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: 03/05/2021
Project: 18-024
Author: Eric Triplett
Sponsor: Citrus Research and Development Foundation

1. Please state project objectives and what work was done this quarter to address them:

We proposed to test foliar fertilization with potassium phosphate to alleviate HLB symptoms and increase yield of citrus.
Foliar phosphate fertilization field trial (Hamilton and Immokalee).
We are currently about one year into scheduled spraying of our mature citrus plots. Four concentrations of potassium phosphate have been applied every three months since July 2019 (Hamilton) and September 2019 (Immokalee). We have currently extracted DNA and analyzed CLas copy number through TerC and 16S qPCR. The copy number of CLas is continuing to decline in the 2mM phosphate treatment group after a nine-months of spraying. We are currently collecting data on the leaf area index of the tree to determine if leaf density has improved with the spring flush. Fruit yield data will also be available next spring.
Foliar phosphate fertilization on CLas negative Valencia trees (Greenhouse)
Our greenhouse experiment continues to test whether foliar application of soluble potassium phosphate can reduce citrate level in the phloem of citrus trees. Treatments consist of foliar application of 6mM and 2mM potassium phosphate and/or 6mM insoluble calcium phosphate applied to the medium. Each treatment has six replicate trees and control trees are treated with 6mM potassium sulfate to standardize potassium application. All treatments receive a base fertilization with all macro and micro nutrients without phosphate. Every three months three leaves are collected from each tree, midribs are excised and sent off to Dr. Killiny’s laboratory in UF CREC for quantification of citrate and phosphorus levels. The greenhouse experiment hosted by Nabil Killiny’s group continues to show measurable differences of CLas titer between treatments. These trees are given a potassium phosphate foliar spray, a calcium phosphate soil drench (to mimic FL grove soils), or no added phosphate other than what is in the typical fertilizer mix. We continue to visually observe the disappearance of HLB symptomology in the treated saplings. We have recently analyzed the titer change of CLas in the plants.
In the lab, qPCR continued from leaf samples to assess HLB titer in both greenhouse and field plants. Organic acid measurements were made in each experiment above.

2. Please state what work is anticipated for next quarter:
Field trials continued. qPCR analysis on first 12 months of samples will be done and analyzed. By the end of the next quarter, we will know whether foliar phosphate fertilzation reduces citrate levels in phloem.

3. Please state budget status (underspend or overspend, and why): Harder than expected to get field trials underway but now full speed ahead. Spending at the expected rate now.


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