Some Grapefruit Showing Less HLB Symptoms

Daniel CooperGrapefruit, Rootstocks, Varieties

Flavia Zambon leads the MAC Project.

A recent update of the U.S. Department of Agriculture Multi-Agency Coordination (MAC) Project evaluating multiple grapefruit varieties on different rootstocks reveals that some combinations have less HLB symptoms than others.

Jackson has one of the lowest ranks for disease index across the combinations. The healthiest trees in the trial include:

  • Flame grafted onto X-639
  • UF N40-16-11-11 grafted onto SuperSour 1
  • UF KW-1-40 grafted onto UFR-5 and X-639
  • UF N40-16-11-7 grafted onto SuperSour 1
  • Rio Red grafted onto SuperSour 1, US-802 and X-639

At least half of the blocks in the MAC trial will reach a mature stage this season, so more information must be collected to draw any conclusions.

Flavia Zambon, University of Florida Institute of Food and Agricultural Sciences assistant professor, leads the MAC Project. It is one of the most robust experimental plots for grapefruit research established for grower-driven decision-making. The project is comprised of more than 12,000 trees across Florida in 40 blocks in collaboration with 12 growers.

Seven grapefruit scions and six rootstocks were chosen, totaling 42 combinations. The selected scions for the trial are Rio Red, Star Ruby, Flame, Jackson, UF GF N-40-11-11, UF Red GF N40-11-7 and Red Pummelo UF KW-1-50. Rootstocks are UFR-4, UFR-5, US-942, US-802, SuperSour 1 and X-639.

One consistent practice in the trials has been the use of oxytetracycline (OTC) on plants more than 2 years old.

Grove management in the trial’s block 27 has resulted in healthier trees compared to the other blocks measured in the same period. The grower of that block has used individual protective covers, just finished the second injection of OTC, and kept weeds suppressed and rows cleaned.

See a 2023 update of the MAC Project and another research project here.

Source: Indian River Citrus League

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