People tend to think of ozone as something in the upper atmosphere that protects the earth's surface from UV radiation. At the ground level, however, ozone is a pollutant that damages crops, particularly soybean.
Establishing the exposure threshold for damage is critical to understanding the current and future impact of this pollutant, said Lisa Ainsworth, a University of Illinois associate professor of crop sciences and U. S. Department of Agriculture Agricultural Research Service plant molecular biologist.
Ozone is highly reactive with membranes and proteins and is known to damage the human lung. It also harms plants, slowing photosynthesis and accelerating senescence. As a result, they take in and fix less carbon, reducing yield.
Ainsworth said that ground level concentrations of ozone are already high enough to damage crop production.
"Ozone reacts very quickly once it enters the leaf through the stomata," she said. "It can form other oxygen radicals and also hydrogen peroxide. Then a series of cascading reactions causes a decrease in photosynthesis, reducing stomata conductance."
The plant's response to ozone mimics a hypersensitive response to a pathogen attack. "At quite high concentrations of ozone, you can get leaf bronzing, stippling of the leaves and necrotic spots," Ainsworth said. "At really high concentrations, you get cell death."
Ainsworth's group conducted a two-year study in 2009 and 2010 at the Soybean Free Air Concentration Enrichment facility at the U of I South Farms. Researchers found that any increase above the ambient concentration, or 38 parts per billion, of ozone was enough to reduce seed yield by roughly half a bushel per acre for each additional part per billion.
"This is significant, especially considering that background concentrations of ozone today vary year to year, anywhere from about 38 to 39 parts per billion to about 62," she said. "That can be 15 bushels per acre from one year to the next that farmers are losing to ozone."
Potential increases in background ozone are predicted to increase soybean yield losses by 9 to 19 percent by 2030.
Frogeye leaf spot
Frogeye leaf spot, which is caused by the fungus Cercospora sojina, is an important pathogen of soybean plants.
University of Illinois plant pathologist Carl Bradley said since 2010 strains of the fungus that are resistant to strobilurin fungicide have been found in Illinois and other states.
"In 2012, we detected strobilurin fungicide-resistant strains of Cercospora sojina in several new states, counties and parishes," he said. "In total, we have detected strobilurin fungicide-resistant strains of Cercospora sojina in 44 counties or parishes in eight states since 2010."
This year, Bradley's research team conducted a soybean foliar fungicide trial at the U of I Dixon Springs Agricultural Center in a field with a history of frogeye leaf spot and a strobilurin fungicide-resistant strain of Cercospora sojina. They found that strobilurin fungicides were not effective, but fungicides from other chemistry classes reduced frogeye leaf spot severity.
"As we plan for the 2013 growing season, it is important to consider the increasing number of detections of strobilurin fungicide-resistant strains of Cercospora sojina across several soybean-producing states and how this problem might affect frogeye leaf spot management decisions," Bradley said.
He recommended several management practices to slow down the development of fungicide resistance including use resistant varieties and cultural practices (crop rotation, tillage) to help manage frogeye leaf spot, don't rely on one class of fungicides to control plant disease and apply foliar fungicides only to control plant diseases.
-- Compiled by Herald-Whig Staff Writer Deborah Gertz Husar