Huanglongbing1 (HLB) or citrus greening, first observed more than a hundred years ago in Asia, is the most serious disease threat to the citrus-growing industry worldwide due to its complexity, destructiveness, and incalcitrance to management. The bacterium Candidatus Liberibacter asiaticus, which is associated with HLB, is transmitted by the Asian citrus psyllid (ACP). First detected in Florida in 2005, HLB is now widespread in the state and threatens the survival of the Florida citrus industry despite substantial allocation of research funds by Florida citrus growers and federal and state agencies.
In Florida, HLB research is overseen by the Citrus Research and Development Foundation (CRDF), a nonprofit corporation created in 2009 through the initiative of the state’s citrus industry. In 2017, at the request of CRDF, the National Academies of Sciences, Engineering, and Medicine conducted a review of the foundation’s research portfolio, with the goal of identifying ways to reconfigure HLB research to accelerate the development of tools and strategies to abate disease impacts and prevent the collapse of the Florida citrus industry (see the full statement of task in Chapter 1, Box 1-1).
In its review, the committee found that research supported by CRDF and other agencies has expanded knowledge of every aspect of HLB, yet
1 Huanglongbing (HLB), which means yellow (huang) shoot (long) disease (bing), was unanimously adopted as the official name of the disease by the International Organization of Citrus Virologists (IOCV) at the 13th Conference of IOCV in Fuzhou, China, in 1995 (http://iocv.org/huanglongbing.htm; accessed February 22, 2018).
there have been no breakthroughs in HLB management. The reasons for the lack of breakthroughs in HLB management, despite the investments in research, are complex. Other than research on ACP in Florida, most of the available information on HLB prior to 2005 was based primarily on research performed outside the United States so researchers faced a steep learning curve. The disease itself is intractable for a variety of reasons related to the citrus host (its perennial nature, the lack of resistance in any citrus relative, and the difficulty of breeding to produce HLB-resistant cultivars); the pathogen (especially the inability to culture it in the laboratory); the insect vector (and its major role in transmission); the complexity of pathogen, vector, and host interactions; the lack of a good model system; as well as the current approach to HLB research.
The committee’s analysis of HLB research outcomes allowed the committee to identify progress and pitfalls in major research areas, and to identify research efforts that the committee believes should be continued or initiated.
The committee regards the following as critical to achieve progress toward a viable HLB solution:
- Building on knowledge generated through previous research
- Supporting research on factors that influence adoption of management practices proven effective
- Greater collaboration and more-frequent venues for information sharing by scientists
- Timely and systematic communication of research outcomes and evaluation of research progress
- Increased research coordination by CRDF and other funders of HLB research
Citrus growers, particularly in Florida, still need short-term solutions to sustain the industry while researchers continue to generate longer-term approaches for managing HLB, so support of basic and applied short- and long-term research is needed. Longer-term HLB solutions are likely to involve citrus variety improvement, derived primarily from new molecular techniques such as gene editing, and those efforts should focus on targets mediating molecular interactions among plant, bacteria, and vector.
Because a single breakthrough discovery for managing HLB in Florida is unlikely, funders should support the development of sets of management approaches that can be combined in different ways, optimized, and validated for use in different locations and conditions. This approach, founded on integrated pest management, would allow optimization of management for each grower.
Economic and sociological factors that impact decision making and behaviors of growers, processors, and the public will influence the adoption and success of future HLB management efforts; hence, CRDF should consider funding these research areas and creating accessible databases to support sociological and economic modeling of HLB-related research outcomes and application projections.
During its review the committee observed inconsistency in laboratory and field experimental designs and sampling methods. Because inconsistency limits the comparison of findings across teams and institutions and the use of previous research to inform further exploration, the development of community-accepted standards to conduct, evaluate, and assess research should be supported. Improved consistency in reporting research outcomes is also needed to reduce constraints in reviewing research progress and delays in applying new information to HLB solutions. CRDF should develop a standardized format and procedure, and set a timeline for mandatory reporting of project progress and final reports, to include publications and presentations, outcomes, practical applications, and impacts.
Despite commendable efforts of multiple agencies to coordinate funding and encourage appropriate interstate, interagency, and interdisciplinary collaborations, the committee noted that decisions about research funding priorities and allocations occur largely within the domain of each agency, and it recommends that CRDF and other agencies work together to create an overarching HLB research advisory panel to develop a fresh, systems approach to HLB research prioritization and the strategic distribution of resources for research leading to effective HLB management.
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