Biocontrol challenges in field condition: streptomycetes in the battle against Fusarium infection and mycotoxins

Among the mycotoxins produced by Fusarium species, deoxynivalenol (DON) is probably the most common contaminant of cereal grains worldwide. Furthermore, the emerging Fusarium mycotoxin enniatin B (ENB) obtained the attention of the European Food Safety Authority regarding its potential interactions with other mycotoxins leading to possible negative synergistic effects on both human and animal health. Control strategies and efficient methods for mycotoxin containment are needed and beneficial microorganisms could be a promising and environmentally friendly strategy. Two Streptomyces spp. strains were previously screened for their ability to reduce Fusarium head blight (FHB) by F. graminearum [1, 2] and DON production in vitro on grains [3]. According to our previous results, the growth of the two streptomycetes in liquid culture was not affected by either DON or ENB, even at the highest dose of exposure (88 µg/mL and 50 µg/mL, respectively). Additionally, after ENB exposure, no changes in the metabolite profile of one of the strains were detected, further suggesting the weak impact of ENB on our bacterium. Given the future application, in this work we explored the Streptomyces spp. strains' efficacy in field conditions by using F. graminearum and F. avenaceum as producers of DON and ENB, respectively. Therefore, a field experiment was conducted on common wheat (Triticum aestivum “A416”) in 2024 in northern Italy in a randomized design. During wheat anthesis, the spikes were inoculated with bacterial spores in combination with the conidia of Fusarium graminearum and Fusarium avenaceum. Three weeks after infection, the two Streptomyces spp. strains reduced significantly the severity of F. graminearum disease in wheat by 44% (p< 0.001) and 31% (p< 0.05), respectively. On the other hand, the FHB symptoms caused by F. avenaceum did not show a significant reduction by our bacteria. Additionally, performing the simultaneous infection of both F. graminearum and F. avenaceum in wheat, both Streptomyces spp. strains reduced the severity of the disease, but not significantly. The results suggest that the presence of F. avenaceum, as a major producer of ENB, may interfere with the efficacy of our strains in the field, diverging from the in vitro results on interaction between ENB and the two Streptomyces spp. strains. This highlights the challenges and complexities of field trails, where multiple factors can co-exist. Further investigations are needed to understand the role of F. avenaceum and its secondary metabolites during the infection, potentially including the plant response, to better assess the effectiveness of biocontrol agents in field conditions.