Photochemical air pollution (smog) is harmful to California agriculture, as has been widely recognized since the 1940s, when growers in the Los Angeles Basin reported silvering, bronzing, and other discolorations on leafy crops such as lettuce, spinach, and Swiss chard. Severe smog injury on these crops rendered the produce unmarketable or greatly reduced its value.

This research was supported by the U.S. Environmental Protection Agency, the California Producers Cotton Seed Distributors, and the Kern County members of Western Oil and Gas Association. Staff at the USDA Cotton Research Station and the UG Cooperative Extension, Kern County, office provided valuable advice and assistance.