gallers on Cydonia

1a On shoots, thinner twigs or leaves => 4

1b On thicker, woody stems => 2

2a Cancer-like malformation => 3

2b Thicker branches with club- or thick spindle-shaped swellings: Viscum album

3a Bark with ± expanded irregular, cracked proliferations, occupied by white woolly aphids: Eriosoma lanigerum

3b Rotund or flattened oval proliferations on basal stem parts, soon lignified, surface cork-like and granulated. Bacterial cancer: Agrobacterium tumefaciens

4a On leaves and shoots => 5

4b Bark of thin twigs with small nodules, with coarse surfaces: Acalitus phloeocoptes

5a Several leaves at shoot tips irregularly disfigured => 10

5b Leaves with locally defined galls => 6

6a Galls bulge-like or slightly bladder-like, soon covered with fungal sori => 7

6b Leaf blade with many rotund pocks, weakly protruding on both sides; these with narrow exit on underside: Eriophyes pyri

7a Galls bulge-shaped, yellowish, often on leaf veins, occasionally on young stems; cylindrical, tube-shaped, with sori which soon lacerate and rip open from apex downwards => 8

7b Leaf blade often with several weak bladder-like swellings, which display a white-grey cover at maturity consisting of many asci: Taphrina bullata

8a Lateral walls of peridium cells with obliquely or transversely running ridges => 9

8b Lateral walls of peridium cells with rotund tubercles: Gymnosporangium clavariiforme

9a Sori on leaf underside usually only weakly arched. Peridium protruding about 2 mm, later on frayed to its base, its cells 18–24 µm across. Aeciospores 19–26 x 19–22 µm: Gymnosporangium confusum

9b Sori on leaf underside usually distinctly arched. Peridium protruding up to 3 mm, later on frayed to its base, its cells 31–35 µm across; aecia 30–39 x 25–30 µm: Gymnosporangium tremelloides

10a Leaves strongly curled, clumped at the ± stunted shoot tips. Caused by aphids => 11

10b Leaf margin deflected downwards: Cacopsylla mali

10c 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: Podosphaera leucotricha

11a Leaves in spring ± twisted along the midrib, more or less tightly rolled downwards, sometimes developing loose clusters at the shoot tips. Aphid green, with short, thick green siphunculi: Rhopalosiphum oxyacanthae

11b Similar malformations; often clusters more compact. Leaves usually also deflected over the tip. Aphid with rather long black siphunculi: Aphis pomi

11c Leaves are curled and yellowed. Aphids with red body fluids: Dysaphis pyri