Dichotomous table for gallers on Sisymbrium ...
by Hans Roskam

Localised to expanded, one- or all-sided, ± curved swellings on stems of vegetative or flowering shoot. Inserted leaves stunted, bent over their length, pale green, thickened and brittle, inserted flowers ± atrophied in all their parts. Down loose, conidiophores branched. Sisymbrium spp.: Hyaloperonospora sisymbrii-loeselii

[Plant Parasites of Europe's main entry for the gall adds the following information]

Underside of the leaves with a greyish white fungal down of erect conidiophores that apically several times are branching, each branch ending upon an ovoid conidium.

Brassicaceae, monophagous
Sisymbrium altissimum, irio, loeselii, luteum, officinale, volgense.