Pourreza, who earned his Ph.D. at the University of Florida in 2014, worked on early detection of Huanglongbing disease of citrus. Huanglongbing, an incurable disease that is spread by Asian citrus psyllid, has seriously impacted citrus production in Florida. The disease has been found in commercial and residential sites in all counties with commercial citrus.
Early detection allows growers to remove infected trees before the disease can spread to healthy trees. Currently HLB infection is confirmed when leaves with yellowing and blotches are submitted for PCR testing, which is expensive and time consuming. However, the yellowing can be also symptomatic of other conditions, such as nutrient deficiency.
“We discovered we could see the symptoms of Huanglongbing using a camera, a set of cross-polarizers and narrow band lighting before it is visible to the human eye,” Pourreza said.
He said the yellow blotches on HLB-infected leaves are caused by starch accumulation.
“If we could detect abnormal levels of starch in the leaf, we could tell it is affected with HLB,” Pourreza said. “Starch showed the ability to rotate the polarization plane of light. We used this optical characteristic to develop the sensing methodology.”
Pourreza said the team has patented the technique and is working on developing a commercial product. He is seeking funding to continue the research in California, where, to date, HLB has only been detected in isolated Los Angeles neighborhoods. Asian citrus psyllid is found in important California commercial citrus production regions from the Mexican border to as far north as Placer County.
Pourreza is based at the UC Kearney Agricultural Research and Extension Center in Parlier.
A tell-tale sign of spring in California is a flush of new leaf growth on citrus trees. Because the feathery light green leaves are particularly attractive to Asian citrus psyllids (ACP), the leaves' emergence marks a critical time to determine whether the pest has infested trees.
“We encourage home citrus growers and farmers to go out with a magnifying glass or hand lens and look closely at the new growth,” said Beth Grafton-Cardwell, UC Agriculture and Natural Resources (UC ANR) citrus entomologist. “Look for the various stages of the psyllid – small yellow eggs, sesame-seed sized yellow ACP young with curly white tubules, or aphid-like adults that perch with their hind quarters angled up.”
Pictures of the Asian citrus psyllids and its life stages are on the UC ANR website at http://ucanr.edu/acp. If you find signs of the insect, call the California Food and Agriculture (CDFA) Exotic Pest Hotline at (800) 491-1899.
Asian citrus psyllids are feared because they can spread huanglongbing (HLB) disease, an incurable condition that first causes yellow mottling on the leaves and later sour, misshapen fruit before killing the tree. ACP, native of Pakistan, Afghanistan and other tropical and subtropics regions of Asian, was first detected in California in 2008. Everywhere Asian citrus psyllids have appeared – including Florida and Texas – the pests have found and spread the disease. A few HLB-infected trees have been located in urban Los Angeles County. They were quickly removed by CDFA officials.
“In California, we are working hard to keep the population of ACP as low as possible until researchers can find a cure for the disease,” Grafton-Cardwell said. “We need the help of citrus farmers and home gardeners.”
Grafton-Cardwell has spearheaded the development of the UC ANR ACP website for citrus growers and citrus homeowners that provides help in finding the pest and what to do next. The site has an interactive map tool to locate residences and farms that are in areas where the psyllid has already become established, and areas where they are posing a risk to the citrus industry and must be aggressively treated by county officials.
The website outlines biological control efforts that are underway, and directions for insecticidal control, if it is needed. An online calculator on the website allows farmers and homeowners to determine their potential costs for using insecticides.
There are additional measures that can be taken to support the fight against ACP and HLB in California.
- When planting new citrus trees, only purchase the trees from reputable nurseries. Do not accept tree cuttings or budwood from friends or relatives.
- After pruning or cutting down a citrus tree, dry out the green waste or double bag it to make sure that live psyllids won't ride into another region on the foliage.
- Control ants in and near citrus trees with bait stations. Scientists have released natural enemies of ACP in Southern California to help keep the pest in check. However, ants will protect ACP from the natural enemies. Ants favor the presence of ACP because the psyllid produces honeydew, a food source for ants.
- Learn more about the Asian citrus psyllid and huanglongbing disease by reading the detailed pest note on UC ANR's Statewide Integrated Pest Management website.
- Assist in the control of ACP by supporting CDFA insecticide treatments of your citrus or treating the citrus yourself when psyllids are present.
- Support the removal of HLB-infected trees.
The site’s launch coincides with the announcement last month by the California Department of Food and Agriculture that six more psyllids were found in three Tulare County yellow sticky traps. In 2012, three psyllids were found on two traps and an eradication program ensued.
The new website was developed by Beth Grafton-Cardwell and Matt Daugherty, UCCE specialists in the Department of Entomology at UC Riverside, Karen Jetter, economist with the UC Agricultural Issues Center, and Robert Johnson programmer with the UC Agriculture and Natural Resources Informatics and GIS Statewide Program program. Funding for the site was provided by UC Agriculture and Natural Resources.
The website includes information on the psyllid's distribution in California, monitoring methods and treatment options. For example, the website suggests citrus farmers and homeowners not rely on yellow stick card traps to monitor for the pest.
“At certain times of the year, the yellow sticky cards are totally unattractive to Asian citrus psyllid,” Grafton-Cardwell said. “The cards are just color, but citrus flush is an attractive color and also emits irresistible volatiles or smells.”
The website advises farmers to regularly conduct systematic visual surveys along the margins of their orchards looking carefully at new green shoots and conducting tap sampling. To tap sample, spray a plastic surface with soapy water, position the plastic sheet underneath a branch and tap the branch above to dislodge adult ACP. The insects will stick to the filmy plastic where they can be studied with a magnifying hand lens to determine if they are psyllids.
If psyllids are found, the UC website outlines the immediate action that is required.
Adult psyllids should be placed in a container with 90 percent alcohol and reported to the county agricultural commissioner’s office so the insects can be tested for huanglongbing disease. Immature stages of the pest should be left on the tree so the ag commissioner’s office can make an official regulatory collection.
“Florida and Texas don’t have exactly the same insecticides that are available in California and the environmental conditions are different,” Grafton-Cardwell said. “Over time we will compare different treatments and determine how long they will protect the trees. Any new developments will be posted to the website.”
Because the goal in the San Joaquin Valley is eradication, Grafton-Cardwell recommends aggressive action against a psyllid infestation.
“The best approach is using two broad-spectrum insecticides within a short period of time,” she said. “No one insecticide will kill all the stages of the pest."
Another key to successful eradication is area-wide treatment coordination. Grafton-Cardwell suggests farmers work closely with their treatment liaisons and treat their orchards in a coordinated manner.
“The bigger the area being sprayed at the same time, the better,” Grafton Cardwell said. “If we get a blanket effect over the whole area, that’s been shown in Florida and Texas to have the greatest impact on the psyllid populations.”
The aggressive psyllid treatment aims to buy time for researchers to find long-term strategies for maintaining the California citrus industry in the presence of ACP and, especially, with the incurable and fatal citrus disease they spread, huanglongbing.
The July ACP find prompted the California Department of Food and Agriculture to quarantine 178 square miles in the Porterville area, placing severe restrictions on the movement of local citrus nursery stock and citrus fruit outside the area. It also prompted several hundred farmers and pest control advisers to gather July 30 at the International Agri-Center in Tulare for an update on the threat.
Like a scene from the film “Scared Straight,” Ken Keck, former executive director of the Florida Department of Citrus and now director of the California Citrus Research Board, admonished the California farmers to learn from the Sunshine State’s mistakes.
“I feel like the ex-con in front of a room of 17-year-olds,” Keck said. “All I can say is, ‘prevent, prevent, prevent.’”
Asian citrus psyllid is established in Southern California. Efforts in the southern part of the state are focused on managing the psyllids to reduce the likelihood they will find a tree infected with huanglongbing. Huanglongbing (HLB) is an incurable and fatal disease of citrus spread by ACP.
By working together and following science-based treatment strategies, UC Cooperative Extension specialist Beth Grafton-Cardwell believes farmers and CDFA can still eliminate ACP in the San Joaquin Valley, where the bulk of the state’s commercial citrus is grown.
At the Tulare meeting, Grafton-Cardwell explained the eradication strategy she developed by studying Asian citrus psyllid and huanglongbing disease management programs in Florida and Texas, another state where the pest is well established.
“We want to slow the spread of psyllid into new areas,” Grafton-Cardwell said. “We want to prevent psyllids from finding HLB infected trees.”
Although it is unlikely the pest and disease can be kept at bay indefinitely, Grafton-Cardwell said the battle will buy time for researchers to discover long-term approaches for maintaining California’s citrus industry in the presence of ACP and HLB.
Developing citrus varieties resistant to HLB through traditional breeding or genetic modification will take too long, she said. Scientists are considering such futuristic solutions as inserting HLB resistance into a mild form of the Tristeza virus and inoculating trees with the virus to fight HLB.
“Everyone is racing to come up with tactics to fight ACP and HLB,” she said. “I believe we will eventually be using multiple approaches, such as a repellent spray to keep ACP off the trees and perhaps breeding ACP that can’t transmit HLB and then flooding the population with these incapacitated psyllids.”
To fight ACP and HLB, growers and homeowners can access detailed information on the pest’s distribution, monitoring methods and treatment options on a new website created by Grafton-Cardwell with funding from UC Agriculture and Natural Resources.
For tap sampling, spray a plastic surface with
The website advises farmers and homeowners to regularly conduct visual surveys and tap sampling (see video on right) in their orchards. “Yellow sticky card traps are not very attractive to psyllids,” it says.
If psyllids are found, immediate action is required.
Adult psyllids should be placed in a container with 90% alcohol and reported to the county agricultural commissioner’s office so the insect can be tested for HLB. Immature stages of the pest should be left on the tree so the ag commissioner’s office can make an official regulatory collection.
An ACP find should also trigger rapid and wide treatment with the most effective pesticides possible.
Hailing Jin, an associate professor of plant pathology and microbiology at the University of California, Riverside, recently published a paper in the journal Molecular Plant in which she reports having profiled small ribonucleic acid (sRNA) from citrus plants, some of which were affected by HLB.
Her research showed that several sRNAs were found to have been induced specifically by HLB, meaning they could potentially be developed into early diagnosis markers for the disease.
The study also showed that in a three-year field trial in southwest Florida diseased trees suffered from severe phosphorus deficiency and that application of phosphorus solutions to the diseased trees significantly alleviated HLB symptoms, improving fruit yield.
In the trial, 19 healthy sweet orange trees were grafted with HLB-positive bark or leaf pieces. As controls, five trees were mock-inoculated with pathogen-free healthy tissue. Phosphorus solutions were applied to the 19 HLB-positive trees three times a year. After two years of treatment, the diseased trees displayed significantly reduced HLB symptoms.
“Compared with the mock-treated plants, the phosphorus-treated trees had a greener appearance and more vigorous growth,” Jin said. “Fruit yield increased approximately two-fold compared with the mock-treated plants.”
She cautioned that the application of phosphorus solutions did not cure the trees. Her research suggests, however, that additional phosphorus application may help diseased trees look healthier and improve fruit yield.
Jin was joined in the research by Hongwei Zhao, Ruobai Sun, Chellappan Padmanabhan, Airong Wang, Michael D. Coffey, Thomas Girke, Timothy J. Close, Mikeal Roose and Georgios Vidalakis at UC Riverside; and researchers at Nanjing Agricultural University, China; the U.S. Department of Agriculture; Fujian Agriculture and Forestry University, China; and the University of Florida.
The research was supported by a grant to Jin from California Citrus Research Board./span>/span>