Seasonality in Apple Leaf Molybdenum Contents During the Growing Season and Stages of Greatest Need in This Nutrient

Molybdenum (Mo) is an important nutrient participating in nitrogen, sulfur, iron, and copper metabolism, as well as a cofactor in the biosynthesis of auxin and abscisic acid. Apple leaf Mo seasonality and periods of apple tree demand remain unclear due to insufficient data. This study aimed to identify peak leaf Mo levels throughout the growing season to distinguish when apple trees require the most Mo. To analyze seasonal patterns, we determined the leaf Mo contents 11–12 times per season on untreated and Mo-treated apple trees over four seasons in 2020–2023. Foliar fertilizing stimulated a significant increase in leaf Mo status. The Mo contents in untreated tree leaves were significantly lower than in trees after foliar fertilizing. However, in fertilized trees, nutrient contents were low after ten-day periods of heavy precipitation. High leaf Mo levels coincided with periods of intense apple shoot and fruit growth (average Mo level of 0.305 mg kg−1 in untreated trees and 0.386 mg kg−1 in fertilized ones) and apple fruit development and ripening (average Mo level of 0.754 mg kg−1 in untreated tree leaves and 1.069 mg kg−1 in fertilized ones). Mo application via foliar fertilizing in July’s heat is essential for apple tree fruit growth and development to counter negative weather effects. Foliar fertilizing significantly increased tree yields in 2020, 2022, and 2023, resulting in substantially higher overall productivity (169.7 kg/tree−1 without vs. 215.6 kg/tree−1 with fertilizer).