Studies undertaken in 2002–2004 on ‘Golden Delicious’ apple fruits showed the presence of amino acids on the surface of their skin. Amount of total free amino acids ranged from 2.5 to 3.0 mg/L. In in vitro bioassays it was found that amino acids as a basic source of nitrogen did not activate germination of conidia of Peltaster fructicola (Johnson). However amino acids stimulated elongation of germ tubes and early hyphal growth. It was confirmed that fruit washings also contained sugars that stimulated both conidial germination and germ tube elongation. We proved also that different carbon to nitrogen proportions significantly affected P. fructicola growth. Proportions of C:N were evaluated according to newly introduced amino acid sugar index (ASI)