Effect of Growing Media pH on Performance of Huanglongbing-Affected Young Citrus Trees

Faisal Shahzad,Tripti Vashisth,Davie M Kadyampakeni

doi:10.3390/agronomy11030439

Abstract

Since the advent of Huanglongbing (HLB) disease, Florida’s citrus production has dropped by 70%. HLB-affected trees decline rapidly under high pH growing conditions. Limited nutrient availability at high soil pH has been speculated to be the cause of such rapid decline in HLB-affected trees; however, such decline is not observed in healthy citrus trees. Therefore, the objective of this study was to understand the nutrient uptake, physiological, and molecular responses of healthy (HLY) and HLB-affected (HLB) sweet orange (Citrus sinensis (L.) Osbeck) trees when grown in media maintained at different pH levels 6.0, 7.0, or 8.0. Overall, the performance of HLY and HLB trees decreased with the increase in pH conditions. HLB trees showed a significant inverse correlation between growth parameters (leaf number, shoot growth, SPAD value, stem water potential) and growing media pH whereas no such correlation was observed for HLY trees. Interesting, superior performance of HLB trees at pH 6.0 coincided with higher accumulation of Fe, Mn, and N in whole plant body as compared to high pH treatments. In contrast, HLY tree performance was significantly better at pH 7.0 compared to other pH conditions. At pH 7.0, stress related genes were upregulated in HLB leaves as compared to HLY leaves, indicating the stress response in terms of leaf abscission, reduced growth, and natural aging process was exacerbated in HLB trees at higher pH treatments. Altogether, all the physiological and molecular observations supports an interaction between HLB trees and pH, where HLB trees at pH 6.0 perform better than at higher pH.

Highlights

Huanglongbing (HLB; aka citrus greening), a bacterial disease caused by Candidatus Liberibacter asiaticus (CLas) and spread by an insect vector, Asian citrus psyllid (Diaphorina citri Kuwayama), has become the biggest challenge for the Florida citrus industry [1,2]
Soil in Florida citrus groves was reported as alkaline due to high bicarbonate accumulation [8] paired with irrigation water pH above 7.0 [11], indicating that prolonged irrigation with high pH water can raise the soil pH
A negative correlation was found between increase of SPAD value and pH treatments in both HLY trees (r = −0.57, p < 0.05) and HLB trees (r = −0.66, p < 0.05)

Summary

Introduction

Huanglongbing (HLB; aka citrus greening), a bacterial disease caused by Candidatus Liberibacter asiaticus (CLas) and spread by an insect vector, Asian citrus psyllid (Diaphorina citri Kuwayama), has become the biggest challenge for the Florida citrus industry [1,2]. Later in the CLas infection, visible symptoms such as blotchy mottle on the leaves, feeder root loss, nutrient deficiency, stunted growth, fruit drop, and yield decline can be observed [1,6,7]. Recent research indicates that the decline in HLB-affected trees is positively correlated to the pH of irrigation water and soil pH [8,9]. The optimum soil pH range for citrus in Florida is 6.0–6.5 [10]; any variation from the optimum range is expected to affect citrus growth. Soil in Florida citrus groves was reported as alkaline due to high bicarbonate accumulation [8] paired with irrigation water pH above 7.0 [11], indicating that prolonged irrigation with high pH water can raise the soil pH. Why HLB-affected trees are more susceptible to high pH growing conditions than healthy trees remains unknown

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Conclusion