In vitro and field evaluation of chemical and biological products for chestnut brown rot control (caused by Gnomoniospis smithogilvyi) in Michigan

The chestnut brown rot (CBR) fungus, Gnomoniopsis smithogilvyi, is the main cause responsible for the decay of chestnuts and threatens the sustainability of the chestnut market all over the world. The pathogen infects the flowers in the field, with symptoms affecting nut quality at harvest time and during storage. For this reason, orchard management and fungicide applications are crucial factors to prevent infection in the flowers and later in the kernels. Michigan is the leading producer of commercial chestnuts in the USA. After the first isolation of the pathogen in 2016, CBR was identified in 80% of Michigan orchards. This study aims to compare the efficacy of different fungicides and biofungicides for the inhibition of G. smithogilvyi. For the in vitro assay, 15 products belonging to different FRAC groups were tested for fungicide activity against mycelium radial growth and spore germination. In the EC₅₀ evaluation, Inspire Super (difenoconazole 8.4% + cyprodinil 24.1%) had the highest fungicidal activity, followed by Tilt (propiconazole 41.8%) and Cevya (mefentrifluconazole 34.9%). For the field trials, fungicide applications with 6 different products were conducted for two consecutive years (2023 and 2024), with 2 and 4 applications between bloom and harvest. Sysstem Zn (zinc phosphonate 8%) was the most effective treatment based on qPCR assay for the pathogen detection, followed by Inspire Super and Tilt. These results will help the Michigan chestnut industry in preventing CBR infection pre-harvest and will encourage the rotation of fungicides.