Change in susceptibility in the pathosystem of wheat-biotrophic pathogen Blumeria graminis f. sp. tritici (syn. Erysiphe graminis), a causative agent of powdery mildew of wheat, under the action of root treatment of wheat seedlings with various concentrations of abscisic acid (ABA) before and after infection was investigated. It is shown that the magnitude and direction of the effect of exogenous ABA on the number of colonies of the pathogen depends on its concentration and the time of application relative to the moment of infection. Concentration dependence for ABA was variable in form, could be inhibitory and have a minimum or stimulating with a maximum number of colonies at similar concentrations. At the same time, the earlier use of ABA relative to the time of infection was more likely to be inhibitory. Non-monotonicity and variation of the form of concentration dependence probably can explain the ambiguity in the literature in determining the immunomodulatory properties of ABA. In the natural environment, various abiotic stresses induce transit peaks of ABA concentration of different intensity and different dynamics. Their combination with epiphytoty can give a different result of infection. The complex nature of the concentration dependence and the variation in it within a fairly wide range of the immunological state seem to ensure the maintenance of equilibrium in the pathosystem and the chances for survival of both the host plant and the pathogen.