Plant PAN-demic

We’ve all heard more about pandemics in the past couple of years than any of us would care to know. But the plants in your garden are in a PAN-demic of their own. PAN is short for peroxyacetyl nitrate. [You can see why we call it PAN.]

Symptoms of air pollution damage (Mary Ann Hansen, VA Polytechnic Institute, Bugwood.org) CC BY 3.0

PAN is second only to ozone in its toxicity to plants. As a type of air pollution, it is especially hard on smaller plants and young leaves.


What is PAN?

PAN is created when sunlight hits car exhaust and industrial gases. We won’t get into the chemistry of it or the different types of PANs. It’s enough to say that hydrocarbons interact with nitrogen in the air, creating problems for plants. PANs occur in high-traffic areas such as Los Angeles, but their effects are felt all along the western U.S. seaboard and in many other industrialized regions.


​Vulnerable plants

Like other garden issues, not all plants are affected equally. Beets, cane fruits, celery, dill, endive, escarole, fennel, lettuces, melons, oats, peppers, pinto beans, potatoes, spinach, sunflowers, Swiss chard, and tomatoes are all vulnerable to PANs in the air. If you live in an area where PAN might be a problem, broccoli, cabbages, corn, cucumbers, lima beans, onions, radishes, sorghum, and wheat should perform better than more sensitive plants.

Air pollution damage to blackberry (Robert L. Anderson, USDA Forest Service, Bugwood.org) CC BY 3.0

Symptoms of PAN

PAN causes the tissues that make up the underside of leaves to collapse. This damage may appear as bronzing, glazing, or silvering. These discolorations commonly appear in bands or blotches. When pinto beans and tomatoes are affected by PAN, the tissue collapse may affect the whole leaf. In grasses, such as corn and millet, leaves affected by PAN look bleached. Chlorosis, early maturity, premature leaf drop, stunting, and weather flecking also occur.

Air pollution causes weather flecking of tobacco (R.J. Reynolds Tobacco Company, Bugwood.org) CC BY 3.0

PAN can combine with ethylene to make matters worse. Healthy plants produce ethylene as part of the ripening process. But ethylene is also found in vehicle exhaust and industrial fumes, which means there is often too much of it available. Ethylene acts as a plant hormone. And we’ve all seen what happens when people take too many steroids.


Symptoms of PAN combined with ethylene include downward leaf and stem curling (epinasty), inward petal curling and the failure of buds to open (sleepiness), and stunting. Color breaking in blossoms and early petal drop may also occur.


Bottom line: air pollution is bad for all of us.