Official laboratory of Agricultural Institute of Slovenia confirmed a new harmful organism Gnomoniopsis smithogilvyi (sin. Gnomoniopsis castaneae; Ascomycota:Fungi) on sweet chestnut in Slovenia in October 2016. Based on grower's report an infection of orchard (1.3 ha, 12 years old trees, European x Japanese hybrid Precoce Migoule variety) was found in east Slovenia. Survey of an outbreak site resulted in finding of cankers and chestnut fruit rot in the vicinity. Later another orchard of marrons was found infected in west Slovenia. Pest has been neither regulated in Slovenia/European Union, nor recommended for regulation by regional plant protection organisation EPPO. National express pest risk analysis showed that risk of spread exists by movement of infected plants, by insects, including Dryocosmus kuriphilus, and by natural spread of ascospores. Pest is considered to be present in two areas in Slovenia, where marrons are grown. Official delimiting survey is planned for 2017.