B: Tree that was exposed to psyllids for 12 months and then protected for an additional 12 months with an individual protective cover (IPC)
C: Tree that was exposed to psyllids for 12 months and then protected for an additional 12 months with an IPC and treated monthly with brassinosteroids.
By Fernando Alferez and Saoussen Ben Abdallah
Individual protective covers (IPCs) are now being increasingly adopted in Florida to protect newly planted citrus trees from psyllid colonization. The risk of new citrus plantings becoming infected with HLB is especially high, as young trees flush more frequently and attract more psyllids.
However, the adoption of IPCs does not occur in all groves. In some cases, trees are still planted with no protection. Under these circumstances, if trees are left unprotected, they become infected with HLB during the first year and will not enter fruit production. But can these trees be recovered so they become productive as they age?
Treatment with brassinosteroids (BRs) may induce a strong immune response in young trees by activating the salicylic acid pathway. Treatment with BRs in non-protected trees may delay HLB infection and spread in the grove. University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) researchers hypothesized that infected young trees could be recovered by covering them with IPCs, so reinfection would not occur as the psyllids would be excluded, and by treating them with BRs, so immune response would be induced.
A research trial began in February 2023. Ninety Valencia trees grafted on US-942 rootstock were planted and left unprotected and exposed to psyllids. After one year, all trees tested positive for HLB and were declining. Then, in February 2024, one third of the trees were covered with IPCs, and another third of the trees were covered with IPCs and received a monthly dose of BRs at a rate of 6.2 fluid ounces per 100 gallons of water. The remaining 30 trees were left unprotected and not treated, so they served as the control trees.
After one year, there is a clear recovery of the trees that were covered with IPCs. They have better canopy density and larger leaves than the control trees. These effects are maximized in trees that were not only covered with IPCs but also treated with BRs. In this case, bacterial titer has been significantly reduced, and the trees are even blooming for the first time. In contrast, unprotected trees are in clear decline. Researchers will continue monitoring the trees for at least one more year to determine if these changes are sustained over time.
Acknowledgement: This research is funded by U.S. Department of Agriculture-National Institute of Food and Agriculture project 2022-70029-38481.
Fernando Alferez is an associate professor, and Saoussen Ben Abdallah is a postdoctoral associate, both at the UF/IFAS Southwest Florida Research and Education Center in Immokalee.
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