Answer: There are some sound principles behind some people's thinking on this idea. First, diversity in the garden (planting lots of different things) helps minimize pest problems because pests which are specific to certain vegetables have a hard time finding exactly what they need. They're confused by the aromas. Mixing basil with tomatoes (or many other combinations) is essentially diversity at its best. Second, like pests, there are some diseases that attack both tomatoes and peppers (they're botanically related) and putting them side by side is allowing diseases and some pests to spread freely. The principles are true.
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