A naturally derived pesticide used by many organic farmers and home gardeners can produce the classic symptoms of Parkinson’s disease in rats, scientists said this weekend at the annual meeting of the Society of Neuroscience. The scientists said the finding was the best evidence so far linking chemicals in the environment to Parkinson’s. Deborah A. Cory-Slechta of the University of Rochester Medical School said, “I think you’re just beginning to see the tip of the iceberg.” Scientists cautioned, however, that linkages between the pesticide rotenone — which is derived from the roots, seeds, and leaves of various plants — and Parkinson’s in humans have not yet been established, and they noted that the rats in the study were exposed to particularly high levels of the chemical. Rotenone is found in 680 compounds marketed as organic garden pesticides and flea powders, according to Caroline Tanner at the Parkinson’s Institute.