Response of Huanglongbing-affected Citrus Trees to Zinc Fertilization under Microsprinkler Irrigation

Abstract

Zinc plays a key role in structural, regulatory and catalytic cofactor of many enzyme reactions. Confused symptoms of citrus greening (also called Huanglongbing, HLB) disease with zinc (Zn) include small yellow leaves and mottling. Although enough Zn may be present in the soil, deficiency symptoms can develop due to soil depletion or formation of insoluble Zn compounds. This study investigates the response of HLB affected citrus trees to zinc fertilization using various application method and rates. Treatments include three nitrogen (N) rates at 168, 224, and 280 kg·ha-1 N as main plots and 4 subplots including control (no Zn fertilization), foliar applied Zn at = 5.6 and 11.2 kg·ha-1 Zn and soil applied Zn at 5.6 kg·ha-1 Zn. Results of a 2-year study showed that Zn concentration of the plot supplied with highest nitrogen (N) rate was significantly higher (P < 0.0001) than the plots supplied with the two lower N rates. Foliar treatment showed 110% significant increase in concentration of Zn in the leaf compared to soil treatment. High sorption coefficients (KD) were observed at 0–15 cm and 15–30 cm soil depth while 30–45 cm and 45–60-cm soil depths showed no or very negligible sorption. Zn concentration in the leaf tissue was significantly higher in summer compared to fall. Thus, foliar application of Zn at any of the rates improved the Zn concentration in the leaves above the deficiency level and low critical concentration level while soil application optimized Zn availability in the soil for root uptake.