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.”
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:
The California Department of Food and Agriculture (CDFA), through its Citrus Pest & Disease Prevention Program, has developed a mobile-friendly, Spanish-language training video. It's for use by field crew supervisors and farm labor contractors prior to harvest. The new tool trains industry managers and workers, including field crews, with best practices to prevent the spread of the Asian citrus psyllid (ACP) in California's citrus groves.
This video, which stems directly from the agency's in-person train-the-trainer workshops, provides an overview of best practices for huanglongbing (HLB) control. HLB is the deadly citrus disease spread by ACP. CDFA says the goal of the video is to keep HLB from threatening the California citrus industry's livelihood and infecting commercial groves.
The video can be downloaded here.
For additional resources from the Citrus Pest & Disease Prevention Program, visit CitrusInsider.org/Resources. The Citrus Pest & Disease Prevention Program is an initiative funded by California citrus growers and administered by the CDFA.
A Solution for the devastating bacteria causing citrus huanglongbing?
UC Riverside scientists have found the first substance capable of controlling Citrus Greening Disease, which has devastated citrus farms in Florida and also threatens California.
Oranges afflicted with Citrus Greening Disease. (UCR)
The new treatment effectively kills the bacterium causing the disease with a naturally occurring molecule found in wild citrus relatives. This molecule, an antimicrobial peptide, offers numerous advantages over the antibiotics currently used to treat the disease.
UCR geneticist Hailing Jin, who discovered the cure after a five-year search, explained that unlike antibiotic sprays, the peptide is stable even when used outdoors in high heat, easy to manufacture, and safe for humans.
“This peptide is found in the fruit of greening-tolerant Australian finger limes, which has been consumed for hundreds of years,” Jin said. “It is much safer to use this natural plant product on agricultural crops than other synthetic chemicals.”
Currently, some growers in Florida are spraying antibiotics and pesticides in an attempt to save trees from the CLas bacterium that causes citrus greening, also known as Huanglongbing or HLB.
The Asian citrus psyllid, pictured here, spreads the bacterium that causes Citrus Greening Disease. (Mike Lewis/UCR)
“Most antibiotics are temperature sensitive, so their effects are largely reduced when applied in the hot weather,” Jin said. “By contrast, this peptide is stable even when used in 130-degree heat.”
Jin found the peptide by examining plants such as the Australian finger lime known to possess natural tolerance for the bacteria that causes Citrus Greening Disease, and she isolated the genes that contribute to this innate immunity. One of these genes produces the peptide, which she then tested over the course of two years. Improvement was soon visible.
“You can see the bacteria drastically reduced, and the leaves appear healthy again only a few months after treatment,” Jin said.
Because the peptide only needs to be reapplied a few times per year, it is highly cost effective for growers. This peptide can also be developed into a vaccine-like solution to protect young healthy plants from infection, as it is able to induce the plant's innate immunity to the bacteria.
Jin's peptide can be applied by injection or foliage spray, and it moves systemically through plants and remains stable, which makes the effect of the treatment stronger.
The treatment will be further enhanced with proprietary injection technology made by Invaio Sciences. UC Riverside has entered into an exclusive, worldwide license agreement with Invaio, ensuring this new treatment goes exactly where it's needed in plants.
“Invaio is enthusiastic to partner with UC Riverside and advance this innovative technology for combating the disease known as Citrus Greening or Huanglongbing,” said Invaio Chief Science Officer Gerardo Ramos. “The prospect of addressing this previously incurable and devastating crop disease, helping agricultural communities and improving the environmental impact of production is exciting and rewarding,” he said. “This is crop protection in harmony with nature.”
The need for an HLB cure is a global problem, but hits especially close to home as California produces 80 percent of all the fresh citrus in the United States, said Brian Suh, director of technology commercialization in UCR's Office of Technology Partnerships, which helps bring university technology to market for the benefit of society through licenses, partnerships, and startup companies.
“This license to Invaio opens up the opportunity for a product to get to market faster,” Suh said. “Cutting edge research from UCR, like the peptide identified by Dr. Jin, has a tremendous amount of commercial potential and can transform the trajectory of real-world problems with these innovative solutions.”
UV Riverside news release:
Jules L Bernstein
Senior Public Information Officer
Citrus greening disease, or Huanglongbing (HLB), is deadly, incurable, and the most significant threat to the citrus industry. Most HLB research focuses on the tree canopy, but scientists in California studied the impact of HLB on root systems. They recently published the first study to report on the response of two different varieties of citrus to the causal bacterium, 'Candidatus Liberibacter asiaticus' using metabolomics and microbiome technologies.
"Metabolomics is a cutting-edge field of study that provides snapshot information about the metabolism of living things," explains author Emily M. T. Padhi, "while microbiome studies provide valuable information about the microbial communities living in a particular ecological niche - some microbes are beneficial to the host, while others can be harmful."
Padhi and colleagues wanted to see how the root system of two varieties of citrus responded to HLB. They collected roots from healthy and infected Lisbon lemon and Washington Navel orange trees grown in greenhouses at the same time and under the same conditions.
They found that both varieties experienced a reduction in root sugars and amino acids when exposed to HLB. However, they also found differences. While the concentration of malic acid and quinic acid (two metabolites involved in plant defense) increased in the navel roots, they decreased in the lemon roots. They also found that the beneficial bacteria Burkholderia increased substantially in navel plants but not in lemons, which contradicts previous studies.
"Overall, this is the first study to compare two varieties of citrus using a combined metabolomics and microbiome approach and demonstrates that scion influences root microbial community composition and, to a lesser extent, the root metabolome."
There is evidence to suggest that the causal bacterium moves to the root system soon after a plant becomes infected. A key strategy for preserving the health of an infected tree is root system management and research on different responses to HLB may help devise new variety-specific preventative and treatment measures.
Images of the bulk root mass and sample leaves from healthy and 'Candidatus Liberibacter asiaticus' lemon and navel plants.