Foliar phosphate fertilization: a simple, inexpensive, and unregulated approach to control HLB.

Foliar phosphate fertilization: a simple, inexpensive, and unregulated approach to control HLB.

Report Date: 05/02/2022
Project: 18-024   Year: 2022
Category: Horticultural & Management
Author: Eric Triplett
Sponsor: Citrus Research and Development Foundation

1. Please state project objectives and what work was done this quarter to address them:
During this period a manuscript describing this work is nearly completed. The current draft is attached along with this report. Our sense of this work is that this apporach still has promise. Greenhouse results were very encouraging. Foliarly applied phosphate did resude citrate levels in phloem. CLas titers did decline in graft-infected citrus saplings in the greenhouse upon foliar phosphate application. Citrus was also found to get all of its phosphate needs through foliar fertilization.
However, the field experiments were harder. We were not able to identify plots with low infection rates at the beginning of the work. The heavily infected plants may have been too weak to have the full benefits of foliar phosphate fertilization. In the first 3-6 months of the field experiments, declines in CLas titer were observed with the lowest level of foliar phosphate application but the plants did not recover in subsequent quarters.
More work on this would require larger groves with more experimentation on the frequency of foliar phosphate application. We choose application every two months and used that frequency throughout. More frequent application at lower doses may improve the efficay of foliar phosphate treatments. This treatment is also more likely to work on younger trees with less infection. Younger trees were used at the Imokalee site but those trees were highly infected. The trees in Hamilton were more than 25 years old.
I remain confident that nutritional approaches can help this problem. It will just require the correct nutrients applied at the right time. The upcoming manuscript is entitled “A systems biology approach suggests a simple strategy to alleviate citrus greening disease. ”

Abstract of manuscript:
Citrus greening disease is now completely endemic to Florida citrus trees, having spread rapidly across all counties in the past 20 years and causing devastating economic losses. The disease’s etiological agent is the unculturable bacterium, Candidatus Liberibacter asiaticus (CLas). Liberibacter crescens, the closest culturable relative, was discovered to prefer citrate as its most effective carbon and energy source. Plants load citrate in response to phosphorus deficiency. In Florida’s calcareous soils, supplemental phosphate fertilization is very low due to the assumption that it is readily available for plants through the soil. It is likely that citrus trees are loading citrate to mine phosphorus from the soil, which could inadvertently exacerbate CLas infection. In greenhouse experiments, foliarly applied phosphate is easily taken up by the plants, resulting in decreased free-citrate levels in the phloem, and delaying the onset of CLas infection. Our field experiments show improvements in mature trees where CLas infection is well established, and infected psyllids maintain infection.

2. Please state what work is anticipated for next quarter:
None. This report and the attached manuscript respresent our final contributions to CRDF.
3. Please state budget status (underspend or overspend, and why):
We did not overspend on this project.

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