Huanglongbing (HLB) and the Asian citrus psyllid (ACP)
Images below are to help understand the size of the insect, and what plant symptoms look like.
HLB Citrus Disease Compiled Information
Did you know your morning glass of orange juice or lemonade on a hot summer’s day is at risk? Indeed, all citrus trees are at risk from a deadly disease called Huanglongbing (HLB) that is carried and spread by a tiny insect, the Asian citrus psyllid (ACP). Since the first California HLB detection in March 2012, the disease has been detected in backyard citrus trees in Los Angeles, Orange, Riverside, San Bernardino and San Diego counties.
ACP is tiny, about the size of an aphid, which is about 0.125 inch or 3mm. That is very small. For size comparison, take a penny, find the word liberty. It is the size of the last 3 letters. It has wings that can carry it from tree to tree up to 1 ¼ miles on its own, or further on windy days. As it feeds, it can inject the tree with deadly bacteria, Candidatus Liberibacter asiaticus (CLas), that causes the tongue twisting Huanglongbing disease, also called HLB or Citrus Greening Disease, referring to an infected tree producing fruit that does not color and stays green, rather than the leaf color in which the disease turns citrus’s healthy green leaves to yellow and brown. Once a tree gets infected with the disease, it’s a death sentence for the plant. In this author’s personal opinion, it should be called the Citrus Killing Disease.
For this reason, many places in Florida, California and elsewhere, have adopted quarantines as a defense against the disease. The quarantines include restrictions on plant and cutting movements to prevent spread . Fruit needs stems and leaves removed, and fruit is washed prior to transport to other locations.
In the U.S., the disease was first detected in Florida in 2005. The Florida Department of Agriculture was not prepared to find or deal with it until it had spread throughout most of the state. The disease was spread by the insect and by grafting using scion from infected trees. Since then, it has become illegal to graft citrus trees by anyone except approved nurseries following approved conditions with properly sourced material. The disease, once introduced to a tree, will spread throughout the phloem. The phloem is a plant’s equivalent to human arteries. As the disease spreads, it multiplies, blocking the transport of nutrients in the plant, eventually killing it. By the time a tree shows symptoms, the disease has already spread to the roots. Some of the symptoms are asymmetrical blotchy and discolored leaves, misshapen fruit, stunted growth, reduced fruit size, root decline, premature fruit drop, and bad tasting fruit. As most of the symptoms may be attributed to other possible causes, many tree owners may overlook them and not quickly remove the diseased tree, which allows the disease to continue to spread, as the tree acts a reservoir for the bacteria.
The disease is different from most plant diseases in that the carrier psyllid transmits the bacteria directly to approximately 4% of its offspring. The main way ACP acquire the bacteria is through host feeding. Emerging nymphs acquire the bacteria through feeding on the infected trees that the infected mother ACP has fed and passed the bacteria onto. The psyllid produces 300-800 eggs in its life cycle between 15 to 47 days. When it infects citrus trees or related trees, the bacteria propagates there as well. As plants don’t generate antibodies, there is no antibiotic cure. There are antimicrobials for commercial therapy usage, but there is no proof they remove all CLas. Once a tree is infected, the only current options are treatment for ACP and tree removal. Replacement on a future date can be done; however, ACP infected with CLas may still be in the area and may infect the new tree.
As HLB results in billions of dollars in losses for the citrus industry worldwide, there are various attempts at solutions. The main option to fight it is to slow its spread by controlling ACP populations and removing HLB positive trees, allowing more time for scientists to develop a solution. Efforts have been underway since 2011 in California to enlist psyllid parasitoids. A number of predators and parasites feed on ACP. The nymphs are killed by tiny parasitic wasps and various predators, including lady beetle adults and larvae, syrphid fly larvae, lacewing larvae, and minute pirate bugs. Some spiders, birds, and other general predators feed on adult psyllids.
Several species of tiny parasitoid wasps, collected by University of California researchers, were brought to California from 2009 to 2011, host-tested, and have been continuously mass-reared and released. The most promising of these, Tamarixia radiata, strongly prefers ACP nymphs, and under ideal conditions can significantly reduce psyllid populations. It has been mass reared and released since 2011 in California. Females of this tiny wasp, which poses no threat to people, lay their eggs underneath ACP nymphs, and after hatching, the parasitoid larvae feed on and kill the psyllid. To find evidence of this wasp at work, keep an eye out for ACP “mummies”, which look like hollowed-out nymphal shells. This wasp has been released at thousands of sites throughout Southern California since early 2012. More recently, T. radiata releases have been made in Central California and a second wasp (Diaphorencyrtus aligarhensis) that attacks the younger ACP nymphs was released in Southern California. This was only reared and released 2014-2018, it was more difficult to rear than Tamarixia and CDFA has not yet found evidence of parasitism in the field.
Tamarixia and other natural enemies have reduced ACP populations in Southern California, but they have not eradicated the pest and have not halted the spread of HLB. In the absence of ants, these beneficial insects will at least help to reduce psyllids, especially in areas where it is not possible or practical to institute chemical psyllid control measures. Visit the ACP Distribution and Management website to see a map of where these parasitoids have been released in California. [from http://ipm.ucanr.edu/PMG/PESTNOTES/pn74155.html#MANAGEMENT]
Research has found a unique peptide found in the Australian finger lime plant is resistant to the bacteria. A protein has been isolated, which in laboratory conditions has killed or slowed progression of the bacteria (depending on report source.). UCR has partnered with a company to look for a method to deploy the protein into plants to provide resistance or tolerance to the HLB bacteria CLas. Hopefully a possible option comes from this, but it is YEARS AWAY.
Some people think their plant is treated with an insecticide and that it protects them from the disease. Based upon how the disease infects the trees, foliar spraying the tree kills insects through contact, where the psyllid is directly hit by the spray or that feed on recently sprayed leaves, while systemic insecticides would only work after the insect feeds. If the insecticide kills the insect after it bites a leaf, unless it kills the bacteria, it could transmit it from one bite. If it’s a smell or visual based spray hoping to deter the insect from biting the tree, that would just wash or wear off in a short time as the tree normally grows, requiring constant application, and possibly interfering with sunlight on the leaves.
Currently, growers in Florida grow trees expecting only a few years of fruit production before they must remove and replace trees. Or they grow the trees inside fine netting enclosed grow areas, which can be costly. Other options for control are chemicals or covering plants with clay type substance to alter plant color to diminish attractiveness to the insect carrier. Neither of which has proven to work for an extended amount of time.
According to our research, the first documented discovery of the bacteria causing the disease was in 1929. There are actually 3 known variations in the world at this time.
Three Known Variations
The African strain of huanglongbing (Candidatus Liberibacter africanus), which is also sensitive to heat Transmitted by the African citrus psyllid (Trioza erytreae). Typically found in Africa.
The Asian form, Ca. L. asiaticus, (Clas), is heat tolerant and symptoms of the disease can develop at temperatures up to 35 °C. . Transmitted by Asian citrus psyllid, Diaphorina citri Typically found in Asia, USA, the Caribbean, Mexico, and Central America.
American form, Ca. L. americanus (Candidatus Liberibacter americanus) , are heat sensitive, thus symptoms only develop when the temperature is in the range 20–25 °C has been reported in Brazil and Florida.
On the subject of cuttings of citrus, the main recommended source of scion wood is the Citrus Clonal Protection Program or CCPP. Recently, the budwood costs have soared from $0.75 per BUD to $5 per BUD. A small cutting with 3 buds on it costs $15. We feel this is a deterrent for some people, which would then encourage sourcing budwood from local or online sources. We would like to encourage the governing bodies involved to reconsider the pricing, especially for home growers, to encourage them to get cuttings from the disease free source. .
We would also like to encourage the CDFA and USDA-APHIS to put more effort into introducing the resistant Australian finger lime plants into the USA as the variety appears to be resistant to the disease. To order citrus budwood https://ccpp.ucr.edu/onlineOrdersV2/
History of HLB
The Asian citrus psyllid (ACP, Diaphorina citri) was first detected in Florida in 1998.
The HLB bacteria was discovered in Florida in 2005. It spread rapidly due to weather events and lack of processes or quarantines, resulting in trees dying throughout most of the state.
ACP was detected in California in 2008. Fast response by the California Department of Food and Agriculture (CDFA) included survey, diagnostics, treatment, and establishment of quarantines slowed the spread of ACP and HLB.
HLB was detected in California in 2012 in Los Angeles County, introduced by improperly sourced budwood.
ACP has been detected in at least 15 states and territories: Alabama, American Samoa, Arizona, California, Florida, Georgia, Guam, Hawaii, Louisiana, Mississippi, Northern Mariana Islands, Puerto Rico, South Carolina, Texas, and the U.S. Virgin Islands.
The HLB bacteria has been found in Florida, Georgia, Puerto Rico, US Virgin Islands, and portions of South Carolina, Texas, Louisiana, and California.
California has an ACP quarantine in 29 counties where it has been detected. California has HLB quarantines in 5 counties (LA, Orange, Riverside, San Bernardino, and San Diego) where the disease has been detected.
HLB detections are only confirmed when plant samples are collected by trained USDA, state, or county agricultural staff and tested by scientists at the CDFA laboratory following USDA protocols. CDFA staff continuously survey and sample citrus throughout the state. Detection in other countries may be hampered by lack of proper testing facilities or government personnel.
Written by Frank Wagoner and other members of South OC CRFG
Victoria Hornbaker, CDFA Citrus Pest and Disease Prevention Division Director
Natalie DeAngelo of Nuffer, Smith, Tucker