Phenological growth phases of dwarf apple trees: coding and description based on the BBCH scale

BACKGROUND: The BBCH scale (Biologische Bundesanstalt, Bundessortenamt und Chemieindustrie) is a fundamental tool for standardizing plant phenological observations. As apple production shifts towards high-density dwarf orchard systems to enhance yield and facilitate mechanization, a significant gap exists in a unified phenological description framework for dwarfing rootstock apple trees, particularly under desert climate conditions. This study aims to systematically characterize the phenological growth stages of dwarf-rootstock apple using an extended BBCH scale to support precision orchard management. RESULT: In our study, phenological monitoring was carried out systematically through the utilization of optical observation equipment and manual observations throughout the entire growth cycles. The extended BBCH scale with a three-digit coding system was applied, where the first digit indicates the principal growth stage (0-9), the second digit represents a mesostage (1 for spring, 2 for autumn growth), and the third digit describes secondary stages (0-9). We described and illustrated eight main growth stages: bud development (stage 0), leaf development (stage 1), shoot development (stage 3), inflorescence emergence (stage 5), flowering (stage 6), fruit development (stage 7), fruit ripening period (stage 8), and senescence and beginning of dormancy (stage 9). A total of 43 secondary stages were defined. Crucially, mesostages were introduced to differentiate the distinct spring and autumn vegetative growth flushes characteristic of the bimodal growth pattern observed under desert conditions. CONCLUSIONS: The developed three-digit BBCH scale offers a standardized and refined framework for monitoring apple phenology in high-density dwarf orchards. It serves as a vital tool for optimizing the timing of key agronomic practices like irrigation, fertilization, pruning, and pest control, thereby supporting the intelligent implementation of precision agriculture. This framework effectively standardizes phenological observations across diverse environments, laying a foundation for improved yield, fruit quality, and sustainable orchard management.