Well, the less sun, the less bloom......but for SOME bloom, maybe Indian chief for a "red"
and Coronation, for a yellow
or, for a 'lighter" yellow, Flavescens
For a white, Dwarf iris "Snow Maiden" does fairly well, under trees
for a purple, I suppose I. germanica might give a stalk, or two
what most people call "Honorabile" will give some bloom for you
this is a row ,planted under some cedar trees....in about the 'shadiest', driest spot I have .....you can see there is SOME bloom
these are some of the same cultivars in full sun
as to disease....shade seems to cut down on leaf spot, but, as for borers....you either have them, or you don't.......I don't think they area "west coast problem", so they won't be an issue. Good luck !
