FORT PIERCE — Citrus growers in Florida’s “River” production region along the Indian River Lagoon develop unsurpassed flavor and texture in a fresh grapefruit product coveted worldwide.

Growers in the area have watched the industry they love steadily decline in the last two decades. They await scientific results from two active research projects involving more than 39,000 trees, expected to reveal which new varieties will tolerate pressure from the most serious citrus disease worldwide.

The citrus tree disease is called huanglongbing, or HLB, or citrus greening. It has cost Florida millions in fruit sales that once supported the economy with jobs for its citizens. It has hit hard an industry that so many consider a birthright and a career shared by generations of families and their communities.

But among the nearly 40,000 trees researched in two trials—hope also grows. Some trees are expected to stand against the disease, tolerate it, thrive and produce market-ready fresh fruit.

The first experimental citrus grove, the Millennium Block (MB), includes 20 acres of grapefruit, grapefruit-hybrids, oranges, and mandarins. The more than 5,500 trees under study in the MB are entering fruit-bearing years. As the trees mature, they reveal differences in trunk size and foliage. Some trees hold a substantial amount of fruit. Other trees produce low or no fruit.

Some of the 5,500 trees at the UF/IFAS-IRREC pledge promise. “Although trees in both experimental projects show differences in terms of disease tolerance and fruit quality and quantity, the oldest trees are only in the fourth year of growth, and we have had only one harvest from one grove,” said Ronald D. Cave, Director of the UF/IFAS Indian River Research and Education Center.

“The Millennium Block project at IRREC is the most comprehensive trial of the UFR (University of Florida) rootstocks and both legacy standards and newer United States Department of Agriculture Agricultural Research Service (USDA-ARS) rootstocks,” said Peter Spyke, an IRREC Advisory Committee member and longtime Florida citrus grower.

Cave adds that just before the first fruit harvest began in the fall of 2022, Hurricane Ian and Tropical Storm Nicole swept storm gusts across the Indian River area, resulting in early fruit drop.

But more data will come after the 2023, ’24, and ‘25 season harvests. Cave and Spyke agree that a full seven years of data collection and analysis will provide reliable findings that growers can use when they select new trees to give the world its premier fresh grapefruit.

The second citrus experimental project is called “The MAC Project,” or the United States Department of Agriculture Multi-Agency Coordination Project. The MAC project involves 42 blocks evaluating seven grapefruit varieties (scions) grown on six different rootstocks. The MAC Project groves, planted in 2019, are situated across the state and include plantings managed by 16 commercial growers on their lands.

The Millennium Block and The MAC Project were implemented to identify new citrus varieties and rootstock combinations of rootstocks, grafted with scions or the part of a tree from its trunk to its foliage.

Mark Ritenour, a professor of horticulture at IRREC, leads The MAC Project. At the recent 2023 Florida Citrus Show, Ritenour reported early findings from the MB and MAC Project experiments.

“For The MAC project, it appears from the data collected so far that we can eliminate ‘Star Ruby’ from further evaluations,” said Ritenour.

Early research findings for the Millennium Block are presented for two of four research plots in the experimental grove. Trials #1 and #2 consist entirely of grapefruit or grapefruit-pummelo hybrid scions.

“For Trial #1 of the MB so far, we identified 18 out of 54 scion and rootstock combinations with the best internal quality for the 2022-23 season,” Ritenour said.

In Trial #2, Ritenour showed that it is important to evaluate not only total fruit yield but the percentage of the fruit that does not show HLB symptoms because some plots produced decent overall fruit yield, but most of it was not ‘good fruit.’”

“While US-812 was the 7th greatest yielding rootstock from 25 varieties included in the study, only 19% of the fruit was ‘good fruit,’,” said Ritenour.

Ritenour said that the two major storms that had a negative impact on the fruit in the MB and MAC Project fields emphasize the need for additional years of data.

The trees hold hope for the future of Florida’s fresh grapefruit industry. Flavia Zambon, who carried out The MAC Project under Ritenour’s leadership, will begin a new role as assistant professor of citrus production in the fall. In her new position, Zambon will oversee the Millennium Block, The MAC Project, and other research studies, including four Citrus Under Protective Screens (CUPS) protective screenhouses.

“We are very fortunate to have Dr. Zambon join the IRREC team to manage and execute this important trial and other large-scale research projects currently under IRREC supervision,” said Spyke. “We’re looking forward to a year without a hurricane to collect solid data from these robust, one-of-a-kind field trials.”