By Tripti Vashisth, Wesley Webb and Taylor Livingston
As trees become symptomatic for HLB, leaves experience an imbalance of plant hormones. This hormonal imbalance exacerbates HLB symptoms and can lead to more stem dieback, poor vegetative growth and fruit drop. This ultimately leads to reduced bearing wood, resulting in yield losses and tree decline. Thus, the use of growth-promoting hormones can be beneficial for boosting growth in HLB-affected trees. This article will discuss using three PGRS — cytokinin, auxin and gibberellic acid (GA) — on HLB-affected sweet oranges to improve their growth and productivity.
A recent study that compared healthy and HLB trees found that HLB-affected trees have a high accumulation of salicylic acid (a plant defense-promoting hormone) throughout the growth phase of a leaf. They also had high abscisic acid (a plant hormone well known to accumulate during water-deficit conditions). They had low levels of growth-promoting hormones such as auxin, cytokinin and GA. This suggests that HLB-affected plants invest in plant-defense response at the expense of growth and thus exhibit poor growth characteristics. Figure 1 shows some examples of subpar leaf and shoot growth, which can lead to poor photosynthesis in HLB-affected plants. This causes low fruit set and retention of the crop, resulting in economic losses.
In the last two to three years, several PGR trials have been established to identify treatments that enhance growth in HLB-affected trees. Exogenously applied PGRs can potentially improve the health of HLB-affected trees. However, the right timing and concentration of the PGR are critical to achieve the desired effect. This article will discuss two ongoing field trials that will highlight the concept of the right PGR at the right time.
Previous research shows that multiple applications of GA in late summer and early fall can improve the growth characteristics and yield of Valencia trees. Many growers feel encouraged to adopt GA to promote vegetative growth, especially when trees have a natural flushing cycle, i.e., spring (March) and summer (July and August).
Cytokinin, a plant hormone group that is known to promote cell division, can enhance spring and summer growth. A trial was set up at in an experimental grove at the Citrus Research and Education Center (CREC). Trees with severe HLB symptoms (canopy density less than 80%) were selected and treated with one of the following:
- A commercial PGR blend (Ascend, containing auxin, cytokinin and GA)
- A seaweed extract (Stimplex, rich in natural cytokinin).
The treatments were applied every 45 days from March to October. After one year of treatment, seaweed-treated trees increased in canopy density whereas the rest of the treatments decreased (Figure 2). GA-treated trees decreased in canopy density, especially after spring. This is probably because GA treatment at the time of flowering (March) resulted in an increased fruit set, leading to poor vegetative growth. The cytokinin-rich product, as expected, increased vegetative growth without any effect on fruit set. Cytokinin-treated trees developed a better canopy to support crop load.
As expected, the increase in canopy equaled an increase in yield. Stimplex-treated trees produced a 40% higher yield than untreated trees and the rest of the treatments (Figure 3). GA sprays were repeated from March to October every 45 days and still did not achieve the desired benefit. It is likely because the imbalance in the source-to-sink ratio caused higher fruit set at the expense of canopy growth early in the season. This was enough to offset any desirable benefits of treatment. Thus, application timing is very important as PGRs can affect several physiological processes at any given time.
In another study that started in July 2022, researchers employed the idea of the right PGR at the right time based on the tree phenology and desired goal. In this Hamlin study, four treatments were evaluated:
- Untreated control (U)
- Cytokinin April/May (C)
- Cytokinin April/May + GA July/September/November (C + GA)
- Cytokinin April/May + GA July/September/November + Auxin July/September/November (C + GA + A)
Within the first year of application, treatment number 3 (cytokinin in spring + GA) was found to have yielded about 20% more fruit and bigger fruit (Table 1) than the control.
Both of these studies are ongoing, but the first-year findings suggest that using cytokinin in the spring to promote growth and using GA in the summer and fall is beneficial for invigorating HLB-affected trees. The use of GA from January to June should be avoided as it can affect flowering and fruit set, leading to undesirable effects on tree growth. Also, the benefit of synthetic auxin (such as 2,4-D) seems to be limited to cases where fruit drop is a major issue. If the groves don’t drop more than 25% to 30%, the use of 2,4-D can be avoided.
Tripti Vashisth is an associate professor, Wesley Webb is a lab technician, and Taylor Livingston is a biological scientist — all at the University of Florida Institute of Food and Agricultural Sciences CREC in Lake Alfred.
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