In an abundance of caution, some commercial citrus growers in Ventura County, Calif. elected to remove numerous trees after dogs from Florida trained to detect the presence of the bacterium responsible for Huanglongbing (HLB) alerted on over 200 trees.

The dogs are owned by a private company and are trained to sniff for signs of Candidatus Liberibacter asiaticus (CLas), a bacterium believed by researchers to be the cause of the citrus disease responsible for greatly reducing Florida’s citrus production and for infecting residential trees in southern California.

To date HLB has not been confirmed in commercial citrus groves in California. The disease is present in residential citrus throughout southern California, prompting the state to remove trees that test positive for the bacterium to control its spread.

John Krist, chief executive officer of the Ventura County Farm Bureau, said growers in his county remain concerned with the proximity of the disease to their groves and the length of time it takes for trees to be formally diagnosed with the disease through a scientific process known by the industry simply as “PCR.”

The Polymerase Chain Reaction (PCR) test is a scientific method used to amplify DNA and remains the only regulatory-approved means to declare whether a citrus tree has the disease. It can take years for a tree infected with the bacterium to elicit symptoms or contain enough bacteria to test positive.

Dogs as early detection

The trained dogs are considered by some as a viable early detection tool to aid growers in determining whether their trees are infected. Nevertheless, the dogs are not a government-approved option to declare presence of the bacteria. For growers, this means that even a positive “alert” by dog teams will not otherwise open them to quarantine restrictions.

Earlier this year the Ventura County Farm Bureau contracted with F1K9 in Florida to scout commercial groves in the county for HLB, according to Krist. Similar teams of dogs were previously used in California to scout citrus trees in the University of California’s research grove at Riverside, and at the UC Research and Extension Center at Lindcove in Tulare County. In those instances, several trees at UCR were alerted on by dogs while none of the trees at Lindcove were alerted on by the dogs.

To date UCR has not removed those trees from its research grove. Instead, the trees are covered with netting to prevent access by the Asian citrus psyllid (ACP), the tiny insect believed to be solely responsible for the spread of HLB. Those trees are said to be the subject of ongoing testing and research.

In late July and early August, four dogs, along with their handlers, scouted commercial groves in Ventura County, inspecting about 3,500 trees on 20 properties, according to Krist. Out of that the dogs alerted on 211 trees. Those trees have since been removed and destroyed by their owners.

The genesis of the Ventura dog inspection came from an understanding of research led by Dr. Timothy Gottwald, a plant pathologist with the U.S. Department of Agriculture.

The Gottwald research points to the ability of trained dogs to detect the CLas bacterium. These dogs, which a Gottwald article says have been evaluated in laboratory and field experiments, can detect CLas over 95 percent of the time in commercial groves, and over 92 percent of the time in residential citrus. Perhaps more importantly, these dogs can do this well before a PCR test could result in a positive diagnosis.

That is essentially why the Ventura County Farm Bureau, at the request of local citrus growers, requested the dogs. Krist says they cannot afford to wait for the state to find and declare HLB in commercial citrus through regulatory testing.

Krist is concerned that ongoing state efforts to find HLB appear targeted at residential citrus, with little focus on commercial groves.

“They need to start doing a more systematic survey of commercial citrus than they’ve been doing up to now,” Krist says of CDFA efforts.

Response

News of the dogs’ findings rippled through the commercial citrus industry in California.

Victoria Hornbaker, director of the California Citrus Pest and Disease Prevention Division, a new division within the California Department of Food and Agriculture, says there are no confirmed cases of HLB in Ventura County. Moreover, she said the state was not involved in the dog inspection there.

Krist says the goal never was to have the trees PCR tested because growers believe the tests would be negative due to the inability of the PCR test to definitively detect CLas infection in all cases.

California Citrus Mutual President Casey Creamer said that while dogs can be an effective tool, he and others in the industry became concerned by a published news article that implied HLB was found in the commercial groves when that determination cannot be made solely by a dog.

“Our biggest concern was the messaging,” Creamer said, adding that the jury is still out on the effectiveness of the dogs under California conditions.

Citrus Mutual and others in the industry are said to be working on a public response to what many in the industry agree is the inevitable discovery of HLB in commercial citrus in California.

Hornbaker said that testing of plant materials and insects collected in Ventura County in 2015 and 2016 showed “inconclusive” results, meaning some samples taken revealed levels of bacteria too few to declare a positive result under regulatory rules, but enough bacteria that inspectors could not declare a “negative” result. Since then the USDA changed its testing protocols after researchers determined early tests may have “cross-amplified” multiple forms of bacteria in the samples that could have led to a “false positive.” Testing protocols were ultimately tightened to specifically seek only the CLas bacterium regulators are looking for.

What do we know?

Some scientists now believe that the latency of this disease – the period between first infection by ACP feeding activity and when a PCR test will reveal a positive result – may be much longer than the 2-3 years first thought. It’s this latency that concerns growers and led to the use of dogs as an early detection method. Published information by Gottwald suggests these dogs can detect parts-per-trillion levels of the bacteria, which is more sensitive than most laboratory instruments and certainly more sensitive than the PCR test.

A Gottwald article also states that canines studied under Florida conditions, when using potted citrus trees, were able to detect CLas in some trees as early as two weeks after infected psyllids fed on them.

Dr. Beth Grafton-Cardwell, a University of California entomologist and a collaborator in some of the Gottwald studies, is surveying for the presence of ACP in commercial citrus groves in southern California, to include the Ventura County region. She says those surveys have turned up very few psyllids this year, which she attributes to the weather and amplified efforts by growers to treat their groves in unison with approved insecticides as a best management practice. It is believed that a coordinated treatment of groves in a region is the best method to control psyllid populations.

Grafton-Cardwell is one of those scientists who believes that HLB latency may be longer in some cases as the disease may never go systemic in the entire tree, hence the inability for the PCR test to catch it in all cases.

“Everybody here in Ventura County is staring this epidemic in the face and felt a real need to be proactive,” Krist said of the recent dog visit. “Once the disease takes hold, you’re never going to get in front of it. That was the mistake Florida made.”

Krist says there are plans to bring the dogs back in November and again in January to do more scouting of commercial citrus groves.

Source: Todd Fitchette, Western Farm Press