REPORTS: Soybeans Susceptible To PPO Herbicide Injury

Recent heavy rainfall in parts of Indiana have created conditions ideal for herbicide injury from certain pre-emergence PPO inhibitors in soybean fields, two Purdue Extension weed scientists say.

PPO inhibitors are a class of herbicides effective in treating marestail, common waterhemp and Palmer amaranth when used in combination with other herbicides in weed burndowns before crops have emerged. They work by creating toxic levels of oxygen radicals that destroy the lipids of cell membranes, leading to plant death. Under most conditions, soybeans have the ability to metabolize these herbicides quickly enough to reduce the levels of toxic radicals.
PPO herbicides also have soil residual activity, meaning they remain in the soil for a period of time. According to Bill Johnson and Travis Legleiter, that makes them more prone to injuring soybeans under cool, wet conditions.

"The sustained cool, wet soil conditions under which most soybean plants have been emerging this spring are less than ideal for rapid herbicide metabolism and thus have led to some injury symptoms to fields that received one of the PPO herbicides," Johnson said. "Soybean injury also was increased due to heavy rainfall events that splashed herbicides from the soil surface onto emerging soybeans."

 Injury symptoms include stunted growth, crinkled leaves, necrosis of the hypocotyl (lower stem) and cotyledons (first leaves), and necrotic spotting on leaves. Soybeans planted in sandy and coarse soils typically are at higher risk for these types of injuries. But, according to Travis Legleiter, this year's weather conditions have made soybeans planted in most soil types susceptible.

The good news, he said, is that the injury most often doesn't reduce soybean yields.

"In the majority of cases, soybean plants are able to grow out of the initial injury and yield losses don't occur," Legleiter said. "Only in rare cases of severe injury to the hypocotyl or growing point will replanting be required."

Because soybeans can recover, both he and Johnson cautioned farmers about avoiding PPO-inhibitor herbicides going forward.

"The increased amount of injury to soybean this year might cause some of our producers to avoid these particular herbicides in the future," Johnson said. "We have seen exceptional weed control out of these PPO-inhibiting herbicides at our Palmer amaranth research site this season, and would encourage growers to continue to use these valuable tools.

"Producers need to weigh the risk of temporary injury against quality control of problematic weeds, such as Palmer amaranth, common waterhemp and marestail. In the majority of years, these products pose little threat of soybean injury."