FORT PIERCE, Fla. — Early results from a groundbreaking, large-scale citrus trial looking for solutions to the devastating citrus greening disease have given early hope for growers in the Indian River District. The new UF/IFAS research shows tree size does not seem to affect citrus susceptibility to greening.
In the trial, researchers are testing which citrus rootstock and scion combinations will tolerate citrus greening, a deadly global citrus disease that has impacted the Florida citrus industry.
Martin Zapien, a graduate student at the University of Florida Institute of Food and Agricultural Sciences Indian River Research and Education Center (UF/IFAS-IRREC), presented data from the Millennium Block citrus cultivar trial in Fort Pierce, Florida. At his thesis defense, the information Zapien represented was research from a 20-acre grove with grapefruit, navel orange and mandarin cultivars on a wide range of newly released and commercial rootstocks. Planted in 2019, the trees grow in a region where citrus greening is now endemic.
Lorenzo Rossi, UF/IFAS plant root biologist and Tom James, a local citrus industry veteran, supervised Zapien’s research. Advisors on the project were UF/IFAS plant improvement team faculty Fred Gmitter, Jude Grosser and William Castle.
Nearly 80% of Florida’s grapefruit crop is produced by Indian River District growers, who export their crop to Europe and Asia. That’s why scientists study the fruit so closely, said Rossi.
“One of our objectives is to evaluate and compare the early performance of several new grapefruit hybrids grown on three commercial rootstocks under citrus greening conditions,” said Zapien.
In the first two years of growth, the researchers compared UF rootstocks and scions by measuring tree growth and the trees’ ability to tolerate citrus greening. They expect to collect yield and fruit data after the third year, Zapien said.
Rossi said data sets were specific to tree size, bacteria amount, the severity of citrus greening symptoms such as irregular yellow patches in the leaves, and leaf and soil nutrient concentrations.
“We have seen many rootstocks that promote large and small tree size, but we have not seen any correlation between tree size and susceptibility to citrus greening,” Zapien said.
But so far, the research does not support a theory that tree size affects citrus greening susceptibility, but there is a trend in small trees showing less citrus greening symptoms, Zapien said.
Zapien said published research findings prove that high-density plantings produce higher fruit yields by increasing yield efficiency — fruit number per green foliage. The researchers will evaluate if the trees correlate with the published work or if the larger trees in the test grove produce more fruit.
‘Ray Ruby’ grapefruit on UFR-15 rootstock promotes vigorous trees — the trees show the largest canopies and are already flowering. But for UF/IFAS researchers to recommend a particular rootstock and scion combination, more data are required. The experimental grove must be examined for up to four more years before UF/IFAS scientists can make reliable recommendations.
“All trees in the Millennium Block are infected with citrus greening. However, some trees are thriving,” Zapien said. “Trees on sour orange have shown significantly fewer disease symptoms than trees on x-639 and US-942 rootstocks, but we have to consider that sour orange’s drawback is the susceptibility to citrus tristeza virus.”
As to citrus greening severity, ‘Star Ruby’ grapefruit showed only 4% symptoms in the green foliage. In contrast, ‘US Seedless Surprise’ symptoms were 24%. The other varieties fall between the two.
“The data we compiled is nascent as the trees were only two years old at the analysis,” said Zapien. “University researchers will continue to monitor the top performer combinations to determine if the early findings are consistent.”
With research in the UF/IFAS-IRREC experimental grove, Zapien completed a master’s degree in horticultural sciences. Zapien recommends that researchers improve sampling methods to assess the bacteria as the trees mature to advance the research. Zapien and his colleagues will evaluate flowering patterns to determine when the fruit is ready to harvest and reveals market windows. A long-term goal is to measure the yield or the amount of marketable fruit each tree produces.
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