UF scientists make big stride toward greening-resistant citrus trees
by Brad Buck
University of Florida scientists achieved a major milestone in their quest to develop a citrus greening-resistant tree by sequencing the genome of a fruit plant that's a close cousin to citrus trees.
You'd need to print 54,000 pages of copy paper to see the complete genome sequence. But within it, scientists believe they've found genes to lay the groundwork to make citrus more tolerant and even resistant to certain diseases, including citrus greening.
UF/IFAS researchers sequenced the genome from trifoliate orange, in collaboration with scientists from the University of California at Berkeley, the U.S. Department of Energy's Joint Genome Institute and UF's Interdisciplinary Center for Biotechnology Research. The new genome will help those who breed new citrus trees that will survive under today's challenging conditions, including invasive pests, viruses and changing climates. Their research provides a powerful new tool to control the deadly consequences of the greening disease, which has severely damaged the state's multibillion dollar-a-year citrus industry.
“Very importantly, trifoliate orange and its hybrids have genes that can confer high tolerance to citrus greening and resistance to the Asian citrus psyllid, the insect that transmits greening to citrus,” said Zhanao Deng, a professor environmental horticulture and a senior author on the new UF/IFAS-led study. “This genome can be used as a reference template to sequence widely used trifoliate orange hybrid rootstock varieties.”
“Most people – even citrus growers – rarely see trifoliate orange. This is because they usually are the rootstock part of the tree, mostly underground,” said Fred Gmitter, a UF/IFAS professor of citrus breeding genetics and a co-author on the study.
Trifoliate oranges or their hybrids are grown at nurseries, and farmers use them as rootstock to grow the citrus that's above ground. Trifoliate orange and its hybrids were used as the rootstock for more than three million citrus trees in Florida alone in 2018-2019, UF/IFAS researchers say.
Trifoliate orange and its hybrid rootstocks accounted for 82% of the top 20 rootstocks used in the 2018-2019 citrus propagation cycle in Florida.
“Our trifoliate orange genome will allow scientists to develop new tools that can more speedily transfer beneficial genes into sweet oranges, grapefruit and breeding of new scion cultivars, which grow above the ground,” Deng said.
“Releasing the first trifoliate orange genome can be valuable for our citrus gene-editing efforts,” Gmitter said. Scientists are using gene editing to produce canker-resistant and greening-tolerant citrus.
“Because of our high-quality genome, re-sequencing of trifoliate orange hybrid rootstock varieties will be much easier, much quicker and much more cost-efficient,” said Deng. “Re-sequencing will enable development of new breeding tools, such as DNA marker-based selection, genomic selection of new rootstock varieties with resistance and tolerance to citrus greening, citrus tristeza virus and citrus nematodes. The new varieties might give higher yield and fruit quality.”
Citrus breeders want to introduce desirable genes from trifoliate orange into sweet orange, grapefruit and other varieties. It took decades to produce the first citrus scion variety (‘Sun Dragon') from crossing trifoliate orange and transferring some of its genes across multiple generations into sweet orange. With this new information from genome sequencing, that timeline can be dramatically reduced.
This project was funded by two grants from the Citrus Research and Development Foundation (CRDF) and a grant from the USDA/NIFA Citrus Disease Research and Extension (CDRE) program.
To see a video about the research and its implications, click here.
FredGmitter at it. Photo credit: Brad Buck, UF/IFAS.
A recent Ventura County ACP-HLB Task Force sponsored webinar was held, the topics and speakers listed below The PDF's from the speakers are available online
Webinar Agenda 8-13-20
Welcome, and update on status of HLB in California:
Leslie Leavens, chair, Ventura County ACP-HLB Task Force.
Update on area-wide participation rates and CDFA buffer treatments:
Sandra Zwaal, Ventura County Grower Liaison
Final report on 2017-2020 ACP surveying project in Ventura County, and overview of Phase Two research:
Beth Grafton-Cardwell, IPM Specialist and Research Entomologist, University of California-Riverside, and Director of Lindcove Research and Extension Center (retired); and Monique Rivera, Assistant Extension Specialist, Department of Entomology, UC Riverside.
Final report on deployment of detection canines to scout Ventura County commercial groves for evidence of early HLB infection:
John Krist, CEO, Farm Bureau of Ventura County.
Implications of canine detection data for HLB management in Ventura County commercial citrus:
Neil McRoberts, western regional director, National Plant Diagnostic Network, and professor of plant pathology, UC Davis.
For more information, contact us by email at email@example.com.
From the Citrus Industry
The new Sugar Belle hybrid rootstock LB8-9xS13#16 has quite a history, according to University of Florida Institute of Food and Agricultural Sciences citrus breeder Jude Grosser.
“This is one of several projects I did with Orie Lee toward the end of his Florida Citrus Hall of Fame career/life,” recalls Grosser. “We did a lot of brainstorming together, and he was full of ideas and the energy to make them become a reality!”
Before HLB became the dominant scourge of Florida citrus, Grosser was heavily invested in trying to solve blight. Lee had a great interest in blight, because in several of his most productive blocks the annual tree loss to blight was about 12 percent.
“Evidence was showing that pummelo x mandarin hybrids had superior blight tolerance as compared to other rootstock categories, so I made a lot of pummelo x mandarin hybrids,” says Grosser. “Orie was in a hurry to plant rootstock hybrids to test for blight tolerance, and he had just planted a large block at the Alligator Grove. He suggested we plant trees in between each of the planted trees. We made cuttings from about 125 new rootstock hybrids and we tried to get at least seven liners for each rootstock. We grafted them all with Valencia and planted them one year after the original grove was planted — about 12 years ago.”
According to Grosser, the Alligator Grove has never had psyllid control, and about four years after planting, it was evident that nearly all the trees were infected with HLB. “For the past six years, I have been scoring the individual trees for HLB. There were only two of the trees on the S13 parent (salt tolerant HB pummelo x Cleo). During the 2020 scoring, this hybrid had the highest tree health rating (4.25 out of 5), indicating very good ability to transmit HLB tolerance from the rootstock to the Valencia scion,” he explains.
Grosser says it has become evident from multiple trials that Sugar Belle has exceptional HLB tolerance no matter what rootstock it is grown on. “I thought that maybe we could flip this equation and develop a rootstock that any scion could grow on in the presence of HLB,” he says. “Sugar Belle is not used as a rootstock because it does not come back true-to-type from seed and is purported to be susceptible to phytophthora.”
So, Grosser decided to use Sugar Belle as a rootstock breeding parent to test his hypothesis. He made the first crosses with Sugar Belle in 2015, using a salt-tolerant HB pummelo x Cleo hybrid (S13) and a salt-tolerant HB pummelo x Shekwasha (S10) as the pollen parents.
Since then, Grosser has made other crosses using Sugar Belle. As the rootstocks progressed through the screening process, he says one hybrid jumped ahead of the others. So, he planted two trees at the Citrus Research and Education Center (CREC).
“I planted the hybrid #16 tree on the corner of Block 8, just behind our barn, so I could watch it every time I drove up the road to my greenhouses or field plots. I noticed that psyllids were feeding on the tree during every new flush (despite the CREC spray program), and there was visible psyllid damage on leaves from all flushes,” says Grosser. “Despite starting infected and continuous psyllid re-inoculation, this tree has grown off like a normal pre-HLB tree and looks beautiful today, two years after planting. It has set some fruit, so I will begin to get an idea of fruit quality this coming fall.”
Grosser says the second tree, on a different hybrid, is doing okay, but nothing like #16.
“After what I saw the first year, I grafted an infected Murcott (the most HLB-susceptible variety) onto the rootstock and planted it, and it is also growing incredibly well,” he says. “The Murcott tree is now one year old. We tested the original Valencia tree by PCR in December, and the scion had a ct value of 24, indicating a high titer of CLas (the causal agent of HLB); whereas the roots had a ct value of 32, indicating no active infection. We tested this tree again in April, and the ct value went up to 36, indicating no active infection. So, even starting out infected and under heavy psyllid pressure, the rootstock seems to have suppressed the infection, allowing the tree to thrive.”
Although it has only been two years from planting, Grosser is hopeful that the “HLB tolerance, maybe resistance, holds up over time (like it seems to be doing with its parents). I have pathogen-free material of this rootstock, and we are now making cuttings as needed for large-scale evaluations. We have also started this rootstock in tissue culture for micropropagation.”
The University of Florida Institute of Food and Agricultural Sciences and Southeast AgNet are partnering to provide the latest news on citrus-related research in a monthly podcast.
The podcast, “All in for Citrus,” features short interviews with scientists working to find solutions to citrus greening and other devastating citrus diseases.
“This is the latest tactic in a comprehensive communications effort launched this past summer by the UF/IFAS Citrus Team,” said Michael Rogers, statewide citrus research coordinator and director of the UF/IFAS Citrus Research and Education Center in Lake Alfred, Florida.
The podcast complements the new website, citrusresearch.ifas.ufl.edu, and a new monthly newsletter, both launched at the August Citrus Expo held in Fort Myers, Florida.
“We are working hard to get the latest research findings directly to growers in a timelier manner,” said Rogers. “We are also streamlining the research results to focus on what growers could use immediately in their groves or what might be a longer-term strategy.”
LISTEN IN: http://citrusindustry.net/allinforcitrus/
The latest podcast is on protection practices to reduce infection of young trees.
Five UF/IFAS scientists are comparing several insect-management tools. This includes evaluation of individual protective covers (IPCs), which are essentially bag-like covers placed over newly planted trees to keep pests away.
In addition, researchers are also studying reflective mulch, kaolin clay and chemical-based insect pest management. Kaolin clay is a powdery white compound that is sprayed over the trees. Kaolin provides trees with a rough surface, making it harder for HLB-spreading psyllids to grasp. The clay may reduce the transmission of HLB and provide some shade to the plant, allowing better growth.
Through this U.S. Department of Agriculture funded project, the research team hopes to develop new recommendations and provide a comprehensive overview of existing tools that growers can use to combat HLB.
The research project has secured two years of funding. Diepenbrock hopes to receive additional funding in order to study the impacts of these tools on harvest.
This interview with Diepenbrock is part of the June All In For Citrus podcast, a joint venture of UF/IFAS and AgNet Media. Listen to the full podcast.
Last week, UCR issued a news release entitled UCR Discovers First Effective Treatment for Citrus-destroying Disease, which shares the news of a licensing agreement being reached with Invaio Sciences. This is extremely exciting news about potentially promising research that could significantly change the future of the citrus industry, both here and in all citrus growing area in the world. Now it will be important to complete studies on the effectiveness of this therapy in greenhouse and field studies.
As yet, there are no published scientific papers describing the methodology by which they discovered it, or the field or lab trials through which they determined its efficacy. As has been the case so often with ACP-HLB, there have been promising strategies that on further inspection fail to meet the criteria of a “cure”.
If it really works in a commercial field setting, it would be a great complement to the canine surveys, since it would be much easier to clear infection from a tree when it is still in the early stages and has not yet become systemic.
The Citrus Research Board has a wait-and-see approach - Glad that something is in the works to be commercialized, but waiting for more test results. This might be a real breakthrough. Read the CRB's response to the recent news: