This work analyzes a therapy planning problem in an agricultural domain in which where there are no standard treatment protocols. For this reason, the skeletal planning method cannot be applied, and the use of an abductive method is therefore proposed. This work is part of a broader project, the main goal of which is the development of a knowledge-based system for phytosanitary advice. For this knowledge-based system, the CommonKADS methodology has been selected. In order to apply the method, we present a general formalization of therapeutic problems that is domain independent, and afterwards we show how to find solutions for this problem using a treatment task decomposition.