Ozone (O3) naturally occurring in the upper atmosphere (stratosphere) shields plants and animals from harmful solar radiation. At ground level, plant-damaging levels of ozone commonly occur in parts of California during summer and fall, especially in the Central Valley and Los Angeles basin.
Ozone is formed in the lower atmosphere through complex reactions among volatile organic compounds (VOCs), nitrogen oxides, and sunlight. Vehicle emissions and fuel burning are major sources of VOCs. Numerous petroleum-derived products (e.g., many pesticides), plants, and wildland fires also emit VOCs.
Identification
Air pollution damage is difficult to diagnose. Many of its symptoms are similar to and aggravated by those from other stresses as discussed for air pollution.
Ozone-damage symptoms usually are most apparent on younger plants and older foliage. In broadleaf foliage, ozone causes bleaching, mottling, or discolored blotches, flecks, or stippling. Discoloring may be dark, pale, reddish, or a mix of these and other colors between the veins, especially on the upper surface of leaves. Discolored areas can enlarge and extend all the way through leaf tissue, and foliage may drop prematurely.
Flowers can become bleached or necrotic and die. Conifer needles may develop yellow bands, flecks, or mottling. Needle tips can turn brown, reddish, or yellow and may drop prematurely.
Solutions
Minimize VOC emissions by following IPM practices. Consult the pesticide VOCs emissions calculator when choosing a product to apply.
Reducing sources of air pollution, planting species less-susceptible to damage, and using good cultural practices are methods for managing air pollution. For large-scale plantings, species that are low emitters of VOCs can be good choices.
