Space view of earth

Milkweed polka dots

One way we can learn more about air pollution is to study a type of plant that is very sensitive to air pollution. Milkweed is one of these plants. It shows signs of damage from ozone by showing stiples, or dark polka dots, all over the leaves; losing leaves; turning strange colors; or not growing very big. 

Ozone damaged leaf

Before you go out in the field to look for ozone damage to milkweed, and when you double check your leaf samples back in class, remember to review the classic ozone injury symptoms.

Ozone injury is always:

  • on the upper leaf surface only
  • interveinal (between the veins not across the veins)
  • black in color
  • distinct with sharp edges
  • most pronounced on the older and middle-aged leaves

Ozone injury will never:

  • have haloes
  • rub or wash off

And, don't forget that most leaves will have multiple types of damage and that most damage is caused by insects, diseases, nutrient problems or heat stress, rather than ozone.

Okay, now that you've reviewed the classic ozone injury symptoms, follow this slide show and view different milkweed leaves. Can you tell if it's ozone injury?