Early results from a large-scale citrus trial looking for solutions to HLB, also called citrus greening, show tree size does not seem to affect citrus susceptibility to the disease. The trial is being conducted in the Indian River region by researchers with the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS). They are testing which citrus rootstock and scion combinations will tolerate HLB.
Martin Zapien, a graduate student at the UF/IFAS Indian River Research and Education Center, presented data from the Millennium Block citrus cultivar trial in Fort Pierce. He reported on research from a 20-acre grove with grapefruit, navel orange and mandarin cultivars on a wide range of newly released and commercial rootstocks.
The trial was planted in 2019. Lorenzo Rossi, UF/IFAS plant root biologist, and Tom James, a local citrus industry veteran, supervised Zapien’s research. Advisors were UF/IFAS plant improvement team faculty Fred Gmitter, Jude Grosser and William Castle.
“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. “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 added, however, that there is a trend in small trees showing less citrus greening symptoms.
“All trees in the Millennium Block are infected with citrus greening. However, some trees are thriving,” Zapien said. “Trees on sour orange (rootstock) 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 tree canopy. In contrast, US Seedless Surprise symptoms were 24%. The other varieties fall between the two figures.
“The data we compiled is nascent as the trees were only two years old at the analysis,” said Zapien, who completed a master’s degree in horticultural sciences. “University researchers will continue to monitor the top performer combinations to determine if the early findings are consistent.”
Source: University of Florida Institute of Food and Agricultural Sciences
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