaf blade. M. sylvestris, etc.: Phyllocoptes malinus

14a Similar, but not conspicuously pubescent roll of leaf border. M. pumila, sylvestris: Eriophyes malimarginem-torquens

14b Leaves of young shoots with cartilaginous thickening, often on both margins, usually discoloured yellowish or reddish, often running parallel to midrib, usually resulting in an upward roll. Inside the brittle tube are several, at first cream-white, then red midge larvae. Malus spp.: Dasineura mali

= Inquilinous cream-coloured midge larvae living in galls of Dasineura mali: Macrolabis mali

15a Malformations caused by aphids => 19

15b Malformations attributed to other causes => 16

16a Malformations caused by mites, psyllids or spittlebugs => 17

16b Several terminal leaves on shoots stunted, disfigured, their margins ± bent upwards, slightly thickened and often completely covered by a sometimes cotton wool-like, white mycelium. Malus spp.: Podosphaera leucotricha

17a Malformations caused by mites or spittlebugs => 18

17b Leaves variously curled and deflected; with many small spots, soon turning brown, caused by suction feeding of flattened, yellowish to greenish froth-covered nymphs, up to 1.5 mm long, exuding copious honeydew. Malus spp.: Cacopsylla mali

17c Very similar infestation: Cacopsylla picta

18a Leaf blade stunted and soon browned due to many suction spots; in cases of severe infestation both halves are strongly folded upwards and their margins may even come in contact. Infected shoots die off prematurely. Malus spp.: Aculus schlechtendali

= Predating gall midges: Arthrocnodax mali, wissmanni.

18b Stems of saplings shortened. Leaves ± clustered and converging, leaf blades crumpled, locally deep-green. Containing a froth-covered nymph. M. pumila: Philaenus spumarius

19a Leaves folded along their midrib or rolled, distinguished by yellowish to reddish discolouration => 23

19b Rolls of margin or folds without particular thickenings => 20

20a Variously disfigured leaves are loosely inserted often on only slightly stunted shoot ends => 22

20b Shoot tip conspicuously shortened => 21

21a Many leaves on the ± stunted, curved and etiolated shoot tip are strongly deflected and ± curled. Dark-green aphids, ± 2 mm long, with long, dark brown-black siphunculi feed on underside of leaves and move onto shoots. Malus spp.: Aphis pomi

21b Similar galls, only inhabited in spring. M. pumila: Allocotaphis quaestionis

22a Downward leaf roll and curling caused by black aphids. M. pumila: Aphis fabae

22b Midrib of leaves sometimes loosely contorted. Leaf blades deflected downwards, often strongly rolled, making loose clusters; aphid yellowish-green with segmental dark-green striping dorsally and with dark siphunculi that are pale only at the tips. Malus spp.: Rhopalosiphum oxyacanthae

23a Leaves often crookedly rolled downwards on only one side, usually up to midrib, coarsely undulate at tip; discoloured yellowish or mainly intensively red => 24

23b Young leaves at shoot tip strongly rolled, often on both sides, almost intestine-like or ± spirally bent, discoloured yellowish. Often many young shoots infected; in which case the plants look bushy. Malus spp.: Dysaphis plantaginea

24a Aphid galls are usually vacated by end of June; galled leaves already drop in July => 25

24b Galled leaves stay on tree; also inhabited during summer. Malus spp.: Dysaphis devecta

25a The roll galls occurring on cultivated apple and relatives of the next aphids do not differ morphologically; the closely related aphids are only microscopically distinct; main difference is their host alternation. Furcula (mid-thoracic fork) of the wingless aphids is always bifid. Summer hosts are Polygonaceae, Apiaceae or Plantaginaceae => 26

25b Furcula not bifid. Summer hosts are Valeriana species. Malus spp.: Dysaphis brancoi

26a Aphid alternating to Apiaceae or Plantago => 27

26b Exules living on roots of Rumex and Rheum: Dysaphis radicola

27a Aphids alternating to Apiaceae => 28

27b Aphids migrating to Plantago: Dysaphis plantaginea

28a Leaf-roll galls in spring. Exules develop on Anthriscus. Malus spp.: Dysaphis anthrisci

28b Leaf-roll galls in spring. Exules develop on Chaerophyllum. Malus spp.: Dysaphis chaerophylli

28c Rolling and reddening the edges of the leaves. M. domestica and M. orientalis: Dysaphis chaerophyllina

28d Aphids living without host alternation, rolling and blistering leaves. Malus spp.: Dysaphis affinis

28e Rolling and reddening the lateral edges of the leaves. M. domestica and M. orientalis: Dysaphis armeniaca

28f Rolling leaves in E-Eu (North Caucasus, Crimea, Armenia) in spring, but the galls are yellow rather than red. Malus spp: Dysaphis brachycyclica

28g Rolled lateral margins of leaves in spring. M. domestica: Dysaphis flava

28h Rolling and reddening leaves. M. domestica: Dysaphis physocaulis

= Haplorhynchites caeruleus is galling the shoots of Malus sylvestris

29a On flowers or fruits => 30

29b The young caterpillar inhabits buds from 7, 8 onwards, feeds on these and tunnels in shoot during spring. Occasionally the bark reacts at infestation site with further development of conspicuous, sometimes cancer-like swellings. M. pumila: Blastodacna atra

30a On flowers => 31

30b Mainly apical half of infected fruits disfigured, knobbly: Dysaphis plantaginea

30c Scale of young fruits sometimes with many flat, ± pock-like swellings, up to ± 2.5 mm across, usually lighter coloured than surrounding tissue. M. pumila: Eriophyes mali

= Thrips flavus may be causer of distorted apples; not a true gall causer.

31a The corolla of infected buds does not open, but is closed and hood-shaped, at first reddish but soon browning and drying. Containing a single larva. Malus spp.: Anthonomus pomorum

31b Flower peduncle shortened, conspicuously thickened; corolla parts narrowed, variously disfigured, discoloured yellowish to greenish-white. All leaves of the diseased young shoots stunted, often bent upwards, spoon-like and with mealy dusting on both sides. Malus spp.: Podosphaera leucotricha

= Various anomalies of flowers, partially associated with greening, might be attributed to virus diseases.

mod 18.xii.2019