I choose strains that are supposed do well in my geographical area. Northern New England gets lots of cold rain and fog late in the short growing season.
Wiping Stylet/
Neem oil on leaves greatly helps the battle against powdery mildew.
I don't spray buds but some do early on. It degrades fast but I still don't do it.
Every morning I shake off buds from overnight dew and then after every rain.
That is what I do because I found that covering my plants restricts airflow which is a major issue on its own.
Watch out for those damn tiny caterpillars as they will rot your buds worse than rain.
Remember cannabis is a plant and has been getting wet since the beginning of time.