Click for larger image Browning needles on white pine (Pinus strobus) caused by air pollution

Oxidants such as ozone, oxides of nitrogen and peroxyacetyl nitrate (PAN) cause the most direct damage to plants. These are followed by sulfur dioxide. Fluorides, chlorides, ammonia, and chlorine cause occasional local damage. Metallic elements from polluted air also accumulate in plants, but their effects at the concentrations encountered in most localities are minor or unknown.

Injury from air pollutants can be put into two major categories: (1) chronic injury, which is the cumulative effect of long exposure (often over several seasons) to toxic pollutants at low levels on sensitive species or at moderate to high levels on tolerant species; (2) acute injury occurs after short exposure (a few hours to several days) to pollutants at high levels on tolerant species or at moderate to low levels on sensitive species. Acute injury commonly occurs following accidental leaks or spills of gas or volatile liquids. Acute injury results in moderate to severe foliar injury and is the most frequently diagnosed type of air pollution injury. Chronic injury is difficult to diagnose because the foliar symptoms are mild or nonexistent and the trees decline over several years.

Most air pollution results from electric power generation, cars and trucks, and a variety of industrial operations. Air pollutants that are toxic to trees can occur in several forms: invisible gasses, particulates (smoke or dust), or aerosols (fine mists). Invisible gasses are the most common form of pollution toxic to plants (phytotoxic) rather than smoke. Air pollution is not restricted to urban and suburban areas but is also found in remote areas.

Symptoms and Diagnosis

Diagnosis of air pollution damage is difficult. Symptoms can be very similar to those produced by stress from moisture, temperature, or nutrient disorders as well as a variety of biotic factors such as bacteria, fungi, viruses, sucking insects, and mites. Therefore, once these factors are eliminated, one can look to air pollution as the source of the problem. Local meteorological conditions and nearby sources of emissions can often be misleading. The source, the specific sensitivity as correlated with symptom type, and the nature and the movement of air pollutants must be known before accurate diagnosis of injury can be achieved. Sensitive chemical analyses are sometimes needed to detect air pollutants.

Because of the complexity of symptoms, it is suggested that reference material be used to further investigate symptoms.

See individual sheets on Ozone and Sulfur Dioxide for how to diagnose these two problems.

Life Cycle

Specific effects of air pollutants on plant tissue vary with the pollutant, host, time of year, and numerous meteorological factors such as temperature, relative humidity, wind, and solar radiation.

Ozone builds up to phytotoxic levels in the atmosphere during warm, sunny weather when pollutants accumulate in stagnant air. Accumulation often occurs during atmospheric inversions where warm air is trapped in valleys and basins bounded by mountains by a layer of cool air above.

Juvenile leaf tissues are most susceptible to acute injury of fluorides and consequently most severe injury occurs in the spring. Chronic injury, however, occurs progressively during the entire growing season.

Integrated Pest Management Strategies

1. Maintain plant vigor. Plants in good health resist all types of injury better than weakened plants. Therefore, water in times of drought and fertilize to maintain nutrient balance. Soil should be porous, well-draining, and not compacted.

2. Reduce emissions. Reduce emissions from the source when possible.

Organic Strategies

Both of the recommended IPM strategies are strictly organic approaches.

More images:

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Damage to coconut palms (Cocos nucifera) caused by volcanic fumes
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Damage to coconut palms (Cocos nucifera) caused by volcanic fumes