By Ozgur Batuman, Sanju Kunwar and Ana Redondo
In a promising effort to protect citrus from huanglongbing (HLB), University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) researchers are exploring the potential of Australian lime as an interstock graft for Valencia scions with Swingle rootstock. The interstock technique uses various limes, known for their natural HLB-tolerant peptides, between the rootstock and scion to test whether these antimicrobial peptides, or potentially other molecules, can be graft-transmissible and help boost HLB tolerance in commercial citrus trees.
The grafted (healthy) plants carrying different interstocks were prepared in 2022 and challenge-inoculated with Asian citrus psyllids carrying the HLB-causing bacterium (i.e., CLas) in the greenhouse in 2023 (Figure 1). The leaves have been collected regularly since April 2023 to monitor the infection rate in the Valencia scion.
The multiple greenhouse experiment results show that interstock lines UF-SL SUNLIME, R1T98 and R1T55 effectively delay CLas infection, with noticeable reductions in bacterial accumulation for up to 10 months (Figure 1). This delay in disease progression suggests that these interstocks may serve as a buffer, reducing HLB impacts during early infection stages. In contrast, several lime lines, such as R1T41 and R2T61, have been less effective at delaying infection, emphasizing the importance of selecting specific interstocks based on their HLB defense capacity.
All interstock plants are now being transplanted in the Southwest Florida Research and Education Center (SWFREC) experimental grove in Immokalee. To further explore this potential, samples collected in early September will undergo gene expression analysis to determine whether antimicrobial peptides from the interstock lines are present in the scion. This test includes the most promising interstocks (UF-SL SUNLIME and R1T98) and control trees (with Swingle interstock) to compare HLB-related defense responses comprehensively. The results could reveal if beneficial peptides move through the graft and provide protection beyond the interstock zone.
These experiments, designed for rapid data collection, are expected to provide useful insights within 12 to 18 months. The work will support the citrus industry with data on HLB tolerance mechanisms in these graft combinations and determine the chemical composition of the sap, root and leaf metabolites. This research aims to deliver new options for citrus growers, particularly in Florida. It can also be used in California and Texas, helping reduce HLB susceptibility in citrus groves and potentially offering an additional tool for managing this devastating disease.
Acknowledgment: This project is supported with funds from the U.S. Department of Agriculture National Institute of Food and Agriculture (award number 2021-70029-36055).
Ozgur Batuman (obatuman@ufl.edu) is an associate professor, Sanju Kunwar is a post-doctoral research associate, and Ana Redondo is a research coordinator — all at the UF/IFAS SWFREC in Immokalee.
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