A formal model was built in the discrete event paradigm 1) to test the feasibility of the spraying strategy GrapeMilDeWS, 2) to facilitate its transfer and usability, 3) to build a simulator that would allow testing climatic scenarios and scalability of the solution at the estate level. The knowledge elicitation process was made incrementally with the 4 expert designers. The elicitation method uses the graphical language of statecharts as a mediation tool between the expert and the knowledge engineer. Statecharts are both a graphical language (a graph) and a mathematically formal language. We present here the design principles that guided the experts in their design, how these principles have been formalized, the knowledge elicitation methodology that we set up and the questions that arouse from questioning formally the spaying strategy.