Seasonal Variations in Copper, Molybdenum, and Zinc in Apple Leaves and Their Contents in Apple Fruits Under Microbial Fertilizer Application

Sustainable agriculture aims to reduce environmental impacts and enhance the use of renewable natural resources to improve soil fertility and crop productivity. Mineral fertilizer application is a major source of environmental harm, and the intensification of apple production has led to fertilizer rates that may pose ecological risks. Microbial biofertilizers offer a promising means to partially replace chemical inputs, supporting the transition to sustainable horticulture. However, fertilizer efficiency depends on multiple factors, including orchard management systems and the genetic characteristics of scion–rootstock combinations. To evaluate the effects of microbial fertilizers (MFs) on the seasonal dynamics of leaf copper (Cu), molybdenum (Mo), and zinc (Zn) in different apple scion–rootstock combinations under conventional and high-density orchard systems, we established a field experiment in two commercial orchards (total area 2 ha). Azafok application resulted in the highest leaf Cu concentrations in both scion–rootstock combinations across phenological stages (5.54–6.39 mg kg−1). Azafok also exerted a prolonged effect on leaf Zn in cultivar (cv.) ‘Rozhdestvenskoye’ grafted onto rootstock B396 in the high-density orchard (15.30 mg kg−1). The highest leaf Zn in cv. ‘Veteran’ on rootstock B118 was recorded after Enzymocid application (19.97 mg kg−1). For the ‘Rozhdestvenskoye’/B396 combination, the highest seasonal leaf Mo was observed with Azafok (0.635 mg kg−1), whereas in the ‘Veteran’/B118 combination, it was highest following Nitragin application (1.079 mg kg−1). Fertilizer application did not elevate fruit nutrient concentrations beyond permissible safety limits; all values remained within the normal ranges for the respective cultivars. To ensure stable MF effects, microbial consortia and strains with the most consistent and broad affect plant nutrition and status should be selected. Azafok and Enzymocid demonstrated strong antifungal effect in both the ‘Rozhdestvenskoye’/B396 and ‘Veteran’/B118 plantings. The antifungal effect of Nitragin was more pronounced in the ‘Rozhdestvenskoye’/B396 orchard.