How can an in vitro incompatibility of Trichoderma-based products and herbicides impact the parasitism and control of white mold (Sclerotinia sclerotiorum (Lib.) De Bary)?

The integration of management methods for both diseases and weeds depends on the compatibility between the tools. Biological control represents an important strategy to cope with the integrated management of white mold (Sclerotinia sclerotiorum) through parasitism of sclerotia. However, its application in the field is more cost-effective if combined with the herbicide in a tank mix, as long as the products are compatible. Therefore, we aimed at (i) evaluating two compatibility test methodologies (constant exposure and different times) and (ii) two soybean crop seasons to infer the compatibility of Trichoderma-based products. In vitro bioassays were performed to assess the compatibility between herbicides (Haloxifope-p- methyl, Glyphosate N-ammonium salt, Fluasifope-p-butyl, Fomesafem, Chlorimuron ethyl and Imazapyc + Imazapyr) and two biocontrol agents (Trichoderma asperellum and Trichoderma harzianum). Thus, the recommended spray volume for each herbicide was added to the PDA culture medium (Potato-Dextrose-Agar) and then deposited in the center of the plate a disc of mycelium from each antagonist isolate (constant exposure). The tests with time of exposure were marked at times 0, 2, 4, 8 and 16 h (simulating tank mixing) and at the spp of each time were plated on PDA medium. For both tests, the mycelial growth and conidiogenesis of Trichoderma spp. were observed. The combination of herbicide and biocontrol was also tested in the field and sclerotia parasitism, white mold incidence and plant yield were assessed in two field trials. The constant exposure of the antagonists to herbicides revealed that no herbicide was compatible with the T. asperellum or T. harzianum. While in test, exposure time exhibited compatibility with either T. asperellum or T. harzianum, within a period of 2 to 8 h. Conclusively, the integration of biocontrol agents with the herbicide imazapique + imazapyr exhibited significant reductions in white mold disease incidence and conidia germination, along with effective parasitism of S. sclerotiorum and even the least compatible herbicide (glyphosate) resulted in significant reduction in the disease incidence and sustained yield when compared to the untreated control. Therefore, the integration of the biocontrol agent for white mold should always be considered, and the tank mixing of it with the herbicide represents a cost-effective alternative for the grower.