Visible foliar ozone injury for conifer species is expressed at the upper parts of the crown exposed to the light, in the upper side of branches and needles:

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- Chlorotic mottling is the most common symptom described for conifer needles; it is the result of chronic exposure to ozone and can be described as yellow or light green areas of similar size without sharp borders between green and yellow zones. However, not all needles in a fascicle may be uniformly affected.

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- Chlorotic mottling frequently appears only in needles older than 1 year (second-year needles and older).

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- Symptom generally increases with increasing needle age (age effect).

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- Chlorotic mottling is more distinct on light-exposed needle areas in comparison to shaded ones.

- It is easier to observe the mottling, if several needles are held close to each other, forming a “plane” of needles.

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