How to Manage Pests

UC Pest Management Guidelines


Air Pollution

Pathogen: none (abiotic disorder)

(Reviewed 3/17, updated 3/17)

In this Guideline:

Symptoms and signs

High levels of ozone cause a bleached stippling on upper leaf surfaces. Symptoms usually appear on middle-aged and older leaves. Affected leaves may age rapidly and fall off. High concentrations of ozone are associated with low wind velocities and bright sunlight.

Symptoms of peroxyacetyl nitrate on alfalfa leaves resemble those described for ozone injuries but the lesions may be larger. A silver or copper sheen is frequently apparent on affected leaves.

Comments on the Disease

Crop injury from air pollutants results in reduced photosynthetic rates and early aging, which adversely affect crop yield and quality. Many air pollutants (e.g., ammonia, chlorine, hydrogen chloride, hydrogen fluoride, or sulfur dioxide) are capable of causing plant damage, but only the photochemical oxidants (ozone and peroxyacetyl nitrate) are of major concern. They are formed by the reactions of oxygen, nitrogen oxides, and organic molecules in the presence of sunlight. The primary source for these compounds is automobile exhaust, but industrial processes and other forms of combustion contribute to air pollution.


[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Alfalfa
UC ANR Publication 3430


R. M. Davis, Plant Pathology, UC Davis (Emeritus)
C. A. Frate, UC Cooperative Extension, Tulare County (Emeritus)
D. H. Putnam, Plant Sciences, UC Davis

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Statewide IPM Program, Agriculture and Natural Resources, University of California
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