We commonly see signs of air pollution injury in Delaware in summer. Air pollution injury can be easily misdiagnosed as mite injury, pesticide phytotoxicity, or deficiencies. Atmospheric oxidants including ozone, nitrogen oxides, and peroxyacyl nitrates are the main causes of this injury with ozone damage the most common. Ozone is formed by the action of sunlight on products of fuel combustion. It is moved from areas of high concentration (cities, heavily traffic areas) to nearby fields by wind.
Air pollution injury in susceptible ornamental plant develops under the following conditions or situations:
· Ozone levels over 80 ppb for four or five consecutive hours, or 70 ppb for a day or two when vegetable foliage at a susceptible stage of growth.
· High levels of automobile exhausts. Injury is often visible on plants in close proximity to roads, especially those with heavy traffic.
· Humid conditions with cloudy, hazy overcast days and little breeze.
· High concentration of pollutants at ground level and in low lying areas. High pollution indexes in Baltimore, Washington, or Wilmington are good indicators that this is occurring.
· Foggy conditions and heavy dews.
Symptoms of ozone damage to plants range from slow growth to severe leaf browning, followed by premature leaf drop. On pines, the most frequent symptom is an unusual mottled pattern along the needles. In severe cases, tips turn brown and older needles fall off. Deciduous trees exhibit a stippling or flecked pattern on the upper leaf surface. Flecks can be white, tan or black depending on the plant species.
Ozone injury on ash. Photo from the USDA Forest Service - Region 8 Archive, USDA Forest Service, Bugwood.org
Ozone injury on maple. Photo by Robert L. Anderson, USDA Forest Service, Bugwood.org
Ozone injury on pine. Photo by Robert L. Anderson, USDA Forest Service, Bugwood.org
Gordon Johnson, Extension Horticulture Agent, UD, Kent County
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