The oömycetes (Oömycota) are a large group of organisms that superficially look like fungi. There are only two groups which you are likely to encounter in the wild: the Albuginales (White Rusts) and the genus Peronospora and its allies (Downy Mildews). There are lots of other groups which I have left out for simplicity, most of which don't infect plants at all or are generalist hemibiotrophs. I have included Phytophthora as it is one that most people will be familiar with, containing Potato Blight P. infestans and Sudden Oak Death P. ramorum.
On many species in the Brassicacae. The taxonomy is in flux and on most hosts it is just known as Albugo candida agg./sensu lato.
Pustula
Pustula senecionis on Senecio vulgaris.
Coïnfection of Pustula senecionis and Puccinia lagenophorae on Senecio vulgaris.
Pustula spinulosa on Cirsium palustre.
On many hosts in the Asteraceae. The most common by far is Pustula senecionis on Groundsel Senecio vulgaris, which is extremely common in urban areas.
Downy Mildews
Foliicolous Downy Mildews Peronospora, Plasmopara, Plasmoverna, Bremia, etc.
Peronospora
Peronospora trifolii-repentis on Trifolium repens.
Peronospora fulva s.s. on Lathyrus pratensis.
Peronospora hiemalis on Ranunculus acris.
Many species on many different families. This genus also contains the floricolous downy mildews (see below).
Plasmopara
Plasmopara nivea sensu stricto on Aegopodium podagraria.
Plasmopara densa s.s. on Rhinanthus minor.
Symptoms of Plasmopara nivea s.l. on Oenanthe crocata.
Many species, mostly on Orobanchaceae and Apiaceae.
Bremia
Bremia centaureae on Centaurea nigra.
A conidiophore of Bremia centaureae from Centaurea cf. cyanus.
Bremia taraxaci on Taraxacum agg. (zoom in!).
These form sparse to dense conidiophores on Asteraceae.
Floricolous Downy Mildews Peronospora spp.
Symptoms of Peronospora violacea on Succisa pratensis. Note the larger, redder petals and lack of developed anthers.
A conidiophore of Peronospora violacea on a petal of Succisa pratensis. I have used focus stacking to increase the depth of the field; see methods.
A heavy infection of Peronospora radii on Tripleurospermum maritimum.
Sparse to dense conidiophores on the flowers only. Some species alter the development of the flowers, most notably Peronospora violacea. This group is monophyletic and nested within the rest of the genus:
