Effect of enhanced nutritional programs and exogenous auxin spraying on huanglongbing severity, fruit drop, yield and economic profitability of orange orchards

Abstract

Abstract Citrus trees infected with ‘Candidatus Liberibacter asiaticus’, the bacterium associated with huanglongbing (HLB), present asymmetric chlorosis and yellow veins on leaves, nutritional deficiencies due to impaired water and nutrient uptake by a reduced root system, and also premature fruit drop and progressive yield reduction. This motivates growers to adopt additional foliar nutritional and auxin plant hormone derivative 2,4-D sprays to try to mitigate HLB damage. However, long-term studies that compare the yield of both well-nourished asymptomatic (AT) and symptomatic trees (ST) under these programs are absent in the literature. This work presents a long-term field study that compared the effect of several enhanced nutritional programs (ENP) and exogenous auxin spray on the progress of HLB severity and on the fruit drop and yield of AT and ST. Moreover, the benefit–cost ratio of each program was estimated. Programs did not contribute to reduce HLB severity over time (mean final HLB severity\xa0>\xa050%). None of the programs contributed to reduce the percentage of fruit drop (remained higher than 40% in symptomatic branches) nor to increase fruit production of ST. Benefit/cost ratio was, on average, 1.49 times higher in AS than in ST. Grower standard nutritional program had the higher benefit-cost ratio (132.8 and 84.7 for AT and ST, respectively) than the others more costly programs. These observations suggest that ENP and 2,4-D sprays are not important in controlling HLB nor mitigating its damages, and their use, instead of removing diseased trees, can build up inoculum within a region. Diseased trees are inoculum source that psyllid vector Diaphorina citri can acquire the bacteria, transmit to a healthy tree, and, consequently, spread HLB. Rigorous vector control by insecticide sprays and eradication of diseased plants are still the best long-term HLB control.