Chestnut blight is one of the most destructive diseases on species of Castanea genus (Fagaceae). The causal agent of the disease is the ascomycete fungus Cryphonectria parasitica (Valsaceae). In the period 2018-2020, chestnut blight was found on both European (sweet) chestnut (Castanea sativa) and sessile oak (Quercus petraea) trees in mixed chestnut-oak stands planted in the region of State Forest Enterprise (SFE) Simitli (Pirin Mt.) and SFE Petrich (Belasitsa Mt.). The present study provides new data on the first detection of damage caused by Cryphonectria parasitica on Quercus petraea plantations in southwestern Bulgaria. The frequency and impact of the disease, the variety of vegetative compatibility types of the pathogen, as well as the molecular identification and mating type of the pathogen from oak samples were characterized.