Iron Looks Promising To Combat Cercospora Leaf Blight In Soybeans

BATON ROUGE, LA.
   Foliar applications of iron appear to be a viable answer to Cercospora leaf blight in soybeans according to LSU AgCenter research.
   For the past several years, AgCenter plant pathologist Ray Schneider has been applying minor elements to soybean plants to find out if they can control disease pathogens.
   He tried most minor elements, including zinc, copper, manganese, boron, aluminum and iron. “We had good results with some, but we consistently had disease suppression with foliar iron,” Schneider said
   The initial tests used several variations of minor elements, but none were fertilizer grade. But when Schneider began focusing on iron, he started using commercial formulations of foliar nutrients.
   “We needed a form of material that allows the iron to get into the leaves,” Schneider said. He is now using Maniplex iron from Brandt Consolidated.
   “Cercospora leaf blight thrives in hot, dry weather,” Schneider said. “This is a problem because soybeans have no natural resistance to the Cercospora pathogen.”
   To make sure the soybeans are prime targets for a Cercospora infection, Schneider waits until late in the planting season when hot weather is assured.
   Cercospora is not as responsive to fungicides as other diseases, such as frog eye leaf spot and rust, Schneider said. So the results of tests with iron hold promise of combating the disease.
   Schneider has gotten good results in field trials when foliar iron was applied to soybeans at the R-5 growth stage when no Cercospora symptoms were visible. “We know it’s effective before R-6,” he said.
   “We showed that the Cercospora leaf blight pathogen is almost always present in leaves, but with no symptoms,” he said. “Apparently it becomes pathogenic at later stages of soybean growth.”
   Leaf tissue samples indicate low levels of iron, mostly in the upper leaves after the plants pass the mid-reproductive growth stage. “It may be that iron concentrations in the upper leaves are not as high as in the lower leaves, and the disease appears in those upper leaves,” Schneider said.
   The recommended level of iron sufficiency is 50 to 100 parts per million in a tissue analysis. But following foliar applications, iron levels increase in the leaves, and symptoms of Cercospora leaf blight are suppressed when iron is at 280 parts per million.
   “Iron is relatively immobile and doesn’t move well in plants, but we’re looking at test levels higher than the plant needs to be healthy,” he said.
   “In some cases, we get complete disease suppression – as good as or better than with fungicides,” Schneider said. “We still have more to learn about the mechanism of action.”
   Schneider suspects the pathogen needs iron to survive, and when the element is in short supply, Cercospora begins taking iron from the plant cells.
   “In the laboratory, it appears that when the pathogen has low levels of iron, it makes the toxin cercosporin, which damages plant cells,” Schneider said. It’s almost as if the pathogen “steals” iron from the plant.
   Schneider and his team have been researching foliar iron on soybeans in test plots on the Ben Hur farm in Baton Rouge and the Dean Lee Research and Extension Center in Alexandria for the past four years. This year, they’re moving the research to a farmer’s field.
   “The results have been very encouraging,” Schneider said. “The trials are repeatable and are yet to fail.”
   An added benefit, he said, is that foliar iron is less expensive than fungicides for controlling Cercospora.
   Schneider said his next step is to investigate other minor elements to see their effects in foliar applications. His earlier work suggested a different nutrient appeared to suppress rust. ∆