Learning to identify ozone damage

Symptoms of ozone injury develop on the leaves of sensitive plant species during the growing season if ozone concentrations are relatively high for extended periods of time. Ozone injury is cumulative, so symptoms become more severe the longer leaves are exposed to high ozone concentrations. Therefore, symptoms of injury are typically worse in the late summer and early fall.

This example will illustrate ozone injury on a sensitive variety of the snap bean plant, but keep in mind that ozone injury can look slightly different on other types of plants. Next, you will learn the specific characteristics that distinguish ozone damage from damage caused by insects, diseases, and other environmental factors.

Learn more below or play our Ozone Damage Training Game at the bottom of the page

Healthy Leaves

Healthy Leaves

A healthy snap bean leaf is bright green with no signs of damage.

Healthy Leaves


Visible ozone injury on leaves starts as stipple. Stipples only occur on the top leaf surface and are dot-like areas of tan, red, brown, purple, or black. They are typically separate and uniform in size, but may eventually merge and cover much of the leaf surface as ozone exposure continues. Stipples may look slightly different on other types of plants. In the photo to the left, brown spots are stipples, white areas are other types of damage.

Healthy Leaves

Looking for green leaf veins

Ozone damage only occurs between the major leaf veins, not on the veins themselves. If you see damage on or crossing leaf veins, it is not caused by ozone. In ozone-damaged leaves, the veins often stand out as bright green compared to the darker damaged patches.

Healthy Leaves

Yellow and brown patches

As ozone injury accumulates and becomes more severe, additional symptoms may occur. These include leaf yellowing (chlorosis) or brown patches of tissue death (necrosis). These symptoms will be visible on the top leaf surface, and may start to become visible on the back of the leaf.

Healthy Leaves

Older leaves have more damage

Because ozone injury accumulates, signs of injury will typically develop on the older leaves of the plant. You might see some newer leaves that are bright green with no injury at the same time you see older leaves with injury.

Healthy Leaves

Natural senescence

In the fall, plants start to shut down for the winter (senesce) and will drop their leaves. It can be very difficult to distinguish ozone damage from natural senescence during this time.

Healthy Leaves

Other types of injury

Symptoms of disease, insect activity, and environmental factors like nutrient deficiency or drought stress can look very similar to ozone damage. It takes some practice to recognize ozone injury on plants, but with careful observation of the specific symptoms you can learn to accurately diagnose this injury. Keep in mind that symptoms of disease, insect activity, and environmental stress are also likely to be present on the ozone-sensitive plants, even at the same time as ozone injury.

See more examples of damage in our image gallery Learn More

Ozone Damage Training Game

View the pictures below and determine if the leaf has been affected by ozone damage or not by examining the leaf picture then clicking on the YES or NO buttons. Then click on Next Question to view a new leaf picture. Once you are ready head to our Data Entry page to start entering leaf data.

To summarize, ozone injury:
  • Starts as stipple, but can progress to yellowing and patches of tissue death
  • Typically occurs on the top surface of the leaf
  • Only occurs between the leaf veins
  • Accumulates through time
  • Affects older leaves