HLB Tolerance Data Expected in Early 2023

Josh McGill Rootstocks, Varieties

Conclusive data on trials in the Millennium Block, expected in early 2023, may reveal which of more than 5,500 trees can tolerate HLB, researchers say. The Millennium Block is at the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) Indian River Research and Education Center (IRREC) in Fort Pierce.

The Millennium Block features 154 new citrus scion-rootstock combinations.

IRREC Director Ronald Cave said the 20-acre grove, established in 2019, features 154 new citrus scion-rootstock combinations — some of which will produce market-ready fruit. Leading the rootstock research are Cave, the principal investigator; citrus grower Pete Spyke; and Bill Castle, a UF/IFAS professor emeritus.

“In one trial, we took the worst of the grapefruit, the most susceptible to citrus greening (another name for HLB), and matched it with different rootstocks,” said Spyke. “We didn’t pick the grapefruit that tolerates the disease because we are looking for differences, not absolute performance.” 

Cave said the researchers chose navel orange to act as an indicator scion for rootstock effects on orange varieties because it expresses rootstock differences more than standard juice oranges. A rootstock that imparts disease tolerance, climatic stability and high Brix fruit in navels — which struggle with fruit production and Brix issues — will automatically outperform other rootstocks for round oranges grown for processing. The variety UF 950 mandarin was also selected as an indicator scion because it has major issues with fruit production.

“If the research shows high yields with UF 950, it could become the go-to choice for mandarin varieties in general, providing a similar role as Cleopatra mandarin in the past,” said Spyke.

In another trial, the researchers evaluated multiple grapefruit scions. They graft the scions onto the rootstocks. Rootstocks employed in that grapefruit variety trial are standard sour orange, X-639 and US-942. There are 18 different grapefruit and grapefruit hybrids on these three rootstocks in the experimental trial.

The first fruit data and tree growth analyses from the last four years will begin the process of choosing rootstocks that will grow trees that produce high yields of marketable fruit.

“Bottom line, we see some rootstock combinations in all four trials that appear to be growing normally and producing good crops of large, high-quality, delicious fruit,” Spyke said. “Trust in the data from the Millennium Block will require more than one crop.” 

Continued funding for the planting will be important in the future, and the experimental design will provide reliable data, Spyke said.

Spyke said Indian River District grapefruit is a high-value fresh crop exported internationally. There is a strong interest among legacy growers to continue to produce grapefruit locally. If the Millennium Block shows data to support a grapefruit-like variety tolerant to HLB, growers may be interested in that option.

A field day will be held at the Millennium Block Oct. 19. Learn more about it here.

Source: UF/IFAS

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