The behaviour of the model and the interactions at the single-vine scale was considered an
essential first step prior to incorporating the role of the host, i.e. crop structure and heterogeneity in vine vigour into powdery mildew invasion at larger scale (Calonnec et al., 2005; Tessieret al., 2006). Because of the complexity involved in the accurate description of the geometry of cluster development (Fanizza & Colonna, 1996) and the high correlation between damage on bunches and the amount and dynamics of the disease on leaves, the spread of disease was restricted to the leaf population only.