The apple tree and its pests – The dynamic and structural components of the tree’s architecture influence the dynamics of infestation and infection

As part of our research into the long-term resistance of apple trees to pests, our work has focused on powdery mildew (Podosphaera leucotricha (Ell. et Ev.) Salmon) and the grey aphid (Dysaphis plantaginea (Passerini)). Controlling these two pests is essential in orchards to maintain high-quality production. The effects of two components of tree architecture were investigated on two cultivars from INRA’s breeding programmes, ‘Pitchounette’ and ‘Ariane’. The first factor is shoot growth dynamics, primarily in relation to leaf susceptibility; this is likely linked to a trophic aspect (the attractiveness of the leaves to aphids) and/or the development of ontogenic resistance in leaves that have completed their expansion phase (powdery mildew). The second factor is the structure of the branch, which influences its colonisation by the grey aphid. Our results confirm the difference in susceptibility to pests generally observed between the two cultivars studied (‘Pitchounette’ being less susceptible than ‘Ariane’). Generally speaking, tree management has little effect on the epidemiology of powdery mildew, whilst it significantly alters infestation by the grey aphid. For powdery mildew, we confirm the importance of ontogenic leaf resistance: young leaves are more susceptible to secondary infections than older leaves. For the grey aphid, the order of branching – which is positively correlated with the number of intersections between two adjacent growth points – is an important factor in explaining the significant effect of tree training on infestation dynamics. In conclusion, this study confirms the fundamental role of genetic selection in the search for cultivars that are low-susceptible or resistant to pests. Tree management is one of the factors that can partially modulate the dynamics of pest infestation and infection