Researchers Develop Early Warning System for Citrus Plant Disease

Summary

Researchers at the University of Florida have devel­oped a pos­si­ble early detec­tion method for cit­rus green­ing dis­ease, also known as HLB, which has dev­as­tated the Florida cit­rus indus­try. The method involves visu­al­iz­ing sali­vary sheaths left by the Asian cit­rus psyl­lid on cit­rus leaves and could help alert grow­ers to the dis­ease before it spreads fur­ther, although scal­ing it up for com­mer­cial groves may be a chal­lenge.

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.

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.

“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.”

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.

“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.

“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.”

“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.

• The American Phytopathological Society


• Citrus Pest and Disease Prevention Program