Irrigation scheduling is an important issue for crop management, in a general context of limited water resources and increasing concern about agricultural productivity. Methods to optimize crop irrigation should take into account the impact of water stress on plant growth and the water balance in the plant-soil-atmosphere system. In this article, we propose a methodology to solve the irrigation scheduling problem. For this purpose, a plant-soil interaction model is used to simulate the structural-functional plant growth conditioned by water status. The system dynamics is driven by a delay differential system. By considering a price for the crop yield and for the water resource, an optimal control problem can be formulated in order to find the optimal irrigation strategy. The solution is obtained by dynamic programming. In order to handle the delay term of the system due to the continuous mechanism of plant senescence and the curse of dimensionality, the iterative approach of dynamic programming is used.