Influence of Foliar Cu Application Rate on Copper, Potassium and Zinc Concentrations in Soil and Citrus Tissues.

Due to its intensive use in citrus groves to control foliar diseases, copper (Cu) concentrations have increased in soils and plant tissues. Copper is absorbed by plants via roots and their exudates or through leaf after foliar applications. The objective of this study is to determine the optimal range in foliar concentrations of Cu to improve growth of greening affected sweet orange (Citrus x sinensis (L.) Osbeck) and the impact of excessive nutrients on both affected and non-affected citrus trees. Two sets of citrus tree kept free of the disease were evaluated for Cu application rates. All trees tested for presence of the disease by PCR and transplanted into large containers. Copper application rates included: 0 x, 0.5x, 1x and 2x per tree replicated three times applied every 3 weeks from July through October each year. Collected data included leaf, roots, and soil samples. The measurements conducted at mid-summer (July to August) and fall when trees with greening tend to have lower leaf mass (October to December). A mixed blend fertilizer with a total yearly rate equivalent to 165 g K per tree was applied. Total Cu, K and Zn concentrations were determined on dried leaves, roots and soils samples via Inductively Coupled Plasma (ICP). Results showed significant variation in Cu, K and Zn concentrations in both soils and citrus tissues with higher Cu rates. Cu concentration increased by 9, 7.2 and 1.2 times under 2x Cu rate compared to 0x Cu rate in soil, leaves and roots, respectively. Root K and Zn concentrations declined by 60% and 16% when Cu rate increased from 0.5 to 2x, respectively. Also, total K and Zn in citrus leaves declined by 13% and 32% when Cu rate increased from 0.5 to 2x rate, respectively.