gallers on genus Arabidopsis

1a On parts above ground => 2

1b Main- and side roots with conspicuous, spindle- to barrel- shaped succulent swellings. Inside with many cell masses with minute spores. A. arenosa, lyrata subsp. petraea, suecica, thaliana: Plasmodiophora brassicae

2a Malformations caused by fungi, with fructifications developing at surface => 6

2b Malformations caused by animals => 3

3a Malformations of flowers or siliques => 8

3b Spindle-shaped or expanded swellings on stems or petioles => 4

3c Large parts or complete plant disfigured. Shoot severely stunted; flower parts severely disfigured, often greened or reddened. Aphid more frequent on Arabis: A. lyrata subsp. petraea, thaliana: Lipaphis rossi

3d Leaf blade stunted, rolled upwards, curved; covered with abnormal pubescence. A. arenosa: Aceria drabae

4a Swellings ± spindle-shaped and glabrous => 5

4b Shoot severely stunted, like the basal parts of lower leaves variously swollen, spongy, pale green. A. thaliana: Ditylenchus dipsaci

5a tem with about 5–7 mm long, 2 mm thick, spindle-shaped swelling. The pith contains a single larva. A. arenosa, suecica, thaliana: Ceutorhynchus atomus and/or C. griseus

5b Rotund, fleshy swelling of the root collar, up to 10 mm; the single gall chamber containing a single legless, curved larva. A. arenosa: Ceutorhynchus assimilis

5c Spindle-shaped gall, 10‒20 mm long, in the leaf stalk (less often in the stem or a thick vein). A. arenosa: Ceutorhynchus chalybaeus

5d Shoot axis or petiole with spindle-shaped, often strongly arched thickening. Side branches of inflorescence ± shortened. A. thaliana: Inducer unidentified – ? lepidopteran

6a Malformations caused by fungi with white fruiting bodies => 7

6b Diseased shoots ± etiolated; internodes ± elongated and narrowed. Leaves often smaller, pale green; leaf blades slightly thickened, on underside over large areas with dense, ± rust brown sori. A. arenosa, halleri, petraea, thaliana: Puccinia thlaspeos

7a Sori densely clustered, bulging, at first closed, glossy, porcelain-like, then rupturing and with mealy dusting; oblong-oval on stems and inflorescence stalks, often arching the stems. On leaf underside rotund, often without malformations. Conidia developing in chains. A. arenosa, halleri, suecica, thaliana: Albugo candida

7b Sori expanded, with loose cover of branched conidiophores. Infected stems pale green, slightly thickened and arched, especially in flowering part; flowers severely stunted. A. thaliana: Hyaloperonospora arabidopsidis

7c Leaves somewhat disfigured with pale spots on the upperside, that correspond with a fungal bloom on the underside, consisting of erect, apically strongly branched conidiophores. A. arenosa, ? halleri, ? petraea: Hyaloperonospora cardaminopsidis

8a Flowers swollen, unopened; containing several pale yellow, jumping larvae. A. arenosa, cebennensis: Contarinia nasturtii

8b Flowers disfigured, ± leafy; abnormally pubescent. A. arenosa: Aceria drabae

8c Siliques slightly disfigured, hardly inflated. Young seed soon filled with yellowish to grey-violet masses of spores. A. petraea: Thecaphora thlaspeos