Flavia Zambon, assistant professor with the University of Florida Institute of Food and Agricultural Sciences, recently provided an update about Millennium Block research at the Indian River Research and Education Center. The block, established in 2019–20, is a large-scale, 5,500-tree trial seeking HLB-tolerant citrus varieties.
Zambon says some varieties in the trial are showing strong fruit attributes without the need for oxytetracycline injections. The block receives standard grower care and slow-release fertilizer.
The Millennium Block research consists of four independent trials designed to evaluate various scions and rootstocks:
- Trial 1: 18 grapefruit scions and three rootstocks
- Trial 2: Ray Ruby grafted onto 31 rootstocks
- Trial 3: Glen F-56-11 navel orange grafted onto 31 rootstocks
- Trial 4: UF-950 mandarin grafted onto 31 rootstocks
Researchers assess fruit quality, yield and overall adaptability under HLB infection. The trees are now in their sixth year.
LATEST OUTCOMES
Zambon reports that the following outcomes have emerged in the 2024–25 season:
- Triumph grapefruit-orange hybrid maintained notable fruit retention while juice quality across multiple varieties remained impressive. Most combinations surpassed 7 Brix, with Triumph and UF-914 averaging above 9.
- Triumph exhibited an expected drop in yield, from 227 boxes per acre in 2023–24 to 92 in 2024-25. This reduction aligns with previous observations of mild alternate bearing.
- UF-914 pummelo hybrid saw a yield decline in Sour Orange, US-942 and X-639 rootstocks, but its fruit quality remained high.
- For the second consecutive year, UFR-15 has emerged as the leading rootstock for Ray Ruby, producing nearly 60 boxes per acre with juice quality exceeding 7 Brix. Meanwhile, UFR-11 produced an exceptional Brix of 8.5, though with increased acidity.
- The biggest setback this season came from a tornado, which affected fruit retention across Trials 1, 2, and 3 — particularly in Glen navels. The mandarins in Trial 4 continue to struggle with holding fruit throughout the season, exacerbated by funding constraints that halted planned gibberellic acid sprays when necessary.
- Despite these obstacles, the resilience of certain varieties suggests that strategic selection of scion and rootstock can help mitigate fluctuations in productivity. Additionally, Triumph’s potential alternate bearing tendencies warrant further exploration to help growers optimize their management strategies.
MOVING FORWARD
Looking ahead to the 2025–26 season, researchers remain optimistic that Triumph will rebound in productivity, offering a valuable case study in alternate-bearing behavior. The Millennium Block will continue to provide growers with critical data to guide decision-making in variety selection, rootstock pairing and fresh fruit quality optimization.
By refining management strategies based on ongoing observations, research will provide growers with the tools to maintain fruit quality and yield under HLB-endemic conditions.
Source: Indian River Citrus League
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