Elsa Sánchez, Associate Professor of Horticultural Systems Management, Penn State; esanchez@psu.edu
Reprinted from The Vegetable & Small Fruit Gazette, August 2010, Volume 14, No. 8, Penn State Cooperative Extension.
The heat wave we just experienced was brutal! It was tough to keep our crops well watered and take care of other management tasks in that heat. By the end of last week, we started getting reports of possible ozone damage on vegetables.
Symptoms of Damage
Ozone damage first occurs on older and mid-aged leaves with new growth generally being unaffected. Most commonly symptoms are found on the upper leaf surface, but with some plant species and cultivars it can be found on the lower surface as well. Symptoms start as small, irregular-shaped lesions that can be dark or light in color. With time, these lesions can grow into each other creating large dead areas. You may also notice stippling, bronzing or reddening of the leaves. The extent of the development of symptoms depends on the duration plants are exposed to ozone, the ozone concentration, the type and cultivar of crop grown and weather conditions. To see some pictures of ozone damage visit http://extension.udel.edu/weeklycropupdate/?p=304. Symptoms can be confused with severe spider mite damage, phytotoxicity from pesticides, nutrient deficiencies, drought stress and some foliar diseases. Also, ozone damaged plants can be more susceptible to diseases. Extension educators can be contacted to help sort this out.
How Ozone Damages Plants
Ozone is an air pollutant which causes phytotoxicity when levels are high enough. Ozone enters leaves through stomata. From there it reacts with various substances in the leaves which yield toxic compounds.
Depending on how extensive the damage is, photosynthesis is reduced which can lead to decreased growth and yields. Plants also can mature quicker than normal. Plants respond differently to air pollutants, like ozone, with tomato, watermelon, squash, potato, string beans, snap beans, muskmelon, beets, carrots, sweet corn, peas, turnips and strawberries being considered more susceptible and cucumbers, pumpkins and peppers less susceptible.
How the Heat Wave May Have Played a Role
Weather conditions are one factor that contributes to the extent of ozone damage. High ozone levels are found when it is hot and sunny over large areas. Ozone also accumulates when air is not moving. Dennis Decoteau, a Penn State Researcher who studies air quality impacts on plants, recorded ozone concentrations high enough to damage plants last week. Additionally, when plants are under stress, such as from heat, they can be more susceptible to ozone damage.
Minimizing Damage
Maintaining healthy plants by limiting plant stress will help to minimize ozone damage. Careful water and nutrient management are a must. Timely irrigation, especially during heat waves, is critical. If symptoms are slight, the plants will likely outgrow the damage.
References
Effects of Ozone Air Pollution on Plants. 2010.
http://www.ars.usda.gov/Main/docs.htm?docid=12462
G. Johnson. 2008. Air Pollution Injury in Vegetable Crops.
http://extension.udel.edu/weeklycropupdate/?p=304
G.J. Holmes and J.R. Schultheis. 2003. Sensitivity of watermelon cultigens to ambient ozone in North Carolina. Plant Dis. 87:428-434. http://apsjournals.apsnet.org/doi/pdf/10.1094/PDIS.2003.87.4.428
G. E. Brust. 2007. Air Pollution Effects on Vegetables. http://mdvegetables.umd.edu/images/Air%20Pollution%20and%20Vegetables.pdf .