- Author: Ben A Faber
Greening Bacterium Causes Changes in Psyllids
Recent studies, including a partnership project between Fundecitrus and the University of California, revealed that the citrus greening bacterium Candidatus Liberibacter asiaticus causes physiological changes in psyllids, posing additional challenges to management strategies. An increase in the number of eggs, more frequent dispersal flights over longer distances and greater attractiveness to the host are some of the changes observed in infected psyllids.
MORE DIFFICULT TO MANAGE
“Epidemiologically speaking, the changes we have been observing in psyllid behavior turn it into a much more problematic insect,” said Fernando Amaral, Fundecitrus agricultural engineer and post-doctoral student at the São Paulo University Luiz de Queiroz College of Agriculture.
The psyllid behavioral changes hinder the development of pheromone tools to attract the insect and improve its monitoring.
“It is becoming increasingly clear that the psyllid undergoes several changes, and this phenomenon not only poses difficulties to management, but also curbs the development of products to capture the insect,” added Fundecitrus researcher Haroldo Volpe.
Studies published between 2015 and 2024 revealed that psyllids infected with the greening bacteria can lay up to 100% more eggs than healthy insects, contributing to the growth of the psyllid population.
FREQUENT FLIERS
The studies also concluded that infected psyllids are more agitated when compared to healthy insects. In order to reach this conclusion, researchers placed adult insects from both groups on a platform. The teams noticed that infected insects flew, on average, after 50 seconds from the beginning of the observation period. The healthy subjects took around 150 seconds.
“Knowing that the insect flies more often and starts flying earlier demonstrates agitation and altered behavior, increasing its ability to spread greening,” said Volpe.
Yet another conclusion of the studies is that infected psyllids perform more frequent dispersal flights.
“Psyllids infected with the greening bacteria have a 45% higher rate of long flights when compared to healthy insects,” said Amaral.
In other words, the infected psyllid will fly longer distances and further spread the disease. Moreover, infected psyllids will also have a greater need to feed (forage) on more shoots and consequently will further disseminate the disease.
SUSCEPTIBILITY TO INSECTICIDES
On the other hand, the susceptibility of infected insects to insecticides is greater than that of healthy insects. Psyllids infected with the greening bacteria require a 20% to 313% lower concentration of insecticides to achieve the same mortality rate as healthy insects. This happens because the bacteria interferes with the psyllid metabolization of these products, which hinders their detoxication process.
CONTINUE COMBATING DISEASE
Fundecitrus General Manager Juliano Ayres emphasized the need for citrus growers to remain aware of the measures used to combat the disease in the field.
“The more diseased plants in groves without appropriate psyllid control, the more contaminated insects there will be and, consequently, the faster the disease will spread,” Ayres said. “Therefore, it is essential to continue to eliminate diseased plants from groves and keep up strict control of the insect on these plants.”
Source: Citricultor, Fundecitrus
CLas-Positive Psyllid Sample in Riverside County
July 26, 2024
An adult Asian citrus psyllid (ACP) sample from a residential property in the San Jacinto Valley area of Riverside County, California, has tested positive for Candidatus Liberibacter asiaticus (CLas), the bacterium that causes huanglongbing (HLB).
The positive sample was collected as part of the Multi-Pest Risk Survey on a residential property in Hemet. It was confirmed positive for CLas on July 17 by the Citrus Research Board's Jerry Dimitman Laboratory. Nymphs were also collected from the property and tested negative for CLas. This is the first confirmed CLas-positive adult ACP found in the San Jacinto Valley area.
An HLB quarantine zone will not be established as a result of this CLas-positive ACP detection. However, California Department of Food and Agriculture (CDFA) staff is conducting surveys and collecting samples from the property and all HLB host plants that are located within a 250-meter radius around the find, per the ACP/HLB Action Plan.
It is crucial that ACP populations continue to be controlled properly in order to stop HLB from spreading, advised California's Citrus Pest & Disease Prevention Program.
While CDFA is not requiring mandatory treatment for area commercial growers, those who wish to take proactive steps to protect their groves or who have additional questions can contact Riverside County Grower Liaison Sandra Zwaal.
In September 2023, a CLas-positive ACP sample was collected from a residential property in California's Ventura County. That sample came from a residential citrus tree in the southwest area of Santa Paula. An HLB quarantine zone was not established as a result of that detection, either. While that first confirmation of a CLas-positive ACP in Ventura County was concerning, HLB was not detected in any Ventura County citrus trees. Learn more here.
Source: Citrus Pest & Disease Prevention Program
- Author: Brad Buck, UF/IFAS
LAKE ALFRED, Fla. — Sometimes in science, a new perspective brings an “a ha!” moment. That's what one senior researcher at the University of Florida Institute of Food and Agricultural Sciences believes happened with his latest research on Huanglongbing (HLB), or citrus greening.
HLB is worldwide, devastating citrus disease caused by Candidatus Liberibacter asiaticus (CLas), a bacterium that settles into the tree's phloem — its interior vascular system — eventually killing the tree. Since first found in Florida in 2005, it has infected virtually every grove in Florida and cost the citrus industry billions of dollars.
UF/IFAS' Nian Wang's most recent research describes in detail how HLB causes damage to citrus trees and presents the case that HLB is a pathogen-triggered immune disease. A pathogen-triggered immune disease is a disease that results from the activation of an organism's immune cells fighting a pathogen (a virus, bacteria, or parasite) that invades an organism, in this case, the citrus plant.
This is the first time that this explanation of HLB symptoms as pathogen-triggered immune responses has been presented and defended. Seeing HLB in this new context may mean finding new solutions to the disease, faster. Pathogen-triggered immune diseases have not been reported in the world of plants that includes over 250,000 species but are common in humans.
Scientists don't fully understand how the pathogen that causes HLB damages infected citrus plants because it has not been cultured in artificial media. With this new evidence that supports a premise that HLB is an immune-mediated disease, researchers can see new light on how to manage HLB.
Through his research Wang, has shown HLB infection stimulates systemic and chronic immune responses in phloem tissue, especially overproduction of reactive oxygen species (ROS), which are part of the plant's immune response. Chronic and excessive ROS production is responsible for systemic cell death of phloem tissues, which in turn causes HLB symptoms. This supports the hypothesis that HLB is an immune-mediated disease.
Antioxidants and immunoregulators are commonly used to treat human immune-mediated diseases. They halt or reduce the process that results in cell death. In citrus, the researchers tested whether growth hormones like gibberellin acid (GA) and antioxidants (uric acid and rutin) could impact cell death triggered by the infection and, therefore, block or reduce HLB symptoms.
The researchers found the GA and uric acid had an encouraging positive impact on infected trees. This has also been supported by other research in process at the UF/IFAS Citrus Research and Education Center.
“Our findings allow us to control HLB by mitigating ROS with integrated horticultural measures, genetic improvements of citrus varieties with antioxidant enzymes, generating non-transgenic HLB resistant/tolerant citrus varieties by editing key genes required for CLas-triggered ROS production, and using CTV-mediated expression of antioxidant enzymes and silencing of key genes required for CLas-triggered ROS production,” said Wang, a professor of microbiology and cell science at UF/IFAS.
Wang's research also tested existing grove management practices that integrate antioxidants, micronutrients (activating antioxidant enzymes), gibberellin (mitigating ROS, regulating immune response, and promoting new growth) and optimized fertilization and irrigation (reducing ROS production) that are available in Florida. All the products tested, except the antioxidants, have already been registered for citrus production, thus can be easily adopted by citrus growers.
Wang's research has also identified the RBOHD gene to be the main producer of CLas-triggered ROS. With this, the researchers can now “edit (using CRISPR technology)” or “silence” RBOHD or specific receptor gene(s) responsible for RBOHD activation and the resulting cell death of phloem tissues and HLB symptom development. It is expected these approaches will allow economic and efficient citrus production in HLB endemic citrus production regions such as Florida and Texas and provide the long-term solution against HLB for all citrus production regions of U.S. (including California) in case that HLB becomes endemic in the future.
Read the whole story:
Key Points About Plant "Immunity"
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Unlike vertebrates, plants do not have an adaptive immune system. Nonetheless, plants can launch specific, self-tolerant immune responses and establish immune memory.
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To promote virulence, pathogens inject effector molecules that target conserved immune signalling hubs into the plant cell. In response, plants have evolved resistance (R) proteins that detect effector-induced perturbations in these hubs, providing the potential to specifically recognize a large number of pathogens with similar infection strategies through a smaller number of R proteins.
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Intraspecific and interspecific plant crosses suggest that autoimmunity can arise from self-reacting R proteins, illustrating the threat of uncontrolled R protein activity. Dynamic transcriptional and post-transcriptional regulation of R protein levels is thought to minimize the risk of autoimmunity in plants.
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Pathogen-infected tissues generate a mobile immune signal consisting of multiple proteins as well as lipid-derived and hormone-like molecules. These signal molecules are transported to systemic tissues, where they induce systemic acquired resistance (SAR). SAR is associated with the systemic reprogramming of thousands of genes to prioritize immune responses over routine cellular requirements.