I found this article to be of interest in better explaining the mechanism of how copper works:
https://cvp.cce.cornell.edu/submission.php?id=140
From what I got out of the article, certain formulations of copper have low solubility in water, so what you’re spraying on your plants is essentially the copper in a suspension (as opposed to a solution.). The copper dries onto the plant. Now, the important thing to remember is that copper has LOW solubility in water, not 0 solubility. So each time, a certain amount of the copper becomes copper ions in the water. These copper ions are able to denature proteins, and since every cell depends on proteins for cell functions (including reproduction), this denaturing kills the pathogen.
And every time it rains, more copper ions are released.
What I found interesting was that water with an acidic ph dissolves much more copper into solution–leading to an increased chance of phytotoxicity. So, it would probably behoove the grower to spray the copper on when the water is as close to neutral as possible in order to lessen the chances for phytotoxicity.
Perhaps the is common knowledge, but I did not know this.
I guess it’s a case of the dose makes the poison. Just the right amount, and the copper kills the pathogens. Too much and the copper kills the pathogen and the plant.
Sorry if nobody else is interested in the kind of thing, but I always find I understand things better when I know the reasons why.