Field evaluation of chemical and biological products to control chestnut brown rot caused by Gnomoniopsis smithogilvyi in Michigan

Abstract

Gnomoniopsis smithogilvyi causes chestnut brown rot (CBR) which is responsible for the decay of chestnuts and is threatening the sustainability of the global chestnut market. The pathogen infects the flowers in the field, with symptoms affecting nut quality at harvest time and during storage. For this reason, cultural and chemical management is crucial to prevent infection in the flowers and later in the kernels. Michigan is the leading producer of commercial chestnuts in the United States. 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 alternative fungicides to control CBR in orchards. Applications with six different active ingredients (propiconazole, tebuconazole, difenoconazole + azoxystrobin, potassium phosphonate, zinc phosphonate and Bacillus subtilis) were conducted for two consecutive years in 2023 and 2024, with two and four crown sprays between bloom and harvest. Zinc phosphonate was the most effective treatment based on qPCR assay for the pathogen detection, together with difenoconazole and propiconazole. No differences were found between two and four applications during the season, supporting the hypothesis of field applications during bloom as the most efficient strategy to control CBR. Chestnut fruits were healthy for human consumption, with no fungicide residues at harvest time. According to our findings, growers may use the selected fungicides (such as propiconazole and difenoconazole) and alternative fungicides (like K or Zn phosphonates) to complement their field practices for CBR control, encouraging the rotation between different active ingredients.