Plant Parasites of Europe

leafminers, galls and fungi

gallers on Rhododendron

Dichotomous table for gallers on Rhododendron

(incl. Azalea)

by Hans Roskam

1a On leaves or flowers => 2

1b Conspicuous, at first whitish, succulent, later on strongly browned, woody proliferations on root collar. Rhododendron spp.: Agrobacterium tumefaciens

2a On shoot tips or leaves => 3

2b The flowers are irregularly ± double. R. ferrugineum, hirsutum, intermedium: Aceria alpestris

3a Malformations of leaf blades => 5

3b Malformations of shoot tips => 4

4a Conspicuous bud-like tuft of leaves on shoot tip. R. ferrugineum: Dasineura rhododendri

4b Terminal buds crippled; often increased in number. Leaves reduced and disfigured. Buds dying prematurely. ‘Shoot tip mite’. R. simsii: Tarsonemus sp.

5a Leaf blade with rolls of margin or pouch-like to spherical galls => 6

5b Leaf blade otherwise weakly disfigured. R. ferrugineum, hirsutum: Cacopsylla rhododendri

6a Roll of leaf margin => 8/p>

6b Succulent galls on leaf blade => 7

7a Leaf blade largely or completely thickened, pale-green, later on densely chalk-white frosted, especially on underside. Sometimes several, almost rosette-like clustered leaves terminally on severely stunted, thickened, young shoots similarly disfigured; galls sometimes also on calyx leaves and even on corolla. “Earlobe disease”. R. caucasicum, indicum, nudiflorum, obtusum: Exobasidium japonicum

7b Up to about cherry-size, succulent, yellowish or often one-sided reddened, later on white frosted, often almost globular galls on leaves, sporadically even on flower parts, as well, sometimes, ± clustered, raceme-like, on shoot tips. R. ferrugineum, hirsutum, x intermedium: Exobasidium rhododendri

7c Galls succulent red-orange. R. luteum, ponticum: Exobasidium horváthianum

8a Leaves on unshortened shoot tip usually narrowly rolled upwards over all their length, lighter; with small 1-celled hairs on both sides. R. ferrugineum, hirsutum, intermedium: Aceria alpestris

8b Leaf margin rolled downwards, ± curled and crippled. R. indicum, molle: Phyllocoptes azaleae

8c Mites living in buds and free on the underside of the leaves. They can cause bronzing and rusting of the young leaves, downwards rolling of the leaf margins and eventually stunting; in nurseries. R. atlanticum, japonicum: Aculus atlantazaleae

8d Rolling of the leaf margin. R. ferrugineum: Aculus thomasi

Last modified 17.v.2020