I have been battling powdery mildew on my plants (and sadly had to pull up all of the kale), which isn't usually a problem for me until late July or August. Also, the dreaded aphids have been causing me problems since early May.
There is little that I can do about the powdery mildew (and I've tried the milk spray and the baking soda spray with no luck. It's just something I have to live with; it makes the plants look ugly and slows their growth, but that's about it as far as I can tell.
But there are things I can do for the aphid problem.