Air Pollution (Ozone)

There is lots of interest in air quality all over the Western United States when smoke from forest fires fills valleys during the summer. Most of the smoke problem is due to particulates, which are not known to damage plants. Ozone, however, is known to damage plants.

Ground-level ozone is also called smog. Smog-forming pollutants are primarily generated by traffic and activities in urban areas, but often the wind blows the pollution to outlying suburban and rural communities. Smog is a problem for the Portlzand-Vancouver and Medford Oregon areas during the summer months. It is created most often on days when the temperature is 90°F or higher, especially when winds are light or non-existent.

All areas of Oregon currently meet the federal air-quality standard for ozone. These levels, however, are based on human health. Sensitive plants can have problems when the ozone concentration is between 50–120 parts per billion (ppb)—120 ppb equals 12 pphm (parts per hundred million)—for extended time periods. The Eugene ozone monitoring station recorded at least 7 days during the summer of 2017 that met this requirement.

Ozone levels in the Puget Sound basin have exceeded the National Ambient Air Quality Standard (NAAQS) on several occasions last century. Sensitive plants grown near Seattle and Tacoma (Washington), Portland (Oregon), Vancouver (British Columbia), or any growing metropolitan area, are at risk.

What are the most sensitive plants? The list is long but includes alder, blackberry, currant, dahlia, grape, lilac, milkweed, ninebark, Oregon Ash, Oregon Oak, petunia, Poplar (including aspen), radish, sequoia, snowberry, sycamore, tomato, and Tulip poplar. Ozone symptoms on plants usually occur between the veins on the upper leaf surface of older and middle-aged leaves, but may also involve both leaf surfaces for some species.

In grapes, it is referred to as “oxidant stipple” or “ozone stipple” and has been observed in Oregon. Symptoms of ozone damage appear predominantly on older grape leaves. Small interveinal spots (0.1mm–0.5 mm in diameter), occur only on the upper leaf surface. The lower leaf surface does not show stipple symptoms but may show necrotic spots if symptoms are severe. Stipple may begin as a yellow color but is most commonly recognized when spots become brownish-bronze to dark brown. Heavily stippled leaves may have a uniform bronze appearance from a distance and necrotic spots up to 2mm in diameter. The tissue in the veins surrounding lesions remains green. Affected leaves eventually drop from the vine. Growers may confuse this with spray damage.

The most common symptoms of ozone damage on field-grown beans is bronzing, in which a purple-brown discoloration develops on the upper surface of the leaves. Bronzing on green beans has been observed in Oregon that could be due to ozone. The symptom was observed late in the season and could easily be overlooked since crops are senescing or have other diseases (such as powdery mildew). Symptoms of air pollution and/or ozone injury have been observed in multiple bean fields in central Washington, particularly in adzuki bean crops.

Many forest and landscape trees are also susceptible to ozone injury. Extensive injury was detected on forest ozone biomonitoring sites in California, with ponderosa and Jeffrey pines, mugwort, skunkbush, and blue elderberry showing injury. Little or no injury was detected in Oregon and Washington.

Ozone injury has not been observed on small fruit crops in the PNW.


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