Dichotomous table for gallers on Centaurea

(incl. Amberoa, Cheirolophus, Leuzea, Mantisalca, Rhaponticum)

by Hans Roskam

1a On above-ground parts => 4

1b On roots and root collars => 2

2a On main- and lateral roots => 3

2b Root collar with an often plurilocular fleshy swelling, up to 15 mm long. One larva per chamber. C. scabiosa, jacea, etc.: Isocolus scabiosae

3a Main root with spindle- or pear-shaped enlargement on all sides, up to 30 mm long, Containing one larva. C. jacea, stoebe, nigra: ? Pseudocleonus grammicus

3b Root nodule galls. Centaurea spp.: Ceratapion onopordi

3c Rarely developing oblong root galls which may contain a reddish-brown caterpillar with dark brown head capsule. C. jacea, nemoralis, etc.: Epiblema scutulana

3d Adventitious roots with small nodule-shaped swellings. Centaurea spp.:
Meloidogyne hapla

4a On capitula and fruits => 28

4b On vegetative parts => 5

4c Infested plants stunted, showing reduced growth, very bushy, with the apical parts of the stems and capitula still green and fresh during the hot and dry season, less spiny than usual, and producing less seed. C. solstitialis:

a Aceria solcentaureae

b Aceria solstitialis

c Aceria squarrosae

5a Plant parts with localised malformations caused by fungi or animals and with extensive malformations caused by animal parasites => 8

5b Complete plants or major plant parts etiolated; internodes ± elongated and lanky; leaves ± narrowed and paler; large areas of the underside occupied by fungal fruiting bodies => 6

6a Sori yellowish or brown => 7

6b Completely diseased shoots ± etiolated; also curved on one side. Especially the leaf undersides with expanded whitish areas of branched conidiophores. C. montana: Bremia centaureae

7a Shoot usually completely diseased; normally remaining sterile. The pycnidia and primary rugose uredinia soon also partially produce darker telia bearing mycelium which pervades the whole plant whilst deforming it. Secondary uredinia and later telia in dot-like sori are widely dispersed over normally shaped plant parts. C. montana, nervosa, nigra, phrygia, triumfetti, uniflora, etc.: Puccinia montana

7b Corresponding malformations with similar mycelium, completely pervading the plant; uredinia and telia confluent C. cyanus: Puccinia cyani

8a On leaves => 17

8b In buds, shoots or stems => 9

9a On stems => 11

9b In buds or shoots => 10

10a Leaves of the shoot tip of different length ± tuft-like stunted; leaf blades narrow, ± rolled, curled and often strongly pubescent. C. jacea, scabiosa, vinyalsii: Unidentified gall mite

= Extensive malformation of shoots of C. montana are caused by the aphid Uroleucon montanivorum

10b Shoot tip or lateral buds stunted, densely white-felt-like pubescent, somewhat thickened at base. Leaf blades of further developing leaves with irregularly distributed covers on both sides. One or several yellow-red larvae. C. jacea, montana, triumfetti: Dasineura centaureae

11a Malformations predominantly of the stems => 12

11b Younger stems locally stunted and sometimes curled; leaves stacked, nest-like converged; leaf blades curled and locally dark green. Shoots sometimes deformed over larger parts. C. cyanus, jacea, montana, nigra, etc.: Philaenus spumarius

12a The gall causers live inside the stem => 13

12b Stems with bulging swellings over various lengths. Galls ± strongly bent, solitary or with several ridges framed by depressions, which contain the gall causer. C. scabiosa: Planchonia arabidis

13a Galls caused by insect larvae => 14

13b Stem, also neighbouring leaf stems, of young plants with pale green, spongy, sometimes curved swellings which may differ in their respective lengths. C. cyanus, Amberoa moschata, etc.: Ditylenchus dipsaci

14a Swellings inconspicuous or absent => 15

14b Stem with conspicuous swellings => 16

15a Outside of stem hardly changed. Galls only visible after dissection; inside of stem usually with several oval, tough-walled larval chambers, up to 5 mm long and 1 mm wide, each with one larva protruding into the pith. C. aspera, diffusa, jacea, micranthos, nigra, phrygia, stoebe, scabiosa: Phanacis centaureae

15b Galls in stems in a form of rounded or elliptical cells in the parenchyma; the stem sometimes slightly malformed. C. adpressa: Isocolus volgensis

15c Similar gall. C. adpressa, orientalis: Phanacis crassinervis

15d Similar gall. C. adpressa, orientalis: Phanacis varians

15e Upper part of the young stem especially stunted in longitudinal direction; basal part slightly spindle-shaped swollen over a length of 3–4 cm. A long, narrow larval chamber in the pith, contains a single caterpillar. C. stoebe, scabiosa: Unidentified ? lepidopteran

= The caterpillar of the leafroller Epiblema scutulana lives in the capitula of several Asteraceae but without causing galls. However, those of the next generation infest the stems and roots of C. jacea and is reported as a gall causer of stems for C. nemoralis

16a Stem in different extensions, sometimes over a length of 10–15 cm, irregularly crookedly swollen. The inside with many solitary or gregariously situated oval, tough-walled larval chambers each containing one larva. C. scabiosa incl. subsp. sadleriana, etc.: Isocolus scabiosae

16b Conspicuous more or less globular one-sided swelling, multi-chambered. C. aspera: Isocolus latreillei

16c Similar usually smooth swelling. 20–25 x 15–18 mm. C. aspera, melitensis, Mantisalca salmantica: Isocolus lichtensteini

16d One-chambered swelling on stem base. C. aspera: Isocolus fitchi

16e Stem strongly swollen and buckled some distance below the flower head. Containing several orange-coloured larvae. C. decipiens: Unidentified gall midge

17a Rotund to oval, only slightly cushion- or callus-like swellings, which are soon occupied by the yellow or brown sori of rust fungi => 25

17b Malformations caused by other gall causers => 18

18a Malformations predominantly of the leaf blades => 21

18b Galls on the midrib or on veins of main- or lesser order => 29

19a Galls not extending onto the leaf blades => 20

19b Veins of different order, more rarely also the midrib or petiole, regularly with solitary rotund-oval or spindle-shaped galls, but also in rows situated gregariously, in the leaf blade extending and visible on both sides. Wall fleshy to cartilaginous, ± pale green. Area around the galls often yellowish or purple coloured. Containing a single yellow larva. Centaurea spp.: Loewiola centaureae

20a Midrib or petiole with rotund, tough-walled, unilocular swelling, up to 8 mm long, visible on both sides. Containing a single larva. C. scabiosa: Isocolus fitchi

20b Midrib in the middle of the leaf blade with lemon-shaped, tough-walled swelling containing a large chamber; terminal part of the spindle recurved. Containing a single larva. C. stoebe: Unidentified gall wasp

21a Malformations by aphids or cercopids => 23

21b Galls small, usually many, wart- or pock-like => 22

22a Leaf blades, especially of basal leaves, with ± rotund-oval lenticular pocks, 1.5 to 2.5 mm long, visible on both sides, often bright green, also reddish to black violet with a fine opening on the upper side. Centaurea spp.: Aceria centaureae

22b Basal leaves on the underside with many semi-globular, golden yellow gleaming, pearl-like warts, hardly 1 mm long. C. jacea: Synchytrium aureum

= The chytrid Synchytrium solstitiale occurs with many orange red warts, up to 0.2 mm across, on the leaves of C. solstitialis. that often are more or less distorted. In a later resting stage the warts are grey

23a Leaf rolls caused by aphids => 24

23b Leaf blades, according to leaf size, completely or partially nest-like deflected; curled mainly close to the site of attack and dark green. Centaurea spp.: Philaenus spumarius

24a Leaves of several shoot tips often rolled over their length. Aphid bright yellowish to yellow-green. C. cyanus, paniculata: Brachycaudus helichrysi

24b Similar malformations caused by black aphids. C. calcitrapa, stoebe, etc.: Aphis fabae

25a Callosities or cushions with brown sori => 26

25b Swellings or hardly thickened areas on the leaf blades bearing aecia and usually also pycnidia. Galls facultatively; on the same host often developed to a different extent, ± reddish coloured. Aecia on underside of leaf, in rotund yellow- to crimson red bordered sori. Peridium broadly deflected, with irregularly incised border. C. nigra: Puccinia arenariicola

26a Minor swellings of leaf veins or petioles, ± rotund-oval, facultative gall causer; only in spring and ± solitary. Only produced by the primary mycelium bearing uredinia and partially also pycnidia. The far more frequent much darker secondary uredinia and telia, produced in little sori, are not cecidogenic. Fungus with various biological forms => 27

6b Leaf blade with rotund compact red brown cushions about 2–5 mm long, often many and usually protruding on the leaf underside, often yellow-bordered. Sori, when on the veins ± oblong-oval; exclusively with densely set, glabrous, 2-celled, dark brown telia. C. scabiosa, also jacea, kotschyana, maculosa, montana, phrygia, etc.: Puccinia verruca

27a Leaf blade on underside develop sori with aecia, often several on the, ± yellow or reddish margined; sometimes with oblong-oval shape on the major veins and only here facultatively inducing minor swellings. Centaurea spp.: Puccinia dioicae

27b Leaf blade on the underside with many small uredinia and telia. Fungus with two kinds of uredinia. The primary uredinia, accompanied by pycnidia, develop on the venation, sometimes on slight, bulge-like swellings. Centaurea spp.: Puccinia calcitrapae

27c Uredinia with two terminal germ pores, warts not developed over the spore surface On C. jacea, etc.: Puccinia hieracii

27d Close to this species is the fungus on Rhaponticum scariosum, occasionally on the veins on the upper side with primary large sori, up to 15 mm long, which are slightly swollen and curved, sometimes accompanied by pycnidia: Puccinia centaureae-rhapontici

27e Uredinia with three equatorially situated germ pores, also with loosely placed fine warts here, as well as on the remaining surface of the spore. Especially on C. scabiosa, but also on many other Centaurea species: Puccinia carthami

28a Galls in ovaries or fruits, capitula externally hardly or not markedly changed => 38

28b Malformations in other parts of the capitula => 29

29a Gall causers are found in the receptacle => 36

29b Malformations in flowers or in the scales of the involucre => 30

30a Substantial parts of the capitula malformed => 31

30b Scales of the involucre of the otherwise not markedly malformed capitula at the base with an oblong-oval, up to 7 mm long and 4.5 mm wide, single-chambered, tough-walled, glossy bulge. Containing a single larva. C. scabiosa: Isocolus rogenhoferi

30c On C. adpressa, solstitialis. Gall at the base of the flower head and/ or on the stem below it. In many cases no deformation of the flower head base and stem was observed and only single galls were found in each infected flower: Isocolus ponticus

30d Same hosts. A large number of galls are developing in one flower head, and these strongly deforming and enlarging the flower head and the stem below. The galls are merging together and forming a hard, lignified conglomerate at the base of the flower head: Isocolus tauricus

30e On C. ruthenica. Galls are formed at the base of flower heads. Usually 8‒10 galls form a conglomerate, up to 30‒35 mm in diameter and, thus the base of a galled flower head is usually enlarged, swollen. Mature galls are lignified, thick-walled, brown: Isocolus ruthenicae

30f On C. (Phaeopappus) trinervia. The usual gall wasp induces galls at the base of flower heads: Isocolus phaeopapucii

30g On Leuzea conifera. Galls inside flower heads. Isocolus leuzeae

30h On C. diffusa, pseudomaculosa, virgata subsp. squarrosa. The galls lie between the involucral bracts and eventually fall off. They are unilocular, 2 mm long, elliptic; the wall is thin, lignified and slightly rough: Isocolus centaureae

30i On C. orientalis. Galls in the capitula, between the achenes: Isocolus flavus

31a Capitula malformed, ± stunted and remaining closed => 33

31b Capitula enlarged, ± wide, gaping. Flowers not developed, malformed, succulent, thickened => 32

32a Ditto on C. bracteata, decipiens, micrantha, scabiosa subsp. sadleriana stoebe: Aceria grandis

32b Ditto on C. jacea: Unidentified gall mite

32c Ditto on C. aspera: Aceria brevisetosa

32d Capitula swollen, malformed, spines reduced. C. aspera: Aceria calathidis

32e Inconspicuous swelling of capitula. C. aspera: Larinus longirostris

33a Malformations caused by gall mites => 34

33b Development of capitula together with their organs remarkably stunted, containing several, usually orange-yellow larvae. C. scabiosa: Dasineura miki

34a Capitula severely stunted, only about 5 mm long and ± disc-shaped, abnormally haired together with the neighbouring parts of the shoot. C. scabiosa: Unidentified gall mite

34b Capitula stunted, remaining ± closed. Flowers ± greened and haired. C. decipiens: Unidenified gall mite

34c Similar, ± greened, but hardly haired, pea-sized malformations of capitula. C. phrygia: Aceria centaureae

35a Extensive parts of receptacle swollen, hardened, with several oblong-oval, perpendicular to the surface, rarely ± inclined larval chambers. Infected capitula often more globular than normal ones. One maggot per chamber with abruptly humped back: various tephritid flies

35b Ditto, with distinct head capsule => 36

36a Beetle larvae feed on flower head; these only malformed on one side, development stunted, ± distorted or skewed. C. jacea, scabiosa, etc.: Larinus planus

= Beetle larvae of Larinus minutus feed on the developing achenes, and pupate in the partly eaten receptacle. Not a true gall. C. diffusa, stoebe.

36b Similar malformation by caterpillars. C. nigra, scabiosa, etc.: Unidentified lepidopteran

= The larvae of the gelechiid moth Metzneria metzneriella are frequently found in the capitula of many Centaurea species, but do not cause galls

37a Fruits ± swollen or elongated, caused by larvae of cynipids which are narrowed at both ends => 38

37b Fruits inconspicuously swollen, slightly elongated. The gall includes occasionally, in the case of young flowers, the under part of the lightly greened corolla tube. Containing a plump, conical, terminally blunt white larva with black mouth parts. C. jacea: Urophora jaceana

38a Fruits oval, up to 8 mm long, swollen, tough-walled, woody; one larva. C. scabiosa: Isocolus scabiosae

38b Fruit weakly swollen, up to 4.5 mm long. Gall tough-walled, ± connate to the receptacle. Containing a single larva. Adult with indistinct carinae on anterior part of mesonotum (comp. next lead). C. jacea, montana, paniculata, stoebe, scabiosa, etc.: Isocolus jaceae

mod 20.xi.2019