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Researchers Develop Early Warning System for Citrus Plant Disease

The new method detects salivary sheafs left by the infecting vectors on the plant, which then allows farmers to determine whether the trees have been infected before they begin to show symptoms of the citrus greening disease.

Dec. 20, 2019
By Imari Scarbrough

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Researchers have taken another step for­ward in com­bat­ing the citrus green­ing dis­ease that has plagued the indus­try.

A pos­si­ble early detec­tion method for the dis­ease, which is also known as citrus huan­g­long­bing (HLB), was announced ear­lier this year by Sheo Shankar Pandey, a post­doc­toral research asso­ciate in plant pathol­ogy, and Nian Wang, a pro­fes­sor of micro­bi­ol­ogy and cell sci­ence.

It’s an improve­ment basi­cally in detec­tion, as far as con­fir­ma­tion. The dif­fi­culty with all of these tech­niques is scal­ing up to make it epi­demi­o­log­i­cally applic­a­ble in a large land­scape.- Tim Gottwald, plant epi­demi­ol­o­gist at the U.S. Department of Agriculture

Both researchers work at the University of Florida citrus research and edu­ca­tion center and pub­lished a paper detail­ing their find­ings in the American Phytopathological Society.

HLB has already dev­as­tated large por­tions of the Florida citrus indus­try.

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“The Asian citrus psyl­lid feeds on citrus leaves and stems, and can infect citrus trees with a bac­te­ria that causes a seri­ous plant dis­ease called Huanglongbing, also known as HLB or citrus green­ing dis­ease,” the Citrus Pest and Disease Prevention Program wrote on its web­site. “While not harm­ful to humans, the dis­ease kills citrus trees and has no cure.”

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It can take months, even years, for signs of infec­tion to become known. By then, the insects may have spread the dis­ease to many more trees. The symp­toms are obvi­ous, but once they have man­i­fested it is too late to save nearby trees.

“HLB-diag­nosed plants exhibit blotchy mottle leaves, stunted growth, yellow shoot, reduced fruit size, corky vein, root decline, and ulti­mately, dieback,” Pandey and Wang wrote.

As trees can be infected for months or years before a grower may notice symp­toms, an early detec­tion method could help alert them to the prob­lem before it can spread fur­ther.

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“We take advan­tage of the fact that Ca. L. asi­ati­cus [the vector that spreads the dis­ease] remains around the ACP feed­ing site imme­di­ately after trans­mis­sion,” Pandey and Wang wrote in the study. “ACPs secrete sali­vary sheaths at their feed­ing sites, which can be visu­al­ized using Coomassie bril­liant blue stain­ing. Epifluorescence and con­fo­cal microscopy indi­cate the pres­ence of sali­vary sheaths between the blue spots on ACP-fed leaves.”

HLB was first reported in 2005 in Florida and over the course of nearly 15 years has caused more than $10 bil­lion in damage to the state’s citrus indus­try.

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“Nearly every plant­ing in Florida from about Gainesville south is infected,” Tim Gottwald, a plant epi­demi­ol­o­gist at the U.S. Department of Agriculture’s hor­ti­cul­tural research lab­o­ra­tory in Florida, told Olive Oil Times.

“The only time that changes is if it’s a new plant­ing, but even new plant­i­ngs, by the time they’re 18 to 24 months, they’re nor­mally 50 to 100 per­cent infected,” he added. “The vector is every­where, the dis­ease is every­where, and the indus­try is in decline. We’ve gone from 243 mil­lion boxes per year to some­where in the 40 mil­lion boxes, prob­a­bly.”

The dis­ease also has been found in South Africa, Indonesia, and the Philippines, threat­en­ing the citrus indus­try of each coun­try. California, another large pro­ducer of oranges in the United States, has a free hot­line for those who believe they have seen the Asian citrus psyl­lid or signs of tree infec­tion.

“Globalization and people moving citrus plants from one place to another have spread this dev­as­tat­ing dis­ease,” the Citrus Pest and Disease Prevention Program wrote.

This new pos­si­ble early detec­tion method will likely help small grow­ers, but be less useful for com­mer­cial groves, accord­ing to Gottwald.

“It’s an inter­est­ing method­ol­ogy,” he said. “It’s an improve­ment basi­cally in detec­tion, as far as con­fir­ma­tion. The dif­fi­culty with all of these tech­niques is scal­ing up to make it epi­demi­o­log­i­cally applic­a­ble in a large land­scape.”

Gottwald added that the method would likely work best for those who can sample more leaves in a small amount of trees.

“It’s not to put down the method at all,” Gottwald said. “It’s a nice little advance­ment to PCR (poly­merase chain reac­tion)… which is a very accu­rate, very reli­able method­ol­ogy. But when you try to take it out into an orchard, the prob­lem you have with PCR and other meth­ods like this is the dis­ease is not com­pletely dis­trib­uted in a single tree, not even in a single leaf.”

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“With a tree with 100,000 or 200,000 leaves on it, you have to pick sam­ples,” he added. “It works very well in a nurs­ery sit­u­a­tion where you’re very inter­ested in a cer­tain tree.”

In spite of this new advance­ment in detec­tion, Gottwald believes that the citrus indus­try still has a long way to go in deal­ing with HLB.

“There’s no short-term solu­tion that I’m aware of,” Gottwald said.