In our gardens, I find issues like blight and powdery mildew to be negligible when determining where to plant things. To truly prevent such problems, agricultural guidelines often dictate plantings to be in appear fields hundreds or thousands of feet (or miles) apart. We cannot afford that in gardens. So I just plant. I do try to prevent things growing near each other that will lead to stunted growth or competition for nutrients. For example, I learned on here about dill not being friendly with many other herbs. But bugs and disease are not something I can effectively combat with spacing crops apart. I could only avoid planting a certain crop, turn to resistant varieties or deploy the use of chemicals (which I try to avoid).it is about blight why potatoes and squash/pumpkins should not be planted together.