Articles
Exogenous application of gibberellic acid and auxin for reduction of premature fruit drop in HLB-affected sweet orange trees
Article number
1448_42
Pages
325 – 334
Language
English
Abstract
It is known that HLB may lead to hormonal imbalances in the tree, which contributes to the exacerbation of disease symptoms.
In infected trees, levels of salicylic and abscisic acids are high, whereas growth-promoting hormones such as gibberellins and auxins are downregulated.
This study aimed to assess the effect of exogenous applications of gibberellic acid (GA3) and auxin (2,4-D) for mitigating HLB symptoms and reducing premature fruit drop in sweet orange trees.
The study was conducted in a commercial sweet orange orchard of late-maturing cultivar ‘Natal’ grafted on citrumelo ‘Swingle’ located in the São Paulo citrus belt.
The initial severity of HLB symptoms ranged from 40.0 to 55.0%. Treatments were composed of GA3 (Progibb 400) at 17 or 33 mg a.i.
L‑1, ethylene inhibitor aviglycine hydrochloride (Retain) at 41.5 or 83 mg a.i.
L‑1, the mixture of the lower rates of both, and synthetic auxin 2,4-D (Aminol 806) at 17 or 33 mg a.i.
L‑1. All products and rates were sprayed three times in May, June, and July (pre-bloom). In addition, the two rates of 2.4-D were also sprayed only once in September (post-bloom), a month before harvest.
Untreated trees were used as negative controls (UTC). Applications were performed using an airblast sprayer at a volume of 1500 L ha‑1. Trees were assessed for the severity of HLB symptoms, premature fruit drop in HLB-symptomatic and asymptomatic marked branches, yield, fruit and juice quality.
None of the treatments reduced symptom severity.
The application of GA3 and 2,4-D reduced fruit drop.
Only 22.5% of fruit drop was recorded for trees treated with 2,4-D 33 mg a.i.
L‑1, followed by GA3 at 33 mg a.i.
L‑1, with 32.9% and 2,4-D at 17 mg a.i.
L‑1 with 33.2%, compared to 68.9% of dropped fruits recorded in UTC trees.
Higher fruit yields were recorded in the GA3-treated trees at full rate, with 69.5 kg tree‑1, and in the half-rate 2.4-D-treated trees, with 60.4 kg tree‑1, as opposed to 54.0 kg in the UTC. However, GA3 and 2.4-D at higher rates resulted in fruit weight reduction from 159 g, in the UTC, to 147.5 and 146.1 g, respectively.
Limonin rates in the juice increased from 1.8 to 3.8 and 4.3 ppm in trees that received three applications of half and full GA3 rates, respectively, and to 3.8 ppm in trees treated three times with 2.4-D at the full rate.
No effect was observed when 2.4-D was sprayed once after blooming in September.
The results showed the potential of GA3 or 2.4-D in mitigating HLB losses.
Adjustments in the application period and rate remain to be further investigated to optimize the results and reduce potential collateral effects.
In infected trees, levels of salicylic and abscisic acids are high, whereas growth-promoting hormones such as gibberellins and auxins are downregulated.
This study aimed to assess the effect of exogenous applications of gibberellic acid (GA3) and auxin (2,4-D) for mitigating HLB symptoms and reducing premature fruit drop in sweet orange trees.
The study was conducted in a commercial sweet orange orchard of late-maturing cultivar ‘Natal’ grafted on citrumelo ‘Swingle’ located in the São Paulo citrus belt.
The initial severity of HLB symptoms ranged from 40.0 to 55.0%. Treatments were composed of GA3 (Progibb 400) at 17 or 33 mg a.i.
L‑1, ethylene inhibitor aviglycine hydrochloride (Retain) at 41.5 or 83 mg a.i.
L‑1, the mixture of the lower rates of both, and synthetic auxin 2,4-D (Aminol 806) at 17 or 33 mg a.i.
L‑1. All products and rates were sprayed three times in May, June, and July (pre-bloom). In addition, the two rates of 2.4-D were also sprayed only once in September (post-bloom), a month before harvest.
Untreated trees were used as negative controls (UTC). Applications were performed using an airblast sprayer at a volume of 1500 L ha‑1. Trees were assessed for the severity of HLB symptoms, premature fruit drop in HLB-symptomatic and asymptomatic marked branches, yield, fruit and juice quality.
None of the treatments reduced symptom severity.
The application of GA3 and 2,4-D reduced fruit drop.
Only 22.5% of fruit drop was recorded for trees treated with 2,4-D 33 mg a.i.
L‑1, followed by GA3 at 33 mg a.i.
L‑1, with 32.9% and 2,4-D at 17 mg a.i.
L‑1 with 33.2%, compared to 68.9% of dropped fruits recorded in UTC trees.
Higher fruit yields were recorded in the GA3-treated trees at full rate, with 69.5 kg tree‑1, and in the half-rate 2.4-D-treated trees, with 60.4 kg tree‑1, as opposed to 54.0 kg in the UTC. However, GA3 and 2.4-D at higher rates resulted in fruit weight reduction from 159 g, in the UTC, to 147.5 and 146.1 g, respectively.
Limonin rates in the juice increased from 1.8 to 3.8 and 4.3 ppm in trees that received three applications of half and full GA3 rates, respectively, and to 3.8 ppm in trees treated three times with 2.4-D at the full rate.
No effect was observed when 2.4-D was sprayed once after blooming in September.
The results showed the potential of GA3 or 2.4-D in mitigating HLB losses.
Adjustments in the application period and rate remain to be further investigated to optimize the results and reduce potential collateral effects.
Publication
Authors
F. Behlau, E.S. Gorayeb
Keywords
plant growth regulators, PGR, mitigation, greening
Groups involved
Online Articles (103)