Okay. First off, I'm in West Virginia, zone 6B--really I think it's 7A now. Been gardening in the same place for 18 years. I've never had a serious problem with blight on my potatoes, but this year many of them did poorly--the ones I'd saved myself in a root cellar. The ones I bought from Tractor Supply did okay--possibly because they went in first. It might have been the lateness that caused uneven sprouting and poor yield in the home-raised ones. And I've had trouble with blights on peppers a couple years but I don't think it's the same ones that affect tomatoes. The tomato blights I get every year, and there are three of them: early blight, late blight, and septoria blight. I don't know if the cages I reuse spread the disease or if it's just present in the soil or the air, but I reliably get it just as the first load of fruit is ripening. All three start low on the plant and climb up. So one thing I do is remove any leaves within three inches of the ground (or run them through the cage bars at a higher level) when the plants are about 16' tall, then do it again with any within maybe a foot of the ground when they're three of four feet tall, and I do mulch. And of course I rotate crops; I have three gardens a couple hundred feet apart so tomatoes take turns in these. But these measures at best slow the spread. I tried a caterpillar tunnel one year, but it didn't help much, and then I had to keep watering the plants, and I had worse bugs that year than any other. Insects are not much of a problem for me; with tomatoes it's only the hornworms and only a couple per year.
The main way I get enough tomatoes is to buy the pricey seed for the blight-resistant hybrids--Defiant and Stellar are the ones that have worked for me--and to plant several different varieties. Defiant is early, then Stellar and Persimmon pick up the slack when is slows. This year I planted Chocolate Stripes in its own bed, but they're all close together. This is one of the especially tasty, dark red and green striped tomatoes, and produces abundantly early, then slackens and usually produces some more late. But it seems to get late blight. For two years it seemed it didn't really hurt production or spread to other plants, but this year the blight knocked out three of my four beds almost completely--because August has been so wet I guess--so maybe I won't plant it next year. And I'll fork over whatever outrageous price they want for a few Defiant seeds, because Stellar alone is not going to give me enough canned sauce.
Oh, I did try copper once, till I read that it can build up and become toxic in the soil, and to be effective you have to hit the plants before there is any sign of blight, and then again after any rain. Okay, forget THAT.