Preventive foliar application of sparingly soluble copper source is effective to protect against Asian soybean rust

Abstract

Application of synthetic fungicides continues an essential approach for mitigating the harmful effects of Phakopsora pachyrhizi (Asian soybean rust, ASR). Nevertheless, the excessive use of these products causes decline of their efficacy and environmental pollution issues. Then, aiming to seek for alternatives on the control of ASR, the efficiency of using copper (Cu) in the form sparingly soluble microparticulate oxide (CuO) on protecting against the disease was compared to soluble sulfate (CuSO₄.5H₂O, CuS). The inoculation with urediniospores of Phakopsora pachyrhizi was performed 5 days after the spray with solutions containing 0.5 g L^− 1 of Cu for either sources or deionized water as a control (CT). Disease symptoms severity was assessed until 30 days after inoculation, when leaf tissue was sampled for studying mesophyll integrity. Further, in vitro tests were conducted to assess the impact of CuO and CuS on the germination of urediniospores; and, a complementary experiment evaluated the concentration of Cu in the sprayed leaves as function of the source. Results revealed that both tested compounds were capable of completely inhibiting urediniospores germination in vitro. However, CuO had greater efficiency to delay the onset of ASR symptoms and to reduce their severity, most likely because the treated leaves maintained greater Cu concentration than those receiving CuS. Moreover, Cu-sprayed leaves of inoculated soybean, regardless of the source, exhibited improved structural organization of the mesophyll compared to the CT. These findings suggest the potential of using CuO as an alternative to CuS for disease management programs with the aid of Cu-based products. Therefore, to refine the recommendations of sparingly soluble Cu sources, further studies remain crucial to reveal the mechanisms by which the protection against the disease is achieved.