Here is the list of foods that, when grown conventionally, contain the largest amounts of pesticide and fertilizer residues:
peaches, strawberries, apples, blueberries, nectarines, bell peppers, spinach, cherries, kale or collard greens, potatoes and (imported) grapes.
These are the foods that we all should definitely buy organically, since unless they’re growing in your backyard, they are truly dirty.
I guess with the kale and collards, it’s to keep the cabbage moths from making holes in the leaves. Organic growers learn to plant marigolds and other plants along with them to help with that problem.
This list is from the Environmental Working Group and their FoodNews.org website. You can download the list as an app onto your iPhone, or you can print The Shopper’s Guide to Pesticides, which fits on a small piece of paper so you can keep it in your pocket.