Foliar applications of iron may provide some opportunities to reduce the impact of Cercospora leaf blight in soybeans.

For the past several years, LSU AgCenter plant pathologist Ray Schneider has been applying minor elements – zinc, copper, manganese, boron, aluminum and iron – to soybean plants to find out if they can reduce the impact of disease pathogens.

"We had good results with some, but we consistently had disease suppression with foliar applications of 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 the material that allows iron to get into the leaves," Schneider said. He is now using a formulation that can be absorbed into the plant tissue.

"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 frogeye leaf spot and rust, Schneider said. So the results of tests with iron applications 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 plant 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 in the tissue.

"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. But we still have more to learn about the mechanism of action."

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 soybean rust.

Schneider said his research program wouldn’t be successful without the work of graduate students Eduardo Chagas Silva and Brian Ward and research associate Clark Robertson.
Rick Bogren