<i>In vitro</i> evaluation of chemical and biologically-based fungicides against <i>Gnomoniopsis smithogilvyi</i> the causal agent of chestnut brown rot

<i>Gnomoniopsis smithogilvyi</i> is the causal agent of brown rot, one of the main diseases of chestnuts that is threatening the sustainability of the global market. Michigan is the leading producer of commercial chestnuts in the United States. After the first isolation of <i>G. smithogilvyi</i> in 2016, chestnut brown rot was identified in 80% of Michigan orchards. The pathogen infects the flowers in the field, with symptoms affecting nut quality at harvest time and during storage. For this reason, fungicide treatments are crucial to prevent infection in the flowers and later in the kernels. This study aims to compare the in vitro efficacy of fifteen different chemical and biologically based fungicides against <i>G. smithogilvyi</i>. The fungicide potential of each product was evaluated for mycelial radial growth and conidial germination by calculating the EC₅₀. FRAC (Fungicide Resistance Action Committee) group 3 fungicides showed the best fungicide activity for both mycelial growth and conidial germination. However, biologically-based fungicides containing <i>Bacillus</i> spp. had no significant differences from FRAC 3, 7, and 11 fungicides. Phosphonates had a statistically lower fungicide activity on conidial germination compared to FRAC 3, 7, 11 and BM02, but same activity against mycelial radial growth. Further investigations should be done to evaluate the potential efficacy of these products in the orchard, considering that there is not always a correlation between <i>in vitro</i> and <i>in planta</i> activity. Our findings will help chestnut growers select chemical and biologically-based fungicides to complement their field practices for chestnut brown rot control, encouraging the rotation between different active ingredients.