Tender vegetable leaves and fruit are susceptible to ozone damage through the tiny openings in leaves called stomata. Stomata help the plant to breathe, take in sunlight and create food through the photosynthesis process. These normal processes are negatively influenced by the chemical pollutants in ozone gases. Vegetables exhibit various symptoms of ozone damage according to their levels of exposure and stage of growth.
Ozone is a colorless, odorless reactive gas made up of three oxygen atoms. It acts as a beneficial sunscreen at high levels in the atmosphere and as a poisonous pollutant at ground levels. It is created by the action of sunlight on the products of fuel combustion. Plants are exposed as a result of traffic and factory activities. Human and plant life are adversely affected by high ozone levels in the air.
According to the University of Delaware extension website, vegetable plants are affected when ozone levels are "over 80 ppb (parts per billion) for four or five consecutive hours, or 70 ppb for a day or two when vegetable foliage is at a susceptible stage of growth." Ozone is carried from areas of high concentration to nearby vegetable fields. High traffic areas and humid, cloudy conditions both contribute to the potential for ozone damage. Cities with high ozone rates grow more damaged vegetable crops.
Symptoms of ozone damage on potato plants vary, but are often black flecking or bronze spots on leaves. Symptoms vary according to the stage of growth. Potatoes show ozone damage in most-recently emerged leaves. They are most susceptible during the period when the potatoes underground are bulking.
Watermelons develop premature yellowing (chlorosis) that becomes brown and black on white spots. On cantaloupe and other melons ozone damage shows as yellow and then white on upper leaves first. Seedless variety watermelon tend to be more resistant to ozone damage than seeded varieties. Squash and pumpkin show the same pattern of damage, but their veins often remain green.
Ozone damage in snap and lima beans causes small bleached spots with a bronze appearance on upper leaf surfaces and pods. The leaves may turn yellow and fall off eventually with high exposure levels. Plants may also set fewer blossoms and beans. Antioxidants and ethylene diurea have been tested and shown some promise as protection against ozone damage.