Millennium Block and MAC Project Updates

Josh McGillResearch, Rootstocks, Varieties

Hurricanes Ian and Nicole in 2022 caused early fruit drop in the Indian River area, where two studies of 39,000 trees that might tolerate HLB are being conducted. Researchers now expect significant data from the studies will come after the 2023, 2024 and 2025 season harvests, but they have already gained some insights from the projects. 

MAC Project
An aerial view of the Millennium Block

The first experimental grove, the Millennium Block (MB), includes 20 acres of grapefruit, grapefruit-hybrids, oranges and mandarins. The more than 5,500 trees under study 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 little or no fruit. The trees are being grown at the University of Florida Institute of Food and Agricultural Sciences Indian River Research and Education Center (UF/IFAS IRREC).

The second project is the MAC Project, formally known as the U.S. Department of Agriculture Multi-Agency Coordination Project. This 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 MB and the MAC Project were implemented to identify new citrus varieties and rootstock combinations.

Mark Ritenour, professor of horticulture at IRREC, leads the MAC Project. “It appears from the data collected so far that we can eliminate Star Ruby (grapefruit) from further evaluations,” said Ritenour.

Early research findings for the MB have been presented for two of four research plots. 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 reported.

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. That’s because some plots produced decent overall fruit yield, but most of it was not what Ritenour termed “good fruit.”

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

Flavia Zambon, who carried out the MAC Project under Ritenour’s leadership, will begin a new role as assistant professor of citrus production at the IRREC in the fall.  Zambon will oversee the MB, the MAC Project and other research studies, including four citrus under protective screen structures.

Source: UF/IFAS

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