Bremia lactucae s. lat.

A complex of Downy Mildew species affecting a wide range of plants in the Asteraceae family, including Lettuce (Lactuca sativa) affected by B lactucae s.str. . Other species in the complex are parasitic on Knapweed, Thistles, Nipplewort, Ragwort and Groundsel, Sow-thistles and Dandelon, and other species; for more details see Chater et al (2020). 

In this genus the sporangiophores are branched and swollen at the apex into disc-like structures which carry the sporangia.

host/microscopy

Photograph both sides of the leaf and the downy mildew on the host plant; ensure you have noted the host species in your comments .  Microscopic examination may be needed for specimens on some host species.

various plants on the Asteraceae