Citrus greening disease can infect an entire tree weeks before symptoms appear
American Phytopathological Society
For the first time ever, scientists have been able to measure the speed of a bacterium that causes the incurable citrus greening disease. Citrus greening disease (also known as Huanglongbing) is the most devastating citrus disease in the world. Afflicted trees grow yellow leaves and low-quality fruit and eventually stop producing altogether, resulting in enormous economic losses to farmers.
Small insects carry the causal bacteria, Candidatus Liberibacter asiaticus (CLas), and inject it when they feed on a tree's sap. CLas also relies on this sap to grow and spread throughout the tree. Using a new statistical modeling analysis and measurement approach, plant pathologists were able to follow CLas on its journey through a tree.
“We found that CLas can move at average speed of 2.9 to 3.8 cm per day. At these speeds a tree that is 3 meters in height will be fully colonized by CLas in around 80 to 100 days, and this is faster than the symptoms appear, which generally takes at least 4 months,” explained Silvio A. Lopes, a plant pathologist based at Fundecitrus, research institution maintained by citrus growers of the State of Sao Paulo in Brazil.
In other words, by the time growers see the symptoms of citrus greening disease, the tree has already been infected from the roots on up, which explains why removing symptomatic branches isn't enough to cure a diseased tree.
Lopes and colleagues also studied the impact of temperature on the speed of colonization. They already knew that CLas does not multiply well in hot or cold environments—but now they have more specific numbers.
“We estimated that 25.7°C (78°F) was the best condition for CLas to move from one side to the other side of the tree,” said Lopes. This is the first time impact of temperature on plant colonization of CLas has been experimentally demonstrated. “The grower can use this information to select areas less risky for planting citrus trees.”
Huanglongbing (HLB) and its causal agent Candidatus Liberibacter asiaticus (CLas) are a serious problem for the US citrus industry, with Florida and Texas already heavily affected, and California at an early stage. Rootstock cultivars with genetic tolerance to HLB improve tree health, fruit production, and fruit quality in HLB-affected orchards, but even the best rootstock available suffers large yield losses when infected. Further gains in tolerance or resistance to HLB are urgently needed to sustain the industry. Field trees in Florida are now all infected with CLas, providing an opportunity to screen thousands of trees for overall performance in environments with high disease pressure. Previous work to create and evaluate over 300 new rootstock hybrids across three HLB- endemic regions in Florida in replicated sweet orange field trials will be leveraged to deliver the best performing HLB-tolerant rootstocks for commercial release, including expanded collection of performance information over the next two years (Objective 1). Top performing rootstocks will be further screened for resistance to Phytophthora and other relevant soil-borne pathogens to ensure adoption in HLB-endemic and HLB-threatened regions. This extensive set of phenotypic data will be integrated with genetic information to identify the genetic control of HLB-tolerance and other important traits, enabling rapid selection of superior tolerant hybrid rootstocks in future breeding cycles (Objective 2). Commercial testing and release of rootstocks will occur in close consultation with industry members and will be disseminated to stakeholders at all levels through virtual and in- person seminars and large industry events.
PERFORMANCE OF 300 HYBRIDS IN ESTABLISHED TRIALS TO MAP HUANGLONGBING TOLERANCE/RESISTANCE GENES AND RELEASE SUPERIOR NEW ROOTSTOCKS
Seymour, D. K.; Deng, ZH, .; Rolshausen, PH, .; Bowman, KI, D.; Albrecht, UT, .
As of November 5, a total of 2,619 trees and 368 ACP have tested positive via PCR for the bacterium that causes HLB. The most recent activity has been in Orange and San Bernardino counties. Infected trees have been or are being removed, additional HLB detection surveys and ACP treatments are applied on a recurring basis to remaining citrus in those areas.
For additional details, please see the updated HLB quarantine and treatment map and table at maps.cdfa.ca.gov/WeeklyACPMaps/HLBWeb/HLB_Treatments.pdf.
For information on regulatory and treatment requirements growers can expect should HLB be detected in or near your citrus grove or packing house, please refer to CDFA's Information for Citrus Growers/Grove Managers, Action Plan for ACP and HLB or this summary flyer.
Mustang Maxx has been approved for Spray and Harvest. For growers who harvest in one ACP Regional Quarantine Zone and pack in another, and use Spray and Harvest as their mitigation for moving bulk citrus, the list of approved ACP materials has been updated. The most recent list and protocols for quarantine compliance can always be found in the Information for Growers/Grove Managers document from CDFA. Please keep in mind this is separate and distinct from protocols and materials for Area Wide Management, discussed above.
Additional ACP/HLB Resources
- Check out the new and improved CDFA Citrus Division website: https://www.cdfa.ca.gov/Citrus/
- General ACP/HLB
oInformation on the state ACP/HLB program including maps, quarantine information, and a signup option for email alerts: citrusinsider.org/
oBiology of ACP and HLB, detection maps and recommendations for monitoring, eradication and management: ucanr.edu/sites/acp/
oUC IPM recommendations for ACP
oWeb-based map to find out how close you are to HLB: ucanr.edu/hlbgrowerapp
oVideo on Best Practices in the Field, available in English and Spanish
oSummaries of the latest research to combat HLB: ucanr.edu/sites/scienceforcitrushealth/
oScience-based analyses to guide policy decisions, logistics, and operations: www.datoc.us
oSign up for program updates from the Citrus Pest and Disease Prevention Division at www.cdfa/signup-email-updates.
oRegulatory requirements for moving bulk citrus: Information for Citrus Growers
oSummary of regulatory requirements in the event of an HLB detection in commercial citrus: citrusinsider.org/Regulatory-Flyer
University of Florida researchers publish award-winning findings on orange grove design to produce fruit under HLB
FORT PIERCE, Fla.— The American Society for Horticultural Science will honor University of Florida scientists for new guidelines citrus growers may apply to their operations. The researchers study ‘Valencia' orange production while trees are under the most serious citrus disease worldwide.
“The 7-year study broke new ground on data we provide to local citrus growers who remain in business despite huanglongbing, or HLB,” said Ronald D. Cave, Director of the University of Florida Institute of Food and Agricultural Science's Indian River Research and Education Center in Fort Pierce (UF/IFAS-IRREC).
A few of the findings citrus growers may employ immediately from the study are:
• High-density plantings produce more fruit—from 86% up to 300% more than trees not planted in high-density configurations under HLB.
• The study showed that advanced management practices that included high tree density, fertigation, and drip irrigation led to higher fruit yield.
• Additional research is needed to determine optimal fertilization rates for high-density sweet orange orchards under HLB-endemic conditions
The award-winning publication, “Sweet Orange Orchard Architecture Design, Fertilizer, and Irrigation Management Strategies under Huanglongbing-endemic Conditions in the Indian River Citrus District,” appears in the December 2020 issue science of the scientific journal HortScience. The paper describes the scientists' hypothesis, the entire research procedure, outcomes, and recommendations for further research.
Members of the American Society for Horticultural Science (ASHS) Fruit Publication Selection Committee wrote about the paper's significance. “Huanglongbing, also known as citrus greening, affects all citrus cultivars and causes serious tree decline. It is currently a major threat to the citrus industry. The results can influence the whole citrus industry to deal with HLB including orchard architecture design, fertilizer, and irrigation management strategies.”
Rhuanito “Johnny” Ferrarezi and his colleagues conducted experiments to determine the variables that promote healthy fresh fruit harvests with the presence of HLB. Ferrarezi, along with fellow professors Mark Ritenour and Alan Wright, will accept the “Outstanding Fruit Publication Award” for papers published in 2020 at the ASHS annual meeting awards ceremony in Denver, Colorado, Aug. 6. Ferrarezi is an Assistant Professor of Citrus Horticulture; Ritenour, Professor of Postharvest Technology; Wright, Professor of Soil and Water Science.
Others who contributed to the research and the award-winning publication are Arun D. Jani, a post-doctoral research assistant; Thomas James III, who manages citrus research groves; and Cristina Gil, an agricultural research assistant.
“We strive to keep citrus growers in business even though HLB is reducing the profitability of infected trees over time,” said Ferrarezi. “The point is to sustain younger trees for a number of harvests to produce the healthy, delicious fruit that made the Indian River District famous.”