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]
Gall
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.

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

- Willem N. Ellis: (2021) Plant Parasites of Europe^©