Sclerotinia spp. a very destructive fungus causing 'white mould' became one of the biggest problems in sunflower breeding in the Republic of Macedonia in 2010. Field monitoring in the region of Bitola show very high infection of around 20-30%. Two types of symptoms where observed during the field monitoring. First symptoms were observed on the leaves of the infected plants in the form of wilting prior to flowering stage. The most characteristic symptoms were observed at the lower part of the stem in the form of a stem cancer. Big variable sclerotia in size and shape were observed inside the stem. The appearance of white mycelium on the infected lower parts of the plant was often observed during the wet weather. Other infected plants showed different symptoms. The stem was longer and thinner than in uninfected plants and the pit was very small around 9 cm. Sclerotia observed inside the stem were not bigger than 2.5 mm. In vitro investigations confirmed the presence of ascomycetes Sclerotinia sclerotiorum (Lib.) de Bary and Sclerotinia minor Jagger for the first time in the Republic of Macedonia. In vitro investigation of antifungal ability of Pseudomonas mediterranea Cattara et al. 2002 against S. sclerotiorum and S. minor showed that it can be a possible antifungal agent against these ascomycetes.