Effect of fungicides and the biological control agent Debaryomyces hansenii on pathogens and the mycobiome of durum wheat grain under drought conditions

Powdery mildew (PM) caused by Blumeria graminis and Septoria tritici blotch (STB) caused by Zymoseptoria tritici are common and devastating diseases of durum wheat. The aim of this study was to analyze the effect of fungicides and a Debaryomyces hansenii yeast strain (a biological control agent, BCA) on pathogens and the mycobiome of durum wheat grain. The experiment was conducted on naturally infected plants. The effectiveness of BCA was tested alone by applying three protective treatments in the stem elongation, heading, and flowering stages. In the integrated approach, BCA was applied after fungicide treatment (proquinazid, prothioconazole, and spiroxamine) in the stem elongation stage.Unprotected plants (control) and plants protected with one or two fungicide treatments were used as reference materials. The severity of leaf, spike, and grain diseases, and grain colonization by fungi were evaluated using phytopathological and mycological methods. The number of fungal operational taxonomic units (OTUs) was determined by next-generation sequencing (NGS). In both years of the experiment, fungicides applied in the stem elongation stage (proquinazid, prothioconazole, and spiroxamine) and the heading stage (prothioconazole and tebuconazole), and BCA treatments applied three times reduced the prevalence of STB (by 55.56% and 35.45%, respectively) and the percentage of kernels with symptoms of infection (by 44.69% and 36.46%, respectively) relative to unprotected plants. Integrated fungicide and biological treatments (Integ) inhibited STB by 47.62%, but the percentage of infected kernels was only 24.71% lower than in the control treatment. When the first fungicide treatment was not applied (Fung 2), the effectiveness of protection against Z. tritici decreased in the second year of the study. Grain treated with BCA was colonized predominantly by D. hansenii, which reduced the number of OTUs of biotrophic (including Blumeria graminis ) and necrotrophic pathogens (including F. avenaceum ). The study demonstrated that the integration of fungicides with biological yeast treatments applied in the flowering stage is a promising approach to controlling diseases in organically grown durum wheat. • Fungicide and Debaryomyces hansenii treatments controlled Septoria tritici blotch • Fungicide and BCA treatments reduced the percentage of infected kernels • Grain treated with BCA was colonized predominantly by D. hansenii • D. hansenii reduced the number of operational taxonomic units of Blumeria graminis