California’s San Gabriel Valley looks to be “ground-zero” in the Golden State for the lethal citrus disease Huanglongbing (HLB).
The California Department of Food and Agriculture (CDFA) confirmed two cases of HLB in two citrus trees in the city of San Gabriel in early July.
This raises the stakes in the state’s fight against the Asian citrus psyllid (ACP), the vector known to spread the bacterium Candidatus Liberibacter that causes the deadly disease.
Citrus experts suspect more cases of HLB could be announced in the coming weeks as live psyllids collected in the area are testing positive for the bacteria. In each case the trees – a kumquat and a lime – were in residential neighborhoods and not connected with any commercial growing operation.
San Gabriel is about 15 miles northwest of Hacienda Heights; the suburban community that in 2012 had California’s first recorded case of HLB. The communities are less than an hour away by car from commercial citrus groves and the University of Riverside’s citrus research facilities and farm.
The San Gabriel Valley is one of several regions in southern California that served as the epicenter for the state’s fledgling citrus industry in the late 19th and early 20th centuries.
An HLB quarantine remains in effect in Hacienda Heights. As of deadline there was no announced HLB quarantine boundary for the San Gabriel area.
The HLB discoveries seemed to not surprise citrus industry officials, who in the past few months have been made aware of research from Texas suggesting HLB could soon be found in more trees in California.
Research out of Texas plotted Ct scores from Texas PCR tests that were inconclusive for HLB. The scores fell in a numerical range between definitely positive (under 32) and definitely negative (over 40) for the bacteria. According to Victoria Hornbaker, an agricultural biologist with the CDFA, those scores presented an indication of risk for HLB based on what was seen in Texas.
Earlier tests inconclusive
Hornbaker told citrus growers in Exeter on July 1 that all those inconclusive tests in southern California with Ct scores between 32 and 40 lead to a resampling of sites and “everything has come back negative for HLB.”
Nine days later the CDFA announced the first San Gabriel HLB find. Two days after that the second San Gabriel HLB case was announced.
At the same meeting Hornbaker told citrus growers and their crop consultants that there are concerning “hot spots” in southern California and one in Tulare County that inspectors continue to watch. The indication is that the Tulare County case is in a residential neighborhood.
With the two reported HLB confirmations CDFA officials say their labs are working at capacity to conduct polymerase chain reaction (PCR) tests on plant materials and psyllids collected in the field.
CDFA Spokesman Jay Van Rein says protocols mandate sampling every host plant within 800 meters (about a half mile) of an HLB find. Given that many and perhaps most of the yards in this region of southern California have some sort of citrus plant in them, the testing takes time.
Some citrus officials have reported that because PCR samples can be “inconclusive” for the disease further tests are run to confirm or deny presence of the disease.
Because HLB does not move evenly through citrus trees, there can be parts of an infected tree that could test negative simply because the bacteria is not present in a significant enough quantity in that segment of the tree.
Some of the tell-tale signs of HLB can include a mottled yellowing of leaves that is asymmetrical. This can be confused with a nutrient imbalance where yellowing is present but the yellowing is symmetrical on the leaves.
There can also be a premature fruit drop and a greening of fruit.
The best thing for residents or growers to do if they spot questionable symptoms on their trees is to contact their local Ag commissioner or the CDFA.
Once a tree has been identified positive for HLB, California officials remove the tree, double-bag it, and dispose of it in a landfill or by burial, Van Rein says.
Nurseries and stores selling citrus within the HLB quarantine zone are given two options by the State of California regarding their citrus. One option is to place all plants in an insect-resistant structure for two years where they are tested for HLB. The other option is to destroy the plants. Van Rein says businesses selling citrus are opting to destroy the plants.
Commercial citrus growers in California have been made aware of the issues surrounding the ACP and HLB in various meetings over the past several years, including best management practices to avoid spreading the psyllid from grove to grove.
Much is at stake for California’s multi-billion citrus industry given what was witnessed in Florida since HLB was discovered there.
According to the USDA, Huanglongbing is confirmed in California, Florida, Georgia, Louisiana, South Carolina and Texas. Quarantines for HLB are established across the entire states of Florida and Georgia with smaller quarantine zones in the other states. Quarantine boundaries can be found online.
John Gless is a citrus grower with groves in Riverside and Kern counties. While his Kern County groves south and east of Bakersfield are believed to be psyllid-free it's a different story for his Riverside County groves.
“They’re all over the place in Riverside County,” he said of the tiny pest that spreads HLB.
According to University of California experts there are effective insecticides available to treat conventional groves. Current organic practices and products are not that effective in combatting the pest, according to university experts.
“We are using a real aggressive spray program in our southern California groves to combat the ACP,” Gless said.
Gless also serves on the board for California Citrus Mutual (CCM), a membership trade organization of citrus growers that has been out front on the ACP/HLB issue in California.
According to CCM Spokesperson Alyssa Houtby, while the HLB discovery is troubling, it is a testament to the protocols state officials and the industry have put into place to find the disease. Because there is no cure for HLB, the goal is to remove infected trees once they are positively identified in hopes of reducing the chances of HLB being spread.
“We believe the program is working, though it is much like trying to find a needle in the haystack,” Houtby says.
With the discovery of the ACP in California in 2008 California’s citrus industry went to work to gather information on the pest and the disease it vectors. By 2012 when HLB was discovered in Hacienda Heights the pest had spread from the Mexican border to Santa Barbara and Tulare counties. Since then the ACP has been discovered along the Central Coast and as far north as San Jose.
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The long-standing goal of California’s citrus industry and the CDFA has been eradication of the ACP in those areas outside of southern California, where the pest spread quickly through residential neighborhoods that are largely untreated for pests and disease.
In southern California ongoing efforts continue to introduce parasitic wasps in an attempt to control psyllid populations and perhaps slow the spread of HLB.
Huanglongbing is not known to be a human health risk, though the disease does make fruit too bitter to consume as it kills the tree within several years of infection.