Aphids at crossroads: when branch architecture alters aphid infestation patterns in the apple tree

Plant architecture highly constrains pest infestation but is rarely considered in studies on plant–insect interactions. We analysed the relationships between apple tree architectural traits manipulated by tree training and within-branch development of Dysaphis plantaginea (rosy apple aphid, RAA), a major apple pest, during its multiplication wingless phase in spring. We hypothesised that the degree of branching had an effect on RAA within-branch infestation. In an experimental apple orchard, the infestation by aphid wingless forms was surveyed in two consecutive spring seasons within branches manipulated to design contrasted architectures differing in shoot numbers, shoot density and branching orders. Whatever the branch management system, aphid infestation was higher on long versus short, fruiting versus vegetative, and growing versus non-growing shoots. Either less infested shoots or less severe infestation were observed in the most branched system. A pattern of within-branch short-distance infestation was confirmed. Moreover, the number of branching points between two shoots exerted a high constraint on this infestation pattern. Beside possible trophic effects due to plant growth patterns already documented in the literature, a high degree of branching is likely to be a key-architectural trait to constrain within-branch aphid infestation. This opens new perspectives on the manipulation of branch architecture as a mean giving partial control of pests towards sustainable fruit production.     

Genetic determinism of anatomical and hydraulic traits within an apple progeny

The apple tree is known to have an isohydric behaviour, maintaining rather constant leaf water potential in soil with low water status and/or under high evaporative demand. However, little is known on the xylem water transport from roots to leaves from the two perspectives of efficiency and safety, and on its genetic variability. We analysed 16 traits related to hydraulic efficiency and safety, and anatomical traits in apple stems, and the relationships between them. Most variables were found heritable, and we investigated the determinism underlying their genetic control through a quantitative trait loci (QTL) analysis on 90 genotypes from the same progeny. Principal component analysis (PCA) revealed that all traits related to efficiency, whether hydraulic conductivity, vessel number and area or wood area, were included in the first PC, whereas the second PC included the safety variables, thus confirming the absence of trade-off between these two sets of traits. Our results demonstrated that clustered variables were characterized by common genomic regions. Together with previous results on the same progeny, our study substantiated that hydraulic efficiency traits co-localized with traits identified for tree growth and fruit production.     

Light interception in apple trees influenced by canopy architecture manipulation

Improvement of light penetration within tree canopies has been a constant objective of fruit tree architecture manipulation through the setting up of training systems. Recently, centrifugal training, i.e. the removal of fruiting shoots in the tree centre and on the underside of branches, has been proposed to improve fruit size and colour as well as return-bloom as compared to conventional solaxe-trained trees with equivalent crop loads. The present study was conducted to quantify the benefits of centrifugal training on light interception by the fruiting shoots via computer-assisted three-dimensional representations of foliage geometry. Data were collected on six 5-year-old apple trees cv.Galaxy, trained either with solaxe or centrifugal training systems, using an electromagnetic 3D digitiser. The 3D distribution of the foliage in the tree canopy was recreated by combining both the spatial locations of shoots (as measured from 3D digitising) and foliage reconstruction. Light interception efficiency properties of the trees were characterised by silhouette to total area ratio (STAR) values computed from images of the 3D mock-ups. Compared to the solaxe system, centrifugal training significantly improved the STAR of the whole tree by 20%. It also increased both leaf area and STAR of the fruiting shoots by approximately 15%, regardless of their position in the canopy. In this paper, we discuss the role of this enhanced light interception by the canopy in increasing the autonomy of the fruiting shoot, i.e. improved fruit size and colour, and return-bloom.