Provided by American Phytopathological Society

While humanity is facing the COVID-19 pandemic, the citrus industry is trying to manage its own devastating disease, Huanglongbing (HLB), also known as citrus greening disease. HLB is the most destructive citrus disease in the world. In the past decade, the disease has annihilated the Florida citrus industry, reducing orange production for juice and other products by 72%. Candidatus Liberibacter asiaticus (CLas) is the microbe associated with the disease. It resides in the phloem of the tree and, like many plant pathogens, is transmitted by insects during feeding events. Disease progression can be slow but catastrophic. Symptoms begin with blotchy leaves, yellow shoots, and stunting, and progress into yield decline, poor quality fruit, and eventually death.

Currently, the only thing citrus growers can do to protect their crops from HLB is control the insect vector. Dozens of researchers are trying to find ways to manage the disease, using strategies ranging from pesticides to antibiotics to CLas-sniffing dogs. Understanding the plant microbiome, an exciting new frontier in plant disease management, is another strategy.

Dr. Caroline Roper and first author Dr. Nichole Ginnan at the University of California, Riverside led a large research collaboration that sought to explore the microbiome's role in HLB disease progression. Their recent article in Phytobiomes Journal, "Disease-Induced Microbial Shifts in Citrus Indicate Microbiome-Derived Responses to Huanglongbing," moves beyond the single-snapshot view of the microbial landscape typical of microbiome research. Their holistic approach to studying plant-microbe interactions captured several snapshots across three years and three distinct tissue types (roots, stems, and leaves). What is so interesting about this research is the use of amplicon (16S and ITS) sequencing to capture the highly intricate and dynamic role of the microbiome (both bacterial and fungal) as it changes over the course of HLB disease progression.

Ginnan et al. surmised that HLB created a diseased-induced shift of the tree's microbiome. Specifically, the researchers showed that as the disease progresses, the microbial diversity increases. They further investigated this trend to find that the increase in diversity was associated with an increase in putative pathogenic (disease-causing) and saprophytic (dead tissue-feeding) microbes. They observed a significant drop in beneficial microbes in the early phases of the disease. Arbuscular mycorrhizal fungi (AMF) were one such beneficial group that the authors highlighted as showing a drastic decline in relative abundance.

The depletion of key microbial species during disease might be opening the door for other microbes to invade. Certain resources may become more or less available, allowing different microbes to prosper. Dr. Roper and Dr. Ginnan hypothesize that when HLB begins, this depletion event triggers a surge of beneficial microbes to come to the aid of the citrus tree. They suspect that the microbes are initiating an immune response to protect the host.

As the disease proliferates, the citrus tree and its microbiome continue to change. Dr. Ginnan, the lead author on this study, found that there was an enrichment of parasitic and saprophytic microorganisms in severely diseased roots. The enrichment of these microbes may contribute to disease progression and root decline, one side effect of HLB.

Survivor trees, or trees that did not progress into severe disease, had a unique microbial profile as well. These trees were enriched with putative symbiotic microbes like Lactobacillus sp. and Aureobasidium sp. This discovery led the researchers to identify certain microbes that were associated with slower disease progression.

Dr. Ginnan says their "aha" moment during the research was in the data analysis. "Originally we were looking for taxa that increased and decreased in relative abundance as disease rating increased," she said. "Our differential abundance analysis ended up revealing clear enrichment patterns replicated in multiple taxa." This is the moment they began to develop the individual patterns they were seeing into a broader disease model.

This research is the foundation for future projects and collaborations that the authors are excited to continue to develop. They are motivated by the potential function of the microbiome to manage crop diseases. In the near future, they hope that these discoveries and an understanding of beneficial microbes can help establish a microbiome-mediated treatment plan to protect crops from diseases like HLB. In addition, the model they've developed can be applied to understanding diseases of other tree crop systems.

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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 (kaplanj@csus.edu) at California State University, Sacramento.

Source: Citrus Pest & Disease Prevention Program

Plants get bacterial infections, just as humans do. When food crops and trees are infected, their yield and quality can suffer. Although some compounds have been developed to protect plants, few of them work on a wide variety of crops, and bacteria are developing resistance. Now, researchers reporting in ACS' Journal of Agricultural and Food Chemistry have modified natural plant alkaloids into new compounds that kill bacteria responsible for diseases in rice, kiwi and citrus.

Currently, no effective prevention or treatment exists for some plant bacterial diseases, including rice leaf blight, kiwifruit canker and citrus canker, which result in substantial agricultural losses every year. Scientists are trying to find new compounds that attack bacteria in different ways, reducing the chances that the microbes will develop resistance. Plant compounds called tetrahydro-β-carboline (THC) alkaloids are known to have antitumor, anti-inflammatory, antifungal, antioxidant and antiviral activities. So, Pei-Yi Wang, Song Yang and colleagues wondered whether derivatives of THC alkaloids could help fight plant bacterial diseases.

The researchers used a THC alkaloid called eleagnine, which is produced by Russian olive trees and some other plants, as a scaffold. To this framework, they added different chemical groups to make a series of new compounds, two of which efficiently killed three strains of plant pathogenic bacteria in liquid cultures. The team then tested the two compounds on rice, kiwi and citrus plant twigs and leaves and found that the new alkaloids could both prevent and treat bacterial infections. The researchers determined that the compounds worked by increasing levels of reactive oxygen species in the bacteria, which caused the bacterial cells to die.

How it works

The abstract that accompanies this paper can be viewed here.

Antibacterial Functions and Proposed Modes of Action of Novel 1,2,3,4-Tetrahydro-β-carboline Derivatives that Possess an Attractive 1,3-Diaminopropan-2-ol Pattern against Rice Bacterial Blight, Kiwifruit Bacterial Canker, and Citrus Bacterial

Hong-Wu Liu, Qing-Tian Ji, Gang-Gang Ren, Fang Wang, Fen Su, Pei-Yi Wang*, Xiang Zhou, Zhi-Bing Wu, Zhong L and Song Yang*