Seed Coating with Thyme Essential Oil or <i>Paraburkholderia phytofirmans</i> PsJN Strain: Conferring Septoria Leaf Blotch Resistance and Promotion of Yield and Grain Isotopic Composition in Wheat

Septoria leaf blotch (SLB) is considered one of the most devastating diseases affecting global wheat production. Biostimulant application is among the modern approaches in plant protection to overcome the impact of SLB&#8217;s fungicide resistance. In this manner, the effect of coating seeds with thyme essential oil or <i>Paraburkholderia phytofirmans</i> PsJN strain on SLB severity and yield components (spikes/m², straw yield (SY), grain yield (GY) and thousand kernel weight (TKW)) were assessed under field conditions for 3 years. The effect on physiological traits and nitrogen and carbon isotope composition (&#948;¹⁵N_grain, &#948;¹³C_grain) and nitrogen and carbon content (N_grain, C_grain) of grains was assessed in one year of study. The increasing SLB severity decreased all yield components, increased &#948;¹⁵N_grain and C_grain content and slightly decreased &#948;¹³C_grain as the resulting effect of <i>Zymoseptoria tritici</i> inducing stomatal opening and leaf necrosis. Across the years, both treatments alleviated the SLB adverse impact by reducing SLB severity, increasing spikes/m², SY, GY and TKW. Both treatments ameliorated grain quality by increasing C_grain content and decreasing &#948;¹³C_grain and &#948;¹⁵N_grain. The difference between the performance of thyme oil or PsJN strain in terms of intensity and stability is discussed and considered to be linked to the different triggered systemic resistance and the associated amount of costs deriving from resource allocation towards defense processes.