Scientists say cure for citrus greening ‘unlikely’


National Academy of Sciences report says pest management is best defense from HLB

By Reggie Ellis @Reggie_SGN

EXETER – There has never been more money and resources dedicated to finding a cure for the fatal tree disease known as citrus greening than there will be this year, yet scientists suggest that a cure may never come.

In its report last month to the Citrus Research Foundation in Florida on combating huanglongbing (HLB) the National Academy of Sciences stated, “A single breakthrough discovery for managing HLB in the future is unlikely since intensive research efforts over almost 20 years have not led to this result.” Authors admitted that after years of funding and research about the only thing they’ve really learned is how resistant HLB and the pest that spreads it are to efforts attempting to eradicate it.

The report goes on to say the biggest impediments to success are “the inability to culture the bacteria in the laboratory, the lack of advanced diagnostics for early disease detection, and the absence of standardized research methodology that would improve the comparability of results across studies.”

Citrus trees infected with HLB usually die within five years, according to the UC Statewide Integrated Pest Management Program. There is no known cure. “The only way to protect trees is to prevent spread of the HLB pathogen in the first place, by controlling psyllid populations and removing and destroying any infected trees,” UC IPM says on its web site. Psyllids, formally known as the Asian citrus psyllid (ACP), is the pest that spreads the disease when it feeds on citrus leaves and stems. 

While the headlines are extremely negative, the body of the report does contain some nuggets, particularly for the California producer.  Managing psyllids must remain the number one goal which then delays the spread and detections of multiple infections in the opinion of the report’s authors.

For the California grower, a sustained ACP management program is the only cure for defending a grove against HLB infection. California Department of Food and Agriculture (CDFA) took that a step further on Jan. 1 when it implemented an emergency regional quarantine to more effectively protect California citrus from the disease. The quarantine was split into seven zones, with Tulare County falling in Zone 2, which also includes Fresno, Kern, Kings, Madera, Merced, San Joaquin, and Stanislaus Counties.

The decision restricts the movement of bulk citrus within the boundaries of the quarantine in areas where Asian citrus psyllid (ACP), the vector for HLB, have been found. Bulk citrus can only be moved if the growers, grove managers, haulers, and harvesters: sign a compliance agreement, provide a declaration that all fruit is free of ACP to the County Agriculture Commissioner’s office 72 hours prior to choosing a method to ensure citrus is not carrying the pest, ensure pallets or field bins are completely tarped or transported in an enclosed container, and only deliver fruit to an ACP-Program approved packing house or processor.

The decision also restricted the movement of citrus nursery stock. Trees grown outdoors in counties where HLB has been detected cannot be transported into a different quarantine zone. Trees can be moved between counties with partial infestations of the pest to counties where the disease has been found if the nursery stock has been properly treated and tagged for ACP.

Tulare County’s billion dollar citrus industry also has the benefit of geography suited to slowing ACP down. Dr. Beth Grafton-Cardwell, entomologist and director of the University of California Research and Extension Center in Lindcove east of Exeter, said at the annual Citrus Conference last fall that the San Joaquin Valley’s extreme seasons make it difficult for psyllid populations to thrive and limits the amount of thrush, or leaves, on the plant during that time of year, which can starve the psyllid out of a grove.

The Valley also has less densely populated cities, which makes monitoring of residential citrus, where pesticides cannot be applied, easier in an area where residents are more compliant with laws to remove and eradicate infected trees.

Other factors that help Valley citrus growers are routine sprays for other pests that help suppress the psyllid, high participation of growers in coordinated treatments, the public is more closely tied and understands agriculture and its challenges, as well as participation by the public to remove infected trees from their backyards.

The report is far more ominous for Florida, where the spread of the disease will likely continue to outpace research success. Once home to the nation’s largest citrus industry, Florida’s commercial production has plummeted by over 70% resulting in average annual economic losses of 7,945 jobs, $658 million in value-added product, and $1.098 billion in industry output, according to a recent report by the University of Florida. The authors urged the Florida industry to adopt multiple management approaches that can be combined in different ways.

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