Downy mildews are very host specific, and the one attacking grapes
wouldn't attack a serissa. Serissa could have a different downy
mildew; I googled "Serissa peronospora/downy mildew" and got only a few
posts in Italian from Bonsai chat groups. This makes me suspect that
someone once mentioned the word Peronospora in conjunction with
serissas, and other people are repeating it without thinking. I did a
CAB Abstract serach (a search of all agricultural literature for the
last 20 years) for "Serissa" and various words meaning "peronospora"
and I got absolutely nothing. This doesn't mean there isn't a downy
mildew that attacks Serissa, but if there were, I'd expect a mention of
it from China, where the genus originates. Chinese host index data is
comprehensive; they employ a lot of mycologists there!

Looking at the symptoms, that sort of chlorosis can be caused by a lot
of things, including nutrient deficiency or a virus. I'd check for
thrips, too.

Nina.