Growers, researchers fight citrus disease


Issue Date: October 26, 2016
By Cecilia Parsons
Perry Edwards from State College, Pa., center, explains his smartphone-based optical sensor research for the detection of huanglongbing to David Britz of SunWest Fruit at the recent California Citrus Conference in Exeter.
Photo/Cecilia Parsons
California farmers provide much of the nation’s fresh citrus fruit, but face the threat of the plant disease HLB. So far in California, the disease has been found only in residential citrus trees, but it has caused serious losses in Florida and other citrus-producing regions.

An arsenal of weapons to combat the deadly citrus disease huanglongbing was described to growers attending the California Citrus Conference in Exeter. Hosted by the Citrus Research Board, the event highlighted innovations in disease detection and resistance and current strategies for managing the Asian citrus psyllid. It also reinforced reasons for growers to continue their efforts in psyllid suppression.

At stake is California's fresh citrus production. California farmers provide much of the nation's supply of navel oranges, mandarins, lemons and specialty citrus varieties—and have looked on nervously while Florida citrus production plummeted in recent years, as HLB infection spread through that state's citrus production areas.

The bacterial disease attacks the vascular system of plants and leads to loss of productivity; misshapen, off-flavor fruit; and eventually, tree death. The disease was detected in California for the first time in 2012 in a citrus tree in a residential area of Hacienda Heights in Los Angeles County. Since then, 28 residential trees in the Hacienda Heights and San Gabriel areas have been identified as infected, and removed.

There have been no detections of HLB-infected trees in commercial California citrus, but experts on the forefront of the fight believe that given the proliferation of the psyllid, which can carry HLB, it's only a matter of time before the disease is found in farmed citrus.

Beth Grafton-Cardwell, a University of California citrus specialist based at the UC Lindcove Research and Extension Center, told the conference HLB can spread fast because of psyllid's efficiency in transmitting it.

Citrus trees become infected when psyllids carrying HLB bacteria feed on new flush. Newly hatched psyllid nymphs immediately pick up the bacteria when they begin to feed on the new flush.

That the disease spreads fast is only part of the control challenge, Grafton-Cardwell said. It is also difficult to detect in a tree before that tree becomes a reservoir of infection. It takes nine months to two years for bacteria to move through the tree to be detected through current test methods, she said.

U.S. Department of Agriculture plant pathologist Tim Gottwald said the Asian citrus psyllid and HLB have spread worldwide into citrus production areas at a faster rate than citrus trees can evolve to resist the disease. Gottwald, who has been a plant disease researcher in Florida through the HLB devastation, said four control strategies must be in play to suppress the disease: psyllid control, nursery sanitation, early detection and removal of infected trees.

"If we lose any of these, there is immediate collapse of the system and the disease will take off," he said.

Requiring all citrus trees to be raised in screenhouses ensures they do not have the disease when planted for production, he said.

Gottwald also shared information on models to pinpoint where HLB is most likely to be found, noting that such models have proved reliable when used to predict where human diseases such as ebola and dengue fever would be introduced in the United States.

Use of models directs sampling efforts toward the most likely sources of HLB infection, Gottwald said. Predictive models have directed HLB searches to residential areas with high populations of residents from countries where HLB is common. In the San Joaquin Valley, citrus near high population areas such as Fresno would be more likely to become infected, he said.

With models targeting the most likely sites for HLB infection, the state can concentrate efforts there, Gottwald said, rather than trying to sweep entire regions looking for the disease.

Development of reliable early detection methods for HLB infection is vital, he said. There are several ongoing projects funded by the Citrus Research Board dedicated to early detection, including a polarized-imaging technique and an optical sensor technique that does not require plant samples.

Gottwald highlighted efforts in using trained dogs to sniff out HLB-infected trees. Dogs can detect the volatiles given off by infected trees, he said. There is a history of using dogs to sniff out citrus canker disease, with a 98 percent accuracy rate in the field. Gottwald said there are currently 16 dogs in training for HLB detection.

Removal of infected or suspected citrus trees is vital to slowing spread of HLB, Gottwald said. It has been proven in Brazil, he said, that removal plus vigilant treatment programs can keep infection rates below 3 percent.

Grafton-Cardwell described control of the psyllid as only a temporary approach to slowing the spread of HLB. Quarantines to manage flow of plant material and organization of area-wide management teams to coordinate pesticide treatments are currently in play in much of the San Joaquin Valley, but challenges remain in early detection of the disease.

In Southern California areas where psyllids are endemic, there are pesticide treatments three to four times per year. Residential areas are treated if they are near commercial citrus plantings. There are also releases of a parasitic wasp, Tamarixia, which preys on psyllids.

Populations of the parasitic wasps introduced in Southern California have persisted, Grafton-Cardwell reported, but they are not achieving a level of control needed to reduce spread of the pest. In addition, in many cases, Argentine ants are interfering with the wasps' efforts to parasitize the psyllids.

The biggest problems for psyllid control, Grafton-Cardwell said, are untreated urban areas, retail nurseries and movement of bulk citrus.

She said three objectives will be pursued in the next two years with the help of $1.4 million in federal funding. The first is improvement in psyllid sampling and better estimates of treatment efficacy in counties with high psyllid populations. Second is to define high-risk hot spots by testing live psyllids. The third is removal of neglected or abandoned citrus trees in residential areas. Grafton-Cardwell said the program would also analyze the impact of area-wide treatments on psyllid populations.

(Cecilia Parsons is a reporter in Ducor. She may be contacted at ceciliaparsons8@gmail.com.)

Permission for use is granted, however, credit must be made to the California Farm Bureau Federation when reprinting this item.