University researchers are finding evidence in commercial citrus of the bacterium responsible for the deadly disease responsible for decimating groves in other parts of the U.S.

Late last year researchers detected Candidatus Liberibacter asiaticus (CLas) – the bacterium known to cause Huangongbing (HLB) – in Asian citrus psyllid (ACP) nymphs collected from commercial citrus in San Diego and Ventura counties.

“The presence of CLas in nymphs is a strong indicator that CLas is present in trees,” said Neil McRoberts, a plant disease epidemiologist and director of the Western Plant Diagnostic Network at UC Davis.

Because the tests performed by researchers were not part of regulatory protocols, no official state response or quarantine was issued.

The California Department of Food and Agriculture’s response was to request samples for regulatory testing.

Last November, Victoria Hornbaker, director of the CDFA’s citrus pest and disease prevention division, wrote in a message to the commercial industry that these research discoveries were “experimental,” and did not meet state testing guidelines.

“These studies’ laboratory tests vary from the federally-approved testing methods and procedures required by CDFA labs that would allow the state to take any regulatory action,” Hornbaker wrote in the Citrus Insider, a publication of the state’s Citrus Pest & Disease Prevention Program. “The CDFA staff is seeking approval for follow-up sampling to allow for official psyllid and plant samples to be collected for further investigation. However, this action is entirely voluntary for the property owners involved.”

Most of the discoveries of positive adult psyllids and nymphs came from San Diego County, McRoberts said. Researchers found positive CLas levels in 11 of 15 commercial citrus blocks spread across Ventura, Riverside, San Bernardino, and San Diego counties.

Growing trend

McRoberts highlighted the increased CLas detections in southern California. Ongoing studies reveal disturbing finds.

There has been a marked increase in positive CLas results, McRoberts says. Perhaps more disturbing is that bacteria loads are also increasing. This matters because it’s the CLas levels that can determine plant and insect infection levels.

McRoberts is cautiously optimistic, with heavy emphasis on the word “cautiously,” that HLB can be managed in commercial groves once it gains a hold.

“It’s not ‘if,’ but ‘when’ it arrives,” he said.

Outside of approved pesticide treatment protocols, he believes growers will develop practical and manageable programs to manage their groves by quickly removing infected trees.

“It may not rip through California like it did in Florida,” he said.

Scientific laboratory

Southern California’s proliferation of urban citrus landscaping has become a useful research lab for scientists like McRoberts. Researchers continue to study how the tiny psyllid lives and breeds in California’s Mediterranean climate, and how the lethal tree disease it spreads replicates in citrus plants.

HLB continues to spread across southern California after being found in 2012 in a heavily grafted residential citrus tree in Hacienda Heights. By early July the State of California has removed and destroyed over 5,500 residential citrus trees under quarantine rules to keep the plant disease from decimating the state’s multi-billion commercial citrus industry.

McRoberts told farmers and industry leaders recently that it’s highly likely CLas is present in commercial citrus groves in the state. To what level and effect, he does not yet know.

Scientists are trying to better understand how the bacteria spread in the trees, and at what point it renders fruit bitter and unmarketable. This is particularly critical of fresh market citrus, which is sold based on appearance.

Trees growing in hotter climates may not suffer the same impacts from the bacterium than those in cooler climates he said. Researchers don’t yet know why, but studies out of Brazil seem to confirm this, he said.

The urban areas of Orange County – namely the cities of Anaheim and Garden Grove – are seeing a marked uptick in HLB cases in residential citrus. McRoberts said as state officials find new infections and begin setting up their concentric circle delimitation areas to survey, they continue to find more infected trees. As of mid-July, more than 3,800 trees across over 2,500 sites in Orange County had infected trees, based on state testing protocols.

What is known

There is growing understanding by entomologists about the insect. Psyllids feed on the newly formed leaves, laying their eggs within that new growth. Insects that feed on infected plant materials can transmit that infection to other citrus trees. Researchers now know that the psyllids feed only on the new growth, which is softer. This is also where they lay their eggs. Once citrus leaves harden off, the insects are unable to feed and do not reproduce there.

This is what makes coastal regions more concerning for researchers and growers. Citrus trees generate new growth more readily and consistently in these regions. This allows the ACP to multiply more readily than in warmer regions like the San Joaquin Valley or desert. It’s in those warmer regions where the new citrus growth does not happen as often, and hardens off quickly when it does, this leaves fewer opportunities for the ACP to establish itself.

Intensive sampling and treatment practices by commercial growers in the San Joaquin Valley have likewise helped manage ACP numbers, McRoberts said.

“The industry there has been aggressive in its control efforts,” he continued.

California citrus farmers have continued to dodge the disease bullet, learning early from the mistakes made by the Florida citrus industry when the insect and disease was first found in 2005. Since then, state citrus industry leaders, state and federal regulators, and university researchers have partnered to eradicate the psyllid from commercial groves in the San Joaquin Valley. While the insect has not been formally eradicated from the valley, trap card finds of the ACP tend to be low.