Combining Trichoderma koningiopsis and chitosan as a synergistic biocontrol and biostimulating complex to reduce copper rates for downy mildew control on grapevine

Plasmopara viticola, causal agent of downy mildew and one of the most severe grapevine diseases, is commonly controlled by multiple fungicide applications, primarily based on copper compounds in organic viticulture. Biostimulants and biocontrol agents could make a major contribution to pesticide reduction. A synergistic biocontrol and biostimulating complex consisting of Trichoderma sp. and chitosan was investigated to achieve copper fungicide reduction in the control of P. viticola. To ensure complex efficacy, the interactions and effects of all agents need to be investigated at the tri-trophic level of grapevine, downy mildew, and biocontrol and biostimulating complex. To guarantee the compatibility of Trichoderma sp. and chitosan, the chitosan tolerance of different copper-tolerant Trichoderma isolates was investigated in mycelial growth and germination tests. A T. koningiopsis isolate proved to be highly chitosan-tolerant was identified for further studies. The timing of spray application with T. koningiopsis and various chitosans was evaluated for efficacy against P. viticola. In the period 0 to 1 day before inoculation with the pathogen, the formation of the sporangia number was significantly reduced by the tested agents. To further describe the mode of action and efficacy of T. koningiopsis and chitosan against P. viticola, the application of single agents on upper and lower grapevine leaves inoculated with the pathogen in comparison to resistance inducers and copper fungicide were tested. Moreover, the combination of T. koningiopsis with different chitosans and a reduced copper rate was investigated on the respective leaf sides of greenhouse plants. Both, a direct and indirect mode of action against downy mildew was demonstrated. Regardless of the treated leaf side, sporulation of downy mildew was clearly inhibited compared to the untreated control. Moreover, microscopic images showed successful establishment of T. koningiopsis and uniform distribution of chitosan on leaves. The tested biocontrol and biostimulating complex might show a promising combatting tool against P. viticola while simultaneously reducing copper rate.