California growers ramp up efforts to fight citrus disease

California growers ramp up efforts to fight citrus disease

California efforts to fight the Asian citrus psyllid range from looking at controls in organic groves, eye-in-the-sky detection of trees and sniffing for volatile organic compounds.

Efforts to combat the Asian citrus psyllid and the deadly disease it can spread range from looking at controls in organic groves, eye-in-the-sky detection of trees in residential areas and sniffing for volatile organic compounds inside a containment facility with strict controls on anyone and anything that enters and leaves the research setting.

MaryLou Polek, vice president for science and technology for the California Citrus Research Board in Visalia, talked of those efforts at a Tulare meeting on citrus that also included discussion of stubborn disease in citrus, an update on California’s irrigated lands regulatory program and advice on troubleshooting micro-irrigation systems.

Polek said a priority in research efforts is early detection and removal of any diseased trees and among the partners in that effort could be NASA’s Jet Propulsion Laboratory. From planes above the citrus variety collection at the University of California at Riverside, JPL is determining whether citrus trees can be detected.

The psyllids can spread huanglongbing (HLB) disease that can ruin the fruit on infected citrus trees and eventually kill them. There is no cure, and growers say HLB could devastate California’s nearly $2 billion citrus industry.

Because of the threat posed by the insect and the disease, Polek said extensive precautions – including mandated showers and changes of clothes – are used for those entering and leaving a “level three containment facility” where research on HLB is being done at the University of California at Davis.

Research on infected plants at that facility and elsewhere is aimed at improving detection in residential areas and commercial groves.

It includes use of a volatile organic compound detector that Polek said has been streamlined for greater portability and easier use in the field. VOC emissions are being tested at two commercial groves in Texas and in Hacienda Heights where an infected tree was found.

There is also research on whether variability in the reflectivity of leaves on diseased trees could be an indicator.

Research in the containment facility and elsewhere is looking at organic controls for the insect pest, including use of a parasitoid, comparison of Pyganic with conventional insecticides for control of the psyllids in winter months and evaluation of other organic controls.  

The citrus meeting also included discussion of stubborn disease in citrus, which has been a problem in the state for decades and which poses a particular threat for younger blocks of citrus.

Ray Yokomi, a research plant pathologist with the U.S. Department of Agriculture in Parlier, talked of improved methods for detecting the disease, which causes stunting in trees and damages fruit. He said a polymerase chain reaction detection process is being used to find the pathogen that leads to the disease.


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The pathogen, a bacterium, grows in the phloem vessels that course through the plant. Yokomi said that because various parts of the plant may be infected, it’s impossible to prune out the infection.

“The only way to eliminate it is to eliminate the tree,” he said.

Yokomi said the disease may be found in 1 percent to 5 percent of citrus in California, but infestations in some blocks could go as high as 40 percent. And the result, he said, is “severe symptoms, serious economic problems and production losses.”

Neil O’Connell, UC farm advisor in Tulare County, said fruit from diseased trees may be rendered unmarketable. It can result in lopsided fruit, irregular coloration, acorn-shaped fruit and a drop in fruit production between 25 and 32 percent.

He said symptoms include stunting of the tree, leaves that point upright and mottled leaves that resemble those on a tree with zinc deficiency.

Both O’Connell and Yokomi talked of the vector for the disease, the beet leafhopper, which often can carry the bacterium from weeds on the orchard floor. For that reason, Yokomi said it is important to keep weeds out of the orchard, especially when the trees are young, and to be aware of crops planted in close proximity to a grove. If a groundcover is used to reduce dust, he said, it should be mowed from time to time.

The two speakers said citrus is not a preferred host for the insect, which also carries the bacteria into crops that include beets, potatoes and carrots. The insect prefers sunny locations and open trees, meaning it is less likely to thrive in a mature grove.

UC irrigation specialist Larry Schwankl talked of the importance of uniform irrigation with micro-sprinklers and the need to cut over-watering in order to avoid pushing nitrates below the root zone, particularly given increased scrutiny from government regulators.

New irrigation regs coming

His talk followed a presentation by Rick Hoelzel, manager of water resources with the Kings River Conservation District, who gave an update on new regulations coming under the state’s irrigated lands regulatory program.

Hoelzel explained how water quality coalitions have formed to address regulation by the Central Valley Regional Water Quality Control Board. He said the Southern San Joaquin Valley Water Quality Coalition is readying “templates” it develops along with commodity groups that will report nitrogen use and look at crop, soil, cultural and irrigation practices.

The coalition will coordinate water quality issues along four watersheds and the Tulare Lake Basin: the Kings, Kaweah, Tule and Kern. It has a membership that covers a million acres of farmland.

Hoelzel said a timetable of actions by regulators could be pushed back as court challenges from environmentalists continue to play out.

Schwankl said over-irrigating some trees can lead to leaching of water – and nitrates – beyond the root zone. Under-irrigating can cut delivery of needed nutrients to the root zone. His emphasis was on arriving at uniformity in delivery of water to trees.

Pressure differences in the system can lead to different application rates. The problem can be caused by poor system design, differences in elevation, clogging and friction in pipes and lines. Schwankl demonstrated devices that can be used to measure pressure and called attention to a website that has tips on system maintenance,

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