Dichotomous table for gallers on Hydrangea macrophylla
by Hans Roskam
1a On parts above ground => 3
1b On root collar or on roots => 2
2a Roots with slender- to broad spindle-shaped galls, a few mm long.
Meloidogyne hapla
2b Root collar or also basal axial parts with conspicuous proliferations, irregularly fissured at surface, almost walnut-size. Agrobacterium tumefaciens
3a Malformations mainly on leaf blades => 4
3b Stem shortened, spongy, thickened and ± distorted; the stalks and basal vein parts at infestation site clustered, leaves bloated, the leaf blades variously curved. Flowers, if present close to infestation, disfigured and ± greened, stalks ± strongly thickened. Ditylenchus dipsaci
4a Curling, arching and upwards development of leaf blade caused by aphids; or malformations of flowers => 5
4b Leaf blade with pustule-shaped galls which are soon surrounded by an expanded area of dead tissue. Cause unknown – ? gall midge
5a Malformations on plants cultivated in greenhouse. Aulacorthum solani and/or Macrosiphum euphorbiae
5b Flowers on variously stunted stems more densely arranged, greened; corolla leaves usually stunted in development. Stamens and ovaries sometimes leafy. Virus disease