Post-harvest spoilage of chestnuts (Castanea spp.) severely impacts the chestnut industry by lowering the quality and subsequent marketability of the nuts. A 2017 survey of Michigan (MI) chestnuts identified Gnomoniopsis smithogilvyi in association with rot symptoms. Gnomoniopsis smithogilvyi, the causal agent of chestnut brown rot (CBR) is characterized by light to severe brown lesions on the endosperm and the embryo and affects the quality and storage capacity of the nuts. Prior surveys in MI have only confirmed the presence of nut rotting saprophytes such as Penicillium spp. and Botrytis spp. DNA barcoding of the ITSrDNA region of the Gnomoniopsis-like cultures isolated from symptomatic nuts resulted in the identification of several genotypes of Gnomoniopsis spp., distinct from G. smithogilvyi. To complete Koch’s postulates we inoculated nuts from 3 cultivars of Castanea spp. (cv. Benton Harbor, Labor Day and Colossal) collected from trees grown in MI and Washington state with 2 strains of G. smithogilvyi. All nuts inoculated with G. smithogilvyi demonstrated CBR symptoms, however the percentage of the nut affected by rot was variable among the cultivars tested. A higher than expected level of rot occurred in control nuts, confounding informal identification of CBR versus other rots. Identification of other fungi from rotting nut tissue is currently under investigation. Understanding CBR caused by G. smithogilvyi and the various genotypes present in MI chestnuts will aide in the evaluation of potential pre- and post-harvest management strategies by the chestnut industry.