UC Riverside scientists are betting an ancient solution will solve citrus growers' biggest problem by breeding new fruits with natural resistance to a deadly tree disease.
The hybrid fruits will ideally share the best of their parents' attributes: the tastiness of the best citrus, and the resistance to Huanglongbing, or HLB, displayed by some Australian relatives of citrus.
There is no truly effective commercial treatment for HLB, also called citrus greening disease, which has destroyed orchards worldwide. The disease has already been detected in California, where 80 percent of the country's fresh citrus is grown. However, it has not yet been detected in a commercial grove.
To prevent that from happening, the National Institute of Food and Agriculture has awarded a UC Riverside-led research team $4.67 million. Chandrika Ramadugu, a UCR botanist leading the project, helped identify microcitrus varieties with natural resistance to HLB about eight years ago.
“HLB is caused by bacteria, so many people are trying to control it with antimicrobial sprays,” Ramadugu said. “We want to incorporate resistance into the citrus trees themselves through breeding, to provide a more sustainable solution.”
Part of the challenge with this approach to solving the HLB problem is that it's possible to breed hybrids that are resistant to the disease but don't taste good, Ramadugu said. “Hence the need to generate a lot of hybrids and screen them for the ones that will be most ideal for the citrus industry.”
Microcitrus, such as the Australian finger lime, tends to have a sharper, more bitter taste than its relative citrus fruits, like oranges. The perfect cross will have just the right mix of genes to give it sweetness and HLB resistance.
Ramadugu's team includes collaborators from Texas A&M University, the University of Florida, Washington State University and the U.S. Department of Agriculture, as well as scientists from UC Riverside's Department of Botany and Plant Sciences.
Currently, the team is studying differences in the genetic makeup of the hybrids they've already bred. Analyzing the new plants' DNA will help the team see whether enough disease resistance has been bred into the fruit, but not so much that the flavor is compromised.
Another challenge with breeding is the time it takes for new citrus varieties to flower naturally, which can be several years. With the help of Sean Cutler, UCR professor of plant cell biology, the team is hoping to accelerate the time it takes for the hybrid plants to bear fruit in a greenhouse.
This way the hybrids can be analyzed for taste much sooner. Clones of the best hybrid plants will then be grown in Florida and Texas field trials.
UC Riverside scientists are using a variety of approaches to fight HLB. While some hope that altering soil and root bacteria will improve plants' immunity to the disease, others are trying to improve HLB resistance by tweaking citrus metabolism, or by using an antibacterial peptide to clear HLB from an infected plant.
The fruit produced through Ramadugu's method will appeal to many consumers because it will not have genes introduced into them by scientists. Breeding has been done for thousands of years to improve crops and is considered a more natural practice.
Additionally, Ramadugu says she's excited about her approach because it will ultimately produce a product useful for growers and consumers.
Researchers from Sacramento State and the University of California, Riverside are requesting input from citrus industry members to help examine the economics of Asian citrus psyllid (ACP) and huanglongbing (HLB) management in California citrus groves.
The research team is looking for growers, advisors and other citrus industry members in California to provide input on overall knowledge of ACP and HLB, how they obtain information on the pest and disease, and how this might influence grove management practices. What is learned from this survey will help advance an economic analysis, contribute to overall understanding of ACP and HLB management, and improve the design and effectiveness of outreach and Extension resources to manage ACP and HLB.
If you are interested in providing input for forthcoming research, please complete the survey here.
This study is part of a U.S. Department of Agriculture Emergency Citrus Disease Research and Extension-funded project investigating microbial biocontrol to help in the fight against Candidatus Liberibacter asiaticus, the bacterium that causes HLB.
As the threat of HLB and ACP continue to put pressure on the commercial citrus industry, researchers across California are working to find the best treatments for this deadly disease. In a previous news release, Victoria Hornbaker, director of the Citrus Pest and Disease Prevention Division said that while these developments are promising to the future of the citrus industry, “It will take some time -- perhaps years -- before the potential treatment is on the market. In the meantime, it is important for industry members to remain vigilant in implementing best practices in the fight against huanglongbing and the Asian citrus psyllid.”
When answering survey questions, it is not necessary to look up records or calculate precise figures. The survey takes approximately 20 minutes to complete. All answers will be kept anonymous, and results will be presented in aggregate.
For questions about the survey or the research project, contact Jonathan Kaplan (firstname.lastname@example.org) at California State University, Sacramento.
As of December 4, a total of 2,196 residential trees and 321 ACP have tested positive via PCR for the bacterium that causes HLB. See the latest HLB map and table for details: maps.cdfa.ca.gov/WeeklyACPMaps/HLBWeb/HLB_Treatments.pdf. As before, the infected trees have been or are being removed, and ACP treatments are applied on a recurring basis to remaining citrus in those areas. To date, no HLB has been found in commercial trees via PCR testing. The HLB quarantine area, however, includes commercial citrus and continues to expand.
Please refer to the CDFA Action Plan for ACP and HLB for information on regulatory and treatment requirements to expect should HLB be detected in or near your citrus grove or packing house.
Best Practices in the Field
In addition to monitoring and treating for ACP, another important way to protect your citrus from ACP and HLB is to follow Best Practices in the field, including insisting that all equipment, vehicles and bins be free of all plant material before entering your property.
Science-based analyses to guide policy decisions, logistics, and operations: www.datoc.us
ACP trap detections have increased recently on the coast and in the Central Valley. While we usually see trap numbers peak this time of year, this fall the numbers have been higher than the last few years. Please stay vigilant in monitoring your trees for ACP, treating for ACP during the Area Wide Management treatment windows, and using an ACP-effective insecticide if possible when conducting other orchard management applications.
Map of HLB Quarantine and Treatment Area in California
Fastidious plant pathogens infect citrus, tomatoes, potatoes, grapes, peppers and other crops grown throughout Texas. Often transmitted by insect vectors, these disease agents cause billions of dollars of damage each year. The U.S. citrus industry alone would save $3 billion per year through control of just one of these diseases — citrus greening. Additionally, the fastidious pathogen that causes Pierce's Disease in grapes is the No.1 threat to the $1 billion wine industry in Texas.
“Currently, invasive fastidious pathogens are causing several major outbreaks in row crops, specialty crops and citrus, with immense costs to Texas and the U.S.,” said Juan Landivar, Ph.D., director of the AgriLife center at Weslaco, which has been involved in efforts to combat fastidious plant pathogens for many years.
Landivar said an expanded effort against fastidious plant diseases would protect the health of crops, environments, economies and people across the country.
A way to grow “unculturable” bacteria
Some plant pathogens can be grown as pure cultures in the laboratory in the presence of artificial nutrient solutions. Being able to culture disease agents in the lab facilitates their study by providing researchers with a reliable supply of experimental material. However, an estimated 99% of bacteria in the world are fastidious, or unable to grow outside their native environment.
“The greatest obstacle to understanding and controlling fastidious pathogens was the inability to cultivate them in a laboratory setting and to screen for lots of potential therapies,” said Leland “Sandy” Pierson, Ph.D., professor and head of Texas A&M's Department of Plant Pathology and Microbiology. “But Dr. Mandadi and his team have developed a breakthrough method as an alternative means to propagate fastidious bacteria. These bacteria are believed to be responsible for Huanglongbing, also known as citrus greening disease, and other insect-vectored diseases such as potato zebra chip and tomato vein greening disease.”
The breakthrough came in the form of the “hairy root” system. This technology utilizes the pathogen-infected host tissues to produce so-called hairy roots that can serve as biological vessels for the propagation of these pathogens in the laboratory.
“Classical microbiological techniques developed early in the 19th century cultured animal and mammalian viruses in host cells, tissues and embryonated eggs,” Mandadi said. “In a similar manner, we hypothesized that plant hairy roots could be suitable for propagating fastidious pathogens. And indeed, hairy roots supported the accumulation and growth of fastidious plant bacteria.”
Microbial hairy roots appear similar to normal root tissues that develop from the plant and mimic a bacterium's natural environment, he said. This allows the growth of the fastidious pathogens in controlled laboratory conditions.
Expedited screening for antimicrobial treatments
While microbial hairy root cultures are not traditional “pure” test tube cultures, they allow on-demand access to the fastidious bacterium in the laboratory. This enables the expedited screening of diverse antimicrobials like chemical inhibitors, immune modulators as well as gene/CRISPR-based therapies.
Other advantages are that hairy root cultures are easy to produce in the laboratory and can be maintained for several months to a year in laboratory growth chambers. Depending on the pathogen and the efficacy of screening, it is also at least four times faster than conventional screening methods, according to Sonia Irigoyen, Ph.D., and Manikandan Ramasamy, Ph.D., both AgriLife Research scientists and co-authors of the study.
In addition, the hairy root bioassays are scalable, so they can be used to pre-screen from a few to several hundred potential therapies simultaneously in a high-throughput manner. The microbial hairy root system can also be used to obtain mechanistic insights into antimicrobial function.
“Use of this technique has already led to the discovery of six new antimicrobial peptides with proven efficacy in plant materials,” Mandadi said. “These antimicrobials, either singly or in combination, could be used as near- and long-term therapies to control citrus greening, potato zebra chip and tomato vein greening diseases.”
Collaborators in the fight
“Typically, the type of breakthrough Dr. Mandadi and his team came up with is unusual for a university system off-campus center, as such centers usually have limited personnel and resources,” Landivar said. “Fortunately, the support we have received from the Texas A&M University System and other funding agencies and collaborators has helped make it possible for the Weslaco center to perform this world-class-level research.”
To expand on his research, Mandadi recently partnered on a new project with Citrus Research and Development Foundation, Bayer, Southern Gardens Citrus, University of Florida and University of California-Davis. That project is funded by the NIFA ECDRE program. The overall goal is to bring together academics, growers and agrochemical industry to discover, develop and commercialize therapies for citrus greening disease.
Mandadi said use of the hairy root system has already been instrumental in finding several potential new treatments for citrus greening and potato zebra chip, as described in the Nature Communications article.
“We hope this technology can be further expanded to find even more therapies against current and emerging fastidious pathogens and, ultimately, with the support of industry, deploy them as field-ready products,” he said.
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.