Ah, summer! The season of sunburns, pool parties, and… lawn droughts. If your once lush, green carpet now looks like a crunchy brown doormat, you're not alone. Let's dive into why your yard is staging a dramatic death scene and what you can do to...
Huanglongbing (HLB), also known as citrus greening disease, is caused by the bacterium Candidatus Liberibacter asiaticus, which is transmitted by the Asian citrus psyllid. The bacterium harms citrus trees in several ways, leading to severe physiological and structural damage.
After the infection, the HLB bacterium invades the phloem, the vascular tissue responsible for transporting nutrients throughout the tree. HLB causes a disruption in nutrient uptake and transport within the tree. This leads to nutrient deficiencies, particularly in essential elements such as nitrogen, phosphorus, and potassium. As a result, the tree experiences stunted growth, yellowing of leaves (chlorosis), and overall nutrient imbalance. The bacterium affects the root system, leading to root loss and dieback. This compromises the tree's ability to take up water and nutrients from the soil, exacerbating the impact of nutrient deficiencies and water stress. Additionally, it affects the photosynthetic process in citrus trees. The bacterium's presence disrupts chloroplast function, reducing the tree's ability to convert sunlight into energy. This results in decreased photosynthesis, further contributing to nutrient deficiencies and overall tree decline. One of the characteristic symptoms of HLB is the yellowing of leaves, often referred to as citrus greening. The bacterium affects the chlorophyll content in leaves, leading to a mottled appearance. This yellowing occurs unevenly on the tree and is a clear indicator of HLB infection. HLB-infected trees experience stunted growth due to the disruption of normal physiological processes. The overall health of the tree declines, leading to a reduction in fruit production and quality. Infected trees may produce smaller, misshapen, and discolored fruit. HLB-infected trees often exhibit premature fruit drop, where fruit drops from the tree before reaching maturity. This is a significant economic impact for citrus growers, as it reduces the yield and marketable fruit. Over time, HLB-infected trees undergo a general decline in health. The combination of nutrient deficiencies, reduced photosynthesis, and compromised vascular function can lead to the death of the tree. The lifespan of infected trees is significantly shortened compared to healthy trees.
It's important to note that there is currently no cure for HLB, and managing the disease involves a combination of strategies. While researchers from all over the world are working hard to get a cure for that disease, citrus growers are increasingly turning to cultural practices (a diverse set of agricultural strategies) as a sustainable and holistic approach to mitigate the profound impact of this relentless disease. Cultural practices encompass a nuanced and multifaceted array of agricultural techniques and approaches used in citrus groves. In the realm of citrus and HLB management, a comprehensive strategy is imperative to create an environment that not only minimizes the likelihood of disease transmission but also nurtures the robustness of citrus trees and makes them more resilient to HLB.
Various agencies in California are making significant efforts on Psyllid monitoring and management, a first defense line to stop the spread of the disease. Regular Monitoring: Instituting a meticulous and frequent monitoring system for Asian citrus psyllid populations is paramount. This proactive approach enables growers to detect infestations early, facilitating targeted interventions to halt the spread of HLB. In addition to Psyllid monitoring and management, harnessing the power of nature through the introduction of beneficial insects establishes a delicate ecological balance. These natural predators act as biological control agents, keeping psyllid populations in check without resorting to chemical pesticides. Swift identification and removal of HLB-infected trees are critical components of disease management. The prompt destruction of infected trees eliminates potential bacterium reservoirs, mitigating the risk of further psyllid transmission to healthy trees. Utilizing beneficial microorganisms as soil inoculants enhances root health and improves nutrient uptake, potentially bolstering the tree's ability to withstand HLB stress.
Another method is to isolate the trees from the Psyllid through growing the Citrus trees Under Protective Screen (CUPS). The system is a closed screenhouse built on a number of acres from 1-5 acres. The system is currently used by some growers in Florida; however, the response of the Californian varieties to grow under such conditions is not known. The first CUPS structure was established at UC Lindcove research and extension center, and we are currently evaluating the performance of some varieties under this structure.
Recognizing that weeds can serve as alternative hosts for psyllids, implementing rigorous weed management practices eradicates potential habitats and minimizes the presence of psyllids in citrus groves. Studies in Florida and California showed a beneficial effect of using wood chip mulching on root growth and weed control. We are collecting more data on these trials, and it will be available for the growers during the next season.
Managing nutrients with precision is vital for supporting citrus tree health. Tailoring fertilization to the unique needs of the orchard ensures trees receive optimal amounts of essential nutrients like nitrogen, phosphorus, and potassium. This fortification enhances natural defenses against HLB. Additionally, implementing consistent and well-timed irrigation practices is crucial. Effective water management alleviates stress on citrus trees, reducing susceptibility to HLB and other stress-related diseases.
Further, incorporating HLB-resistant rootstocks is a crucial aspect of disease management. Selecting rootstocks that exhibit resistance or tolerance to the bacterium contributes significantly to orchard health. Ongoing research endeavors focus on identifying citrus varieties that exhibit inherent resistance or tolerance to HLB. The strategic planting of these varieties adds an extra layer of protection against the disease and its associated symptoms, contributing to the overall health and productivity of citrus orchards. Currently, we are evaluating the effect of a number of HLB resistant rootstocks on the tango mandarin growing under California conditions. These rootstocks include US942, US897, and X639. The evaluation of the resistant rootstock and varieties will continue in collaboration with worldwide researchers.
In the face of the formidable challenge presented by HLB disease, cultural practices emerge as a promising, sustainable, and sophisticated approach to mitigation. By embracing a comprehensive strategy that intertwines vigilant psyllid management, optimization of tree health, meticulous sanitation measures, selective varietal planting, and collaborative initiatives, citrus growers can fortify the resilience of their orchards. The integration of cultural practices, including rootstock selection and proper fertilization, not only contributes to HLB control but also nurtures the long-term sustainability of citrus farming, ensuring a future where citrus crops not only survive but thrive in the face of adversity. As our understanding of cultural practices evolves, the citrus industry can anticipate a resilient, vibrant, and flourishing future.
The International Research Conference on Huanglongbing VII (IRCHLB VII) is just around the corner! The California citrus industry is excited to welcome researchers, regulators, and citrus industry members from around the world to the Golden State. Do not miss these key dates:
Registration Deadline: Friday, March 15, 2024
A hotel room block with special conference rates has been set aside near the convention center and is filling up quickly. Book your room today at the Historic Mission Inn Hotel & Spa.
For the conference agenda, please click here.To register and access more conference details, CLICK HERE..
This Conference has occurred every two years (except for the covid period). Check out the Proceedings from previous conferences!
The proceedings of the “International Research Conference on Huanglongbing” are available from the 2008, 2011, 2013, 2015, 2017, and 2019 conferences. Please use the following hyperlinks to access the proceedings.
UCR wages $11+ million war against citrus greening disease
Three projects win funding to fight tree-killing bacteria
AUTHOR:JULES BERNSTEIN, UCR
With three new grants totaling more than $11 million, UC Riverside is helping lead the fight against citrus greening or Huanglongbing, a disease threatening citrus industries in the U.S. and worldwide.
Citrus infected with Huanglongbing, or citrus greening disease.
The disease is from bacteria transmitted to citrus trees by a tiny flying insect, the Asian citrus psyllid. Infected trees produce no fruit, or fruit that is bitter, small and worthless. Despite intensive research for the past 15 years, there is no known cure for it. It has reduced citrus production in Florida by more than 75%, and it has already been detected in Texas and California.
Because California supplies the country with 80% of its fresh citrus, and because 267,000 acres of Golden State lemons, oranges, grapefruits, and mandarins are at stake if operations are permanently lost, the USDA National Institute of Food and Agriculture is making an emergency investment in citrus disease research with three projects at UCR.
These projects focus on instilling tolerance to the disease with three different approaches: below ground, in the rootstocks, above ground in the shoots and branches, and systemically, with a peptide that would move throughout the tree.
Nymphs of the Asian citrus psyllid, that transmits the bacterium causing citrus greening disease.
The largest of the projects, at $6.8 million, is being led by Danelle Seymour, assistant professor of genetics in the Botany and Plant Sciences Department at UCR. Their focus is on breeding Huanglongbing or HLB-resistant rootstocks, and the project depends on collaboration with Kim Bowman, a citrus breeder at the USDA Agricultural Research Service in Ft. Pierce, Florida.
“In Florida, nearly every single tree is infected. It's terrible for growers, but wonderful for breeding,” Seymour said. “We can't do this research at large scale in California because the disease isn't as widespread here.”
The classic way to improve resistance or encourage new qualities in crops is through genetics, making crosses between one plant that has a favorable trait, and one that doesn't. “We hope the result is better than the parents,” Seymour said. “When you work in wheat or tomato, you can do these crosses and perform evaluations every year. In citrus, it takes 10 - 15 years to evaluate a new generation of trees.”
Because of the long lag time, the research-ready trees in Florida represent an opportunity for Seymour's team to begin examining new crosses now. The breeder, Kim Bowman, has evaluated over 10,000 trees and unique hybrids, from which a handful will be selected for release to growers.
In addition to evaluating these select few new hybrids for their HLB tolerance, the researchers will be watching the Florida-grown trees' responses to the different environmental conditions in California. “Can they perform well in response to different salinity levels in the soil, different humidity, as well as other pests and pathogens that we have here? We'll find out,” Seymour said.
Chandrika Ramadugu, a project scientist also in UCR's Department of Botany and Plant Sciences, is leading a project to develop HLB-resistant scion varieties that can be grafted to rootstocks. A scion is an above-ground portion of a plant, such as a bud or shoot, that can be used for grafting.
For trees, grafting can be equated to an organ transplant. The scion from one tree is attached to the trunk or rootstock of another with the hope of creating a new plant with combined attributes. With its grant of $3.28 million, this project will analyze second-generation hybrids that are bred for ten years using Australian lime as a source of disease resistance.
Ramadugu will evaluate 24 novel hybrids in California, Florida, and Texas to assess resistance to HLB. Ideally, in addition to having enhanced disease tolerance, the new plants will also be able to produce good-tasting fruit.
There is little genetic diversity in cultivated citrus. When new pathogens arrive, the genetic uniformity can result in disease epidemics and dire consequences for the crop. In addition to the potential benefits of this project for the fight against HLB, the new hybrids may also help protect citrus from other pests and pathogens.
A third project, granted $1.36 million, will utilize a peptide found in Australian finger limes that is known to impart HLB resistance. Led by Hailing Jin, Microbiology & Plant Pathology professor, the project is developing ways to infuse trees with the peptide.
“The antimicrobial peptide in the finger limes are more efficient at killing bacteria as compared to antibiotics currently used in the field, and much more stable at high temperatures,” Jin said.
Because spray applications are expensive, Jin's project aims to spread the peptide throughout the trees' insides. In collaboration with University of Florida professor Svetlana Folimonova, Jin's team utilizes a natural citrus virus with almost no symptoms to deliver the peptide into the trees.
“You infect the tree with the virus, and it will spread in areas where the bacteria reside,” Jin said. “It would move systemically through the tree, and it would be very cost efficient for growers. No need to buy more insecticides.”
These grants were enabled by the 2018 Agricultural Improvement Act, which authorized the Emergency Citrus Disease Research and Development Trust Fund to fight HLB. With these and other projects, the USDA is bringing together the nation's top scientists to find scientifically sound solutions to the problem in a financially and ecologically sustainable way.
Asian citrus psyllids can spread the citrus tree-killing disease huanglongbing. All photos credited to UC Regents
Residents urged to check their citrus trees for Asian citrus psyllid
An insect carrying the huanglongbing bacteria, a pathogen that kills citrus trees, has been found on a residential citrus tree in Ventura County, according to the California Citrus Pest & Disease Prevention Program.
“This really is a devastating find,” said Ben Faber, UC Cooperative Extension subtropical crops advisor for Ventura and Santa Barbara counties. “It means that Asian citrus psyllid that is infected with the HLB bacteria is present in the middle of a citrus-growing area that is currently and historically important for lemon production.”
This is the first confirmed case in Ventura County of Asian citrus psyllid testing positive for the bacterium Candidatus Liberibacter asiaticus. The bacterium is associated with huanglongbing, or HLB, also called citrus greening disease. HLB can kill a citrus tree in as few as five years, and there is no known cure or remedy. All commonly grown citrus varieties are susceptible to the pathogen, which means commercial citrus growers should monitor and treat to manage its vector to protect their groves. The additional expenses come at a bad time.
“The disease is a tree killer and it's happening to growers who are already having difficult times with low lemon prices making it difficult for them to stay and continue farming,” Faber said.
An adult psyllid is about the size of an aphid.
The infected Asian citrus psyllid was found in the Santa Paula area, but the UCCE farm advisor asks all Ventura County citrus tree owners to be on the lookout for the tiny, mottled brown insect about the size of an aphid.
“It takes a whole community to address the disease in order to protect all the trees,” Faber said. “If one tree remains, it potentially can infect all the surrounding citrus trees.”
The psyllid takes the bacteria into its body when it feeds on bacteria-infected plants. When a bacteria-carrying psyllid flies to a healthy plant and injects the bacteria into the plant as it feeds, it can spread the disease.
Hamutahl Cohen, UCCE entomology advisor for Ventura County, saw the devastation of HLB on citrus in Florida, where she worked before moving to California.
“After 2005, we saw that HLB spread rapidly across Florida, resulting in tree mortality and increased costs of production,” she said. “Due to HLB, orange yield in Florida decreased about 40%. In California, growers will benefit if the public helps them by monitoring their backyard citrus trees."
As of Sept. 20, HLB had not been detected in any Ventura County citrus trees.
Neil McRoberts, UC Davis professor of plant pathology, urged vigilance, but not panic. “We have long suspected that ACP infected with the pathogen are present in Ventura, the results simply confirm that suspicion.”
ACP nymphs excrete distinctive waxy tubules
The only way to protect citrus trees from the lethal disease is to prevent the spread of the HLB pathogen by controlling psyllid populations and destroying infected trees.
“Not all ACP carry the bacteria, but if one is found, it means either that an infected insect has flown or hitchhiked in,” Faber said. The psyllids can fly pretty far by themselves, but they can move great distances when people move them.
To identify Asian citrus psyllid, see pictures of the psyllid and its life stages on the UC Agriculture and Natural Resources website at http://ucanr.edu/acp and the UC Integrated Pest Management Pest Note at https://ipm.ucanr.edu/PMG/PESTNOTES/pn74155.html. More information is also available at https://californiacitrusthreat.org , a website maintained by the Citrus Pest & Disease Prevention Program, an initiative funded by California citrus growers and administered by the California Department of Food and Agriculture.
Beth Grafton-Cardwell, UC Cooperative Extension citrus entomology specialist emeritus, and Mark Hoddle, UCCE entomology specialist based at UC Riverside, demonstrate how to look for the various stages of the psyllid in the video “Check your citrus trees for Asian citrus psyllid” on YouTube https://www.youtube.com/watch?v=UhqwUQm0zpk.
Report suspected cases of the psyllid or disease to your county agricultural commissioner's office or call the CDFA hotline at 1-800-491-1899.
Commercial growers can get the latest news about ACP at the Citrus Pest & Disease Prevention Program website https://citrusinsider.org.
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