Look for symptoms on mature leaves that are in full sunlight. The leaves must be exposed to the sun to reveal injury. Symptoms should appear over most of the plant that is in full sunlight, rather than on just one or two branches or leaves. The typical symptom of ozone is an upper leaf surface interveinal stipple. If the injury pattern carries through to the underside, then the injury on the upper surface is probably caused by something other than ozone, such as insects, disease, or weather-related factors.
Older leaves show injury.
Look for injury on the more mature foliage. Sometimes, young leaves look sunburned and have an overall red appearance. This is not due to ozone. Shaded portions of overlapped leaves do not show visible ozone injury. Leaves may show signs of premature aging.