The research was carried out in the laboratory of immunity and molecular marking of the All-Russia Research and Development Institute of Oil–Producing Crops. During three years (2015–2017) 40 lots of sunflower seeds obtained in the Volgograd region were analyzed for the presence of pathogenic microflora. The stem blight pathogen in sunflower seeds was identified and its species was determined. It is a fungus of genus Phoma Sacc. – Phoma macdonaldii Boerema, teleomorph Leptosphaeria lindquistii Frezzi. The illustrations of picnidia and pycnospores formed on sunflower seeds are presented, as well as their morphological features confirming the species of the fungus according to the classification of Boerema et al. (2004). The presence of reproductive organs of Phoma macdonaldii on the achenes means that infected sunflower seeds contribute to the spread of stem blight.