Diagnosing crop problems can often be difficult. Identifying disease, insect or nematode problems is not always straightforward. And what about nutrient problems -- herbicide injury, soil compaction, and physiological problems? And then there's air pollution.
    

Air pollution injury can manifest itself in many ways, depending on the pollutant, its concentration, crop species, crop growth stage, weather conditions, soil moisture, and more. Often, leaves of a certain age are affected, or just portions of those leaves that happened to be in the most susceptible stage at the time peak levels occurred.
    

Ozone is a common cause of plant damage. Plants can be damaged by long-term exposure to ozone concentrations of 50-80 parts per billion. In Chatham, for example, ozone levels have reached a peak between 50 and 70 parts per billion for 18 of the first 26 days of June. You can find these readings at www.airqualityontario.com.
    

Cucumbers, onions, peas, tomatoes and watermelons are some of the vegetable crops most sensitive to ozone. White beans are particularly sensitive and are often used as an indicator crop.Sulfur dioxide and nitrogen oxides can also damage plants, but more significantly, combinations of these pollutants with ozone can cause higher levels of injury. Some cruciferous crops and cucurbit crops are quite sensitive to sulfur dioxide. Other sensitive vegetable crops include onions, peas, lettuce, peppers, and tomatoes.
    

Another pollutant, peroxyacetyl nitrate, known as PAN, is known to affect tomatoes in Ontario. It causes a distinctive bronzing or glazed appearance of the underside of the leaves. PAN also injures spinach, Swiss chard and certain varieties of lettuce and beans. PAN is not emitted directly from pollution sources, but is formed by a chemical reaction between nitrogen oxides and other pollutants.
    

OMAFRA has a factsheet on the effects of air pollution on agricultural crops. Search for it online or visit ontario.ca/crops for more information.

Janice LeBoeuf is vegetable crops specialist for OMAFRA, Ridgetown, Ontario.