The Environment Working Group, in conjunction with the Dept. of Agriculture has released its 2013 list of the Dirty Dozen: They are (after testing 28,000 samples):
Apples; Celery; Cherry Tomatoes; Cucumber; Grapes, Hot Peppers; Nectarines (imported); Peaches; Potatoes; Spinach, Strawberry, Sweet Bell Peppers; Kale/Collard Greens; Summer Squash.
They note that Kale/Collard Greens and Squash do not necessarily meet their standards to be "dirty" but are noted to in some cases contain higher than normal levels of pesticides - toxic to the nervous system.
You can also see their "clean list" at ewg.org
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