gallers on Humulus

1a On axial parts of shoots or leaves => 2

1b On ± distinctly swollen roots erupt, at first white, later on pale brown eelworm cysts, dropping, about poppy-seed size. Heterodera humuli

2a Extensive malformations on shoot tips or on higher situated leaves => 4

2b Galls on root collar or on basal parts => 3

3a Root collar with conspicuous distorted buds, up to even 6 cm large, inside white, at surface at first glabrous, greenish to pale brown, later on fissured or wart-like, dark brown proliferations. Also encroaching onto rhizomes. Agrobacterium tumefaciens

3b Basal organs with many warts, hardly 1 mm long, ± golden-yellow, many-celled, ± joining on leaf underside into crusts; or into ridges on leaf veins, -stalks or stems. Synchytrium aureum

4a Malformations caused by fungi fruiting on outside of malformation => 5

4b Malformations caused by aphids. Phorodon humuli

5a Axial parts of often systemically infected terminal or axillary young shoots are severely stunted, ± conspicuously thickened. Leaves densely clustered, leaf blades often greatly reduced, ± deflected, spoon-like, pale green, apically rapidly decreasing in size, covered on the underside with dense, dark violet down of conidiophores. Pseudoperonospora cubensis

5b Almost identical malformations, which are on upperside as well as underside, covered by a mealy white dusting, later on often ± browned mycelium. Podosphaera aphanis