Although I think I must live in Late Blight central, I don't spray, I did up until a few years back as I always used to think if I don't harvest potatoes for me then I'll end up buying some and they will have been sprayed with a lot more than I would have used. But as all we are left with now are copper sprays I'd rather not use them on my veg plot.
It's just a tentative suggestion (and you probably know more about it than me), but did you consider spraying with aspirin?? Have you come across that idea?
Strange, I know, but it's something suggested in James Wong's Grow for Flavour book. Can I say I find some things about JW extremely irritating but this is the most interesting vegetable growing book I've read in a long while. I hate to love it!
According to the book studies show that "spraying with dilute aspirin increases sugar levels in tomatoes by over one-and-a-half times, as well as boosting their Vitamin C content by 50%" by boosting their immune responses. One study has shown 50% resistance to late blight.
I give you the link to all of the studies here (click on 'scientific references').
http://www.jameswong.co.uk/grow-for-flavour/4588092106 I didn't yet have time to trawl through them, but this one may be the one he's referring to for blight
http://www.interesjournals.org/full-articles/use-of-bioagents-and-synthetic-chemicals-for-induction-of-systemic-resistance-in-tomato-against-diseases.pdf?view=inlineHe doesn't link the text to the specific references at all.
Anyway, I'm considering trying it myself - thought yesterday I might have the beginnings of blight, but it seemed unlikely so early on and so DRY so checked out the posts here and yep..
