Most fungicides are best used as preventive, since they are not so good as cure. You only need to spray when conditions are right for fungal growth like long periods of rain where the leaves don't have a chance to dry out and especially after the rain has stopped and the humidity is high. Preventive spraying is done before the disease appears or is just beginning.
Fungicides do not work on everything, so you need to know what plant you are protecting and what disease you are trying to manage. Sometimes you may need to use a different fungicide on another plant or for another disease.
For any chemical you should read and follow the label instructions.
Copper sulfate needs to be used with caution as it is an eye irritant so propper clothing and handling is required. It can be toxic to some plants, so you should test it on a small area first. It can usually be used up to day of harvest.
Happy gardening in Hawaii. Gardens are where people grow.