Significance of Atmospheric Ozone as a Phytotoxicant

Since the recognition of ozone as a major phytotoxicant in crop plants in 1957, ozone type symptoms have been observed in a wide range of vegetation. These observations include leafy vegetables, field, forage, and textile crops, also shrubs, broad-leafed ornamental, fruit and forest trees, and various conifers. Fumigation experiments at various institutions have confirmed the etiological relation of ozone in many of these observations. Such visible injuries fail to provide a reliable index to the damaging impact of ozone on the numerous plants affected. Hidden injury or damage by ozone at sub-necrotic levels has been reported and experimentally established in a number of crop plants. Evidence is available which indicats that citrus varieties are so affected, and that much of the citrus decline in Southern California may be ozone induced. Photochemically induced ozone may prove to be the most persistent, if not the most difficult, of atmospheric phytotoxicants to control. Ozone type injury has been reported from eighteen different states, and from Canada to Japan.