Foliar application of nutrients and silicon for increasing soybean resistance against infection by Phakopsora pachyrhizi

Asian rust, caused by the fungus Phakopsora pachyrhizi, is one of the most destructive diseases affecting soybean production. Foliar fertilization as a strategy to maintain higher amounts of nutrients, besides promoting better plant growth, may potentiate some biochemical pathways for defense reactions against pathogens. This study hypothesized that soybean plants sprayed with a foliar fertilizer containing nutrients (copper, potassium, zinc, and sulfur) and silicon [Sikon Fert (R) and referred to as induced resistance (IR) stimulus] could raise their basal level of resistance to cope with infection by P. pachyrhizi more efficiently. Plants were sprayed with water (control) or with IR stimulus and non-inoculated or inoculated with P. pachyrhizi. The urediniospores germination was significantly reduced from 57 to 100% after exposure to IR stimulus rates ranging from 0.5 to 10 mL/L compared to the control treatment. For inoculated plants, foliar concentrations of sulfur, copper, and zinc were significantly higher by 40, 490, and 13%, respectively, for IR stimulus treatment compared to the control treatment. The area under disease progress curve significantly decreased by 58% for IR stimulus-sprayed plants compared to plants from the control treatment. Infected and IR stimulus-sprayed plants had their photosynthetic apparatus preserved (significant increases for photochemical yield and electron transport rate values at 7 and 11 days after inoculation (dai) and significantly lower values for yield for non-regulated dissipation and yield for dissipation by down-regulation at 7 and 11 dai, respectively) along with great concentrations of chlorophyll a+b and carotenoids than infected plants from the control treatment. Lower concentrations of malondialdehyde and reactive oxygen species (hydrogen peroxide and anion superoxide) indicated less cellular damage imposed by fungal infection and greater activities of defense-related enzymes [chitinase, beta-1,3-glucanase, phenylalanine ammonia-lyase (PAL), peroxidase (POX), polyphenoloxidase (PPO), and lipoxygenase] and more lignin production allowed the IR-stimulus sprayed plants to hamp the infection by P. pachyrhizi more efficiently. In conclusion, the reduction in Asian rust symptoms in soybean plants gained by spraying them with the IR stimulus was a collective contribution of a better nutritional status (higher foliar concentrations of copper, zinc, sulfur, and silicon), less cellular damage, and the interplay of PAL, POX, and PPO activities for more lignin production. Taken together, these findings open the way to explore the possibility of using this IR stimulus in field conditions to reduce the yield losses caused by Asian rust outbreaks in soybean.