Apple resistance to arthropod herbivores: genetic basis and modification by environmental factors

Abstract: Arthropod herbivores reduce the quantity and quality of apple yield. Resistant apple varieties hold promise to increase the sustainability of pest management in orchards, but little is known on the genetic basis of apple resistance to most arthropod herbivores. Knowledge on the apple genome and QTL (quantitative trait locus) analysis is now facilitating the identification of gene regions associated with resistance. 160 F1-progeny plants of a cross of the apple varieties ‘Fiesta’ and ‘Discovery’ were surveyed at three different sites in Switzerland. Herbivore infestation per genotype as a measure of resistance was quantified for the apple aphids Dysaphis plantaginea, Dysaphis cf. devecta and Aphis pomi, the apple rust mite Aculus schlechtendali, and the codling moth Cydia pomonella. The influence of the environmental factor ‘drought stress’ on apple resistance to a chewing and a sap-feeding herbivore (caterpillar; aphid) was studied in laboratory experiments considering different intensities of pulsed drought stress. Significant QTLs for resistance to D. plantaginea, D. cf. devecta, A. schlechtendali, and C. pomonella were detected. SSR alleles associated to the QTLs may be applied to identify and breed resistant apple cultivars. Environmental factors such as within-canopy variation in climate, and neighbourhood-effects affected herbivore distribution in the field. In the laboratory, pulsed drought stress resulted in non-monotonic resistance responses of apple trees. Low-stress plants showed the highest and high-stress plants the lowest resistance. The studies revealed the genetic basis of apple resistance to different arthropod herbivores and the modifying influence of environmental parameters that may impede QTL detection.