نوع مقاله : مقاله پژوهشی
عنوان مقاله English
نویسندگان English
Antixenosis resistance in apple rootstocks against the woolly apple aphid (Eriosoma lanigerum Hausmann) plays a pivotal role in reducing the economic losses caused by this pest. This study aimed to elucidate the mechanisms underlying antixenosis resistance and their association with root traits. The study was conducted over seven years, with the first three years (2017–2019) devoted to the production and preparation of plant materials and the subsequent four years (2020–2023) to resistance assessment. Ten hybrid genotypes derived from crosses between dwarfing native genotypes and commercial rootstocks (M9, M27, and B9) were evaluated at the Kamalshahr Research Station in Karaj, Iran. Biweekly sampling of aphid colonies was conducted, and indices including host preference, prevalence, colony aggregation, and habitat niche were evaluated. Factor analysis identified two principal components, namely host attractiveness and successful establishment (87.1% of the variance) and habitat limitation (12.5% of the variance). Hierarchical clustering categorized the rootstocks into four resistance groups: susceptible, semi-susceptible, semi-resistant, and resistant. M9op3 was identified as the most resistant genotype, exhibiting the lowest host preference index (0.63) and the highest unexploited habitat niche index (0.63). Positive correlations between root sucker density and host preference (r = 0.51), along with negative correlations between root elongation zone length and habitat potential (r = −0.59), confirmed the association of root traits with antixenosis resistance. The results emphasize the importance of integrating morphological root traits, particularly reduced sucker density and increased root elongation zone length, with ecological mechanisms in the development of resistant rootstocks. These findings provide a basis for breeding strategies aimed at selecting resilient genotypes, reducing pesticide use, and promoting sustainable horticulture.
کلیدواژهها English
Extended Abstract
Introduction
Apple (Malus domestica Borkh.) is a cornerstone of Iran's horticultural economy. However, its productivity is severely threatened by the woolly apple aphid (Eriosoma lanigerum), a pest responsible for significant economic losses. Antixenosis resistance, a non-preference mechanism that deters pest colonization, plays a pivotal role in mitigating aphid damage. Historically, breeding programs such as the Malling–Merton (MM) series have developed resistant rootstocks; however, evolving aphid biotypes and ecological variability continue to pose persistent challenges. This study aimed to elucidate the relationship between root morphological traits and aphid ecology to identify hybrid rootstocks with enhanced antixenosis resistance. By combining root architecture analysis with ecological indices, this research sought to promote sustainable apple cultivation by reducing reliance on pesticides and enhancing rootstock resilience. The objectives were to evaluate host preference, colony aggregation, and habitat niche utilization across ten hybrid rootstocks derived from crosses between dwarfing Iranian native genotypes (e.g., Azop and AZ) and commercial rootstocks (M9, M27, and B9). The study also examined how root traits, including density, length, and the extent of the rhizogenic zone, influence aphid behavior, thereby providing a framework for future breeding strategies.
Materials and Methods
The seven-year study (2017–2023) was conducted at the Kamalshahr Research Station in Karaj, Iran, under controlled environmental conditions. Ten hybrid rootstocks were evaluated, including crosses between dwarfing Iranian native genotypes (Azop and AZ) and commercial rootstocks (M9, M27, and B9). Aphid colonies were sampled biweekly across the root, collar, trunk, and branch zones using non-destructive counting methods to minimize disruption to natural aphid behavior. Key resistance indices, including the Colony Preference Index (C-Aphis), Prevalence Index (PI), and Unexploited Habitat Niche (zi), were calculated to quantify host preference, aphid prevalence, and ecological constraints, respectively. Root morphological traits, including root density, length, rhizogenic zone extent, and rooting percentage, were measured using standardized protocols. Statistical analyses included Principal Component Analysis (PCA) to identify latent resistance factors and hierarchical clustering to categorize rootstocks according to resistance levels. Data normality was assessed using the Kolmogorov–Smirnov test, and correlations between root traits and aphid responses were analyzed using SPSS v27. The experimental design incorporated replication across multiple growing seasons to account for environmental variability.
Results and Discussion
Principal Component Analysis revealed two dominant factors explaining 99.6% of the total variance in resistance: Host Attractiveness and Colonization Success (87.1% of the variance) and Habitat Limitation (12.5% of the variance). The first factor was strongly correlated with indices such as C-Aphis (loading = 0.993) and PI (loading = 0.993), emphasizing the role of rootstock attractiveness in aphid colonization. The second factor, associated with zi (loading = 0.997), highlighted ecological constraints limiting aphid exploitation. Cluster analysis grouped the rootstocks into four resistance categories: susceptible, semi-susceptible, semi-resistant, and resistant. Highly susceptible genotypes, such as Azop2-85, exhibited a high C-Aphis value (0.59) and a low zi value (0.59), indicating limited ecological deterrence, whereas semi-susceptible (e.g., B9op) and semi-resistant (e.g., AZ*M27) rootstocks showed incremental improvements. The most resistant genotype, M9op3, exhibited the lowest C-Aphis value (0.63) and the highest zi value (0.63), suggesting strong antixenosis resistance through reduced host attractiveness and enhanced ecological barriers.
Correlations among root traits and aphid responses revealed that greater root density was positively associated with aphid preference (r = 0.51, p < 0.01), possibly due to enhanced vascular access, whereas greater root elongation zone length was negatively correlated with habitat exploitation (r = −0.59, p < 0.01), suggesting that structural characteristics may limit aphid habitat utilization. Shorter root lengths were also associated with reduced habitat potential (r = −0.59, p < 0.01), consistent with the compact root architecture observed in M9op3. These findings are consistent with previous studies emphasizing the role of root morphology in pest resistance, in which sparse feeder roots may limit nutrient access and compact rhizogenic zones may reduce the availability of chemical attractants. The integration of ecological metrics such as zi provides a novel framework for evaluating resistance, complementing traditional indices and improving the precision of rootstock selection.
Conclusions
This study demonstrates that antixenosis resistance is a multifactorial trait shaped by root morphology and ecological dynamics. The resistant rootstock M9op3, characterized by low root density and compact rhizogenic zones, effectively deterred aphid colonization by restricting access to vascular tissues and minimizing chemical attractants. Breeding programs targeting root architectural traits, such as sparse feeder roots and limited rhizogenic zones, may enhance resistance and provide a sustainable alternative to pesticide-dependent pest management. The integration of ecological metrics, such as zi, offers a novel approach to resistance evaluation, supporting precision in rootstock selection and addressing challenges associated with evolving aphid biotypes. The practical implications include reduced reliance on pesticides and greater alignment with Integrated Pest Management (IPM) goals. Future research should investigate the biochemical role of root exudates in aphid behavior and evaluate rootstocks under abiotic stresses, such as drought, to ensure climate resilience. By bridging agronomy and ecology, this study contributes to efforts toward sustainable horticulture and provides practical insights for developing aphid-resistant apple cultivars while balancing productivity and environmental stewardship.
Author Contributions
All authors contributed equally to the development of this manuscript from the initial draft to the final submitted version, including all subsequent revisions.
Data Availability Statement
Data available on request from the authors.
Acknowledgements
This research was supported by grants from Agricultural Biotechnology Research Institute of Iran (ABRII) Project No. 3-33-99-046992.
Ethical considerations
The authors avoided data fabrication, falsification, plagiarism, and misconduct.
Conflict of interest
The author declares no conflict of interest.
