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Iron tissue content suppresses Cercospora leaf blight development in soybean

Eduardo Silva: LSU

<div>Cercospora leaf blight of soybean (CLB), caused by <em>Cercospora kikuchii</em>, <em>C.</em> cf.<em> flagellaris</em> and <em>C.</em> cf.<em>sigesbeckia</em>, is a devastating disease in Louisiana and many other southern U.S. states. <em>C.</em> cf.<em> flagellaris</em> is the main Cercospora species associated with CLB in the U.S. A preliminary micronutrient screening of Al, B, Cu, Fe, Mn, Mo, and, Zn showed that foliar applications of Fe consistently decreased CLB severity. The objective of this study was to determine if Fe foliar applications (Manny Plex Fe and Fe EDTA, Brandt Consolidated, Springfield, IL) would reduce soybean leaf colonization by <em>C.</em> cf.<em> flagellaris</em> and the severity of CLB leaf symptoms (purple and blight). The cultivar Pioneer 95Y61 was treated with four rates of Manny Plex Fe and Fe EDTA under field conditions at R5 growth stage. Leaf tissue analyses and qPCR were performed to verify Fe uptake and fungal leaf colonization, respectively. Results showed a low relationship between leaf colonization by the pathogen and the leaf symptoms assessed (purple and blight). Severity of purple leaf symptoms increased as leaf Fe concentration increased up to 230 mg/kg of dry matter, after this threshold, the purple symptom severity tended to decrease. Blight symptoms were suppressed as the Fe concentration in soybean leaves increased up to 350 mg/kg. Moreover, leaf Fe concentration did not affect the biomass of the fungus<em>.</em> There was a positive relationship between leaf concentrations of Fe and yield. Negative relationships between yield and Cercospora biomass and severity of blight and purple symptoms were observed. Taken together, the results suggested that Fe does not affect pathogen growth, but may influence its virulence. Furthermore, the results showed that disease suppression is only achieved when the levels of Fe is well above the optimal physiological range (> 100 mg/kg) for soybean growth and development.</div>