if all members of the nightshade family get A. solani early blight, should you plant them together or not? my father has his peppers, tomatoes and eggplants all in a row, interspersed, not sure why - good or bad?
Not plant them together, for the reason you suggest - they can all infect each other.
If you have any disease problems, you should rotate your crops and no solanacea should follow any other in the rotation. Where tomatoes have been, no other nightshade should be for three years.
see, I'm treating them with copper sulphate - if they are all in one place, its' easier to spray. also, if you plant them apart, each plant is a disease reservoir. but.....I'm just not sure which way makes sense. nothing i can do about it this year, anyway.
Early Blight can only overwinter on infected crop debris. With good cleanup there is no need to rotate for 3 years. This is not possible for most home gardeners. I planted in the same place this year and none of the fungal diseases I had last year made it through the winter, including early blight, except verticillium wilt which is soil borne.