MISLEADING ZINC DEFICIENCY DIAGNOSES IN POME FRUIT AND INAPPROPRIATE USE OF FOLIAR ZINC SPRAYS

Abstract

Many researchers and field managers consider Zn the most limiting micronutrient for tree fruit production. A threshold concentration of 18-20 ppm in summer shoot leaves is considered adequate in most world fruit growing regions. In a foliar analysis survey of high yielding pear orchards, the mid summer Zn concentrations varied from 12 to 16 ppm. This suggests that either current threshold values for shoot leaves are inappropriately high (less Zn is required) or that mid summer leaf samples do not adequately reflect the Zn status of the tree. In addition to the problems associated with the interpretation of leaf analyses, considerable confusion exists about the most effective timing for Zn sprays. Zinc is recognized as an immobile nutrient. This suggests that movement of Zn from sprayed leaves to storage tissues in the fall and remobilization into new growth in the spring is unlikely. However, some fertilizer guides recommend post harvest fall sprays to alleviate Zn deficiencies. Dormant Zn sprays are also recommended. In a series of experiments carried out using both unlabelled Zn and the stable isotope Zn68, it was demonstrated that neither dormant nor fall applications contribute significantly to the Zn content of newly developing tissues in the spring. Therefore, these application practices are inappropriate. A spring Zn spray clearly affects the contacted leaves, even though very little moves from the sprayed leaves to new organs. Spring applications are the only effective tool to incorporate moderate amounts of Zn into the targeted organs. However, excessive applications of foliar sprays, particularly Zn, often damage fruit. Zinc formulations alone or in combination with other products are dangerous for some varieties under specific weather conditions. Although foliar applications can be a powerful tool under certain circumstances, the prescription must be technically justified.