Effects of plant growth regulators,foliar fertilizers,and bagging on fruit russeting and transverse diameter of Ruixianghong apple

【Objective】Ruixianghong is a new, high-quality, late-maturing apple cultivar widely recognized for its desirable attributes, including a tall fruit shape, vibrant coloration, and good storability. However, it is highly susceptible to fruit rust during development, which leads to surface russeting, flavor deterioration, and severely constrains its commercial value. Fruit rust is characterized by abnormal epidermal cell structure and is closely associated with cuticle development defects, environmental stress, and horticultural practices. Targeted and precise control techniques for this cultivar are not yet fully developed. This study used 3-year-old Ruixianghong/M9-T337 apple trees to systematically evaluate the effects of plant growth regulators (PGRs), foliar nutrients, and bagging treatments on fruit rust index and fruit diameter. The aim was to determine the optimal application parameters and synergistic mechanisms of these measures, establish an effective control system, and ultimately improve fruit surface cleanliness and the marketable fruit rate.【Methods】The experiment was conducted from April to November 2024 in a Ruixianghong apple orchard in Luoning, Henan Province. A single-factor comparative design was implemented with three main treatment categories: (1) PGR treatments, consisting of six concentration gradients of GA4 + 7 (2.5-40 mg · L- 1 ) and seven combinatorial formulations of GA4 + 7 with CPPU or 6-BA; (2) Foliar nutrient treatments, including three single agents—humic acid (Pulvtong), Ascophyllum nodosum extract (Youdelie), and SiO2-based fertilizer (Shilejie)—and four of their combinations with GA4+7; (3) Eight fruit bag treatments of different materials, such as yellow-black laminated paper with red waxed paper and yellow-black laminated paper with red waxed paper. The impact of sunburn variation between east and west orientations on rust formation was also analyzed. Observation samples were selected via stratified sampling. Periodically measured indicators included the fruit rust index (calculated with reference to an area-based grading standard), fruit diameter (measured with a digital electronic vernier caliper), sunburn incidence, and sunburn-associated rust rate. All data were analyzed using one- way analysis of variance (ANOVA) in SPSS 24.0, with significant differences assessed by Duncan's multiple range test (P＜0.05).【Results】The effects of different treatments on rust control and fruit development varied significantly, demonstrating clear concentration and combinatorial effects. Among GA4+7 single-agent treatments, a concentration of 40 mg · L-1 was optimal, reducing the fruit rust index by 38.9% and increasing fruit diameter by 3.5% to 71.06 mm at 180 days after full bloom compared to the control, while lower concentrations (2.5- 10 mg · L- 1 ) showed ineffective. Among combinatorial PGR treatments, GA4 + 7 combined with 1 mg · L- 1 CPPU significantly increased the rust index to 23%, whereas its combination with 10 mg ·L-1 6-BA promoted fruit expansion (transverse diameter 75.03 mm) but weakened rust control. Among foliar nutrients, the single application of 20 mg · L- 1 Shilejie (20% SiO2) was most effective, reducing the rust index by 35.3%. Its combination with GA4 + 7 (Treatment D4) increased the rust index to 16% but resulted in a significantly larger fruit transverse diameter (70.98 mm). Among bagging treatments, the yellow-black laminated paper with red waxed paper double-layer bag (T2) was most effective, yielding a rust index of only 5.31% (a 44.4% reduction from the control), with no significant difference in the 20-day color transition period after bag removal or in final fruit diameter compared to non-bagged fruit. Orientation analysis revealed that fruits on the west side had significantly higher sunburn incidence and associated rust rates due to intense solar radiation than those on the east side. Application of 40 mg ·L-1 GA4+7 reduced sunburn incidence on the west side by 34.8% and the associated rust rate by 34.2%.【Conclusion】Effective control of fruit rust in Ruixianghong apples requires an integrated strategy of“chemical regulation, physical protection, and orientation management.”Foliar application of 40 mg·L-1 GA4+7 during fruit development optimizes epidermal structure, while combined use with 20 mg ·L- 1 Shilejie enhances stress resistance by thickening the cuticle. Employing yellow-black laminated paper with red waxed paper bags achieves the dual objectives of significantly reducing the rust index and supporting normal fruit development. Supplementary shading measures are necessary for fruits on the west side to mitigate radiation stress. This study clarifies the optimal parameters and mechanisms of various control measures, addresses the limitations of single-method approaches, and provides a scientific basis for the standardized production and rust control of Ruixianghong apples.