COpper is nasty in quantity. Whe I was a kid I dealt with the bindweed all over the garden by nicking some copper sulphate from the chemistry lab at school and painting it on the leaves repeatedly till it all died. It never came back
Copper is not systemic in plants - that's why it can't kill active blight - it's more likely that the extremely acid nature of the sulphate killed the leaves. The kids in my classes who spillled it on their books and clothes found that holes appeared after it dried and became more concentrated and lower pH until it destroyed the fibres.
There is no copper sulphate as such in bordeaux mix solution (or burgundy - the harsher version).
The made-up solution contains two new near-neutral compounds - amphoteric colloidal copper hydroxide and calcium sulphate ie. gypsum (via a double-decomposition reaction).
Copper is extremely bitter but this is a good thing because you can't be voluntarily poisoned by mouth unless you are already insensible or you're mad enough to put it in a gel capsule.
Anyone who wants to refer to copper as a poison should first read:
http://en.wikipedia.org/wiki/Copper_in_health.
and:
http://chemse.oxfordjournals.org/content/26/1/85.fulBasically the average 50kg person with a normal metabolism could eat 5mg a day every day with no ill effects - but you would still definitely taste that unless it was dissolved in more than 1.5 litres of water. If it was spread out on the skin of even 20 or 30 tomatoes it would make you retch.
Do I really have to point out that making soup from recently sprayed but unwashed tomatoes is a bad idea?
It is also good that copper doesn't penetrate the plant (so it can only act as a preventative) - it doesn't get to the tubers of potato and it can be polished off a tomato by rubbing gently with a cloth (or clothing).
If your anti-blight wonder chemical can stop blight after it has invaded the plant, then that means you are eating that wonder chemical in your meal. QED.
All things considered copper is pretty much exactly what you'd want as an almost foolproof barrier - but of course this isn't what 'big pharma' wants you to hear.
Copper isn't 100% effective in a bad year - but nothing is 100% effective in a bad year... use copper and hedge several bets at once - accept the occasional losses.
You can always grow oca instead of maincrop potatoes, and you can do a lot for tomatoes with well-ventilated covers plus copper.
Unfortunately there is no obvious single substitute for tomatoes - but between them ground cherry: P.pruinosus and lychee tomato: S.sisymbriifolium cover most of the bases.
Cheers.
