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We studied the role of phytohormones: zeatin, kinetin, and abscisic acid, in the regulation of development of the conidial inoculum of Erysiphe cichoracearum DC. f. phlogis Jacz. and E. graminis DC. f. hordei Marchal. When the pathogen conidia were in direct contact with phytohormones, the intensity of their germination significantly increased. In the presence of cytokinins, the amount of normal appressoria decreased and that of abnormal growth tubes increased. On the phlox leaves treated with cytokinins, the intensity of germination of the conidia increased, as compared to the control, while abscisic acid exerted the opposite effect. The treatment of barley leaves with cytokinins did not affect markedly the development of conidial inoculum, as compared to the control, while abscisic acid significantly decreased the intensity of germination of the conidia. On the leaves of different Phlox species, the degree of germination of the conidial negative correlated with their resistance against the powdery mildew. The role of cytokinins in pathogenesis of biotrophic fungi is discussed.