Call it what you like – citrus greening disease or the California-preferred vernacular “Huanglongbing” (HLB for short) – it’s an insidious disease that kills citrus trees while it robs them of production and fruit quality acceptable for fresh markets and processing.

Yet, while there is much alarm about the disease, research continues into finding a cure or, at the very least, developing trees resistant to the disease. Simultaneous studies are also looking into key markers that could make early detection of the disease possible, thus helping the citrus industry and regulators find and remove infected trees before they become a reservoir to further spread the disease.

Much of this year’s California citrus showcase, an annual event held by California Citrus Mutual (CCM), was dedicated to discussions on the disease and what citrus producers can and must do to protect their industry.

CCM has taken an “all hands on deck” approach to inform growers, the general public and solicit support – political and otherwise – to keep the disease from doing to California’s fresh fruit industry what it is doing in Florida and may very well do in Texas.

Time will tell just how quickly the disease spreads through Texas’ fresh-fruit industry, which is predominantly made up of grapefruit, according to Texas Citrus Mutual President Dale Murden.

Murden and two Florida citrus growers told over 700 California citrus growers, their pest control advisors and other industry representatives at the annual citrus showcase luncheon the lessons learned in those states as the Asian citrus psyllid (ACP) and HLB took hold.

The ACP is a tiny winged bug that is known to feed only on citrus plants. The invasive pest, which is about the size of a gnat, is also capable of vectoring a bacterial pathogen that causes HLB.

History lesson

Florida’s mistake was not taking the ACP seriously when it first appeared near Miami in 1998.

Larry Black, Jr., the general manager of Peace River Packing Company in Fort Meade, Fla., and a citrus grower himself, and Rick Freeman, a grower who sits on the Florida Citrus Mutual Board of Directors, said the psyllid was nothing more than a “nuisance” when it was first discovered there. Because it wasn’t viewed as an economic threat and growers were already fighting citrus canker in the state, the insect was allowed to reproduce and spread unfettered by treatment programs.

Even after HLB was discovered in Brazil in 2004, Black said the disease was far from the minds of Florida growers, who were enjoying near-record production after hitting a peak of 244 million boxes of oranges in the 1998 season.

In 2005 HLB was discovered near the Miami International Airport. Today it is estimated that from 70-90 percent, or more, of citrus trees in the state have the disease.

Murden said the Texas example looks to mimic what happened in Florida; though there are pockets of citrus in Texas that remain clean from the disease.

According to Murden, HLB was found five years ago in Texas, a fact that has him on edge.

“We’re in our fifth year of HLB here,” said Murden. “Our trees still look healthy and our production is up. I’ve seen Florida’s graphs and in their fifth to sixth year after their first find the bottom fell out. We’re at a scary crossroads here in Texas.”

Today

Citrus production in Florida is down about 75 percent from its high of 244 million boxes. The U.S. Department of Agriculture estimates that Florida citrus production this year could be 59 million boxes.

Further challenging the Florida citrus industry is the exploding cost of production related to psyllid treatments. Nutrient treatments and the 12 spray treatments per year growers employ have pushed costs from around $800 per acre to about $2,000 per acre, said Ed Stover, a research horticulturist and geneticist with the USDA-ARS in Fort Pierce, Fla.

“We are actually to the point now where there is serious concern on part of the industry that we can maintain expensive infrastructure like processing plants because they require high throughput of materials to be profitable,” Stover said.

Stover gave a presentation at the showcase where he talked in part about the history of ACP/HLB in Florida and some of the ground-breaking research happening to address the tree disease.

Though nutrient enhancements and other management practices have helped some, Stover says there is no evidence that HLB can be cured with existing technologies.

Stover echoed what the Florida growers said: there is no substitute for holding back the advance of the psyllid in California as a means to keep the disease from overtaking the state’s commercial citrus industry.

So far California has been fortunate. Though at least 14 citrus trees have been discovered with the disease in California, those have all been on residential properties in the San Gabriel Valley, a suburb of Los Angeles. Though the psyllid has made it into the San Joaquin Valley citrus growing region, growers there have been more attentive to treating for the pest.

Of the psyllids that have been found in the San Joaquin Valley that could be tested for the bacteria, officials have not reported any positive finds of psyllids infected with the HLB-causing bacterium.

Research continues

Stover says some research into developing trees that are at least tolerant of the disease – the goal, he says, is a tree impervious to the disease so that spray treatments for the psyllid are unnecessary – shows promise.

“There is widespread agreement that HLB-resistant trees will be the long-term sustainable solution,” he said.

Short-term fixes include trying to find existing cultivars “that are more tolerant” of the disease and can perhaps yield marketable fruit a bit longer than susceptible trees, he said.

Early detection studies into the disease continue in California.

Wenbo Ma, an associate professor in the department of plant pathology and microbiology at the University of California, Riverside, is looking at proteins in citrus plant material as an early indication that the bacteria is present.

The only recognized technology approved by the USDA to determine whether a citrus tree is infected is the Polymerase Chain Reaction (PCR) test, which amplifies strings of DNA to detect bacteria.

Drawbacks to this technology include the time it takes the bacteria to move through the tree and the plant sample tested. For instance, a diseased tree could test negative because the plant material sampled did not contain any or enough bacteria to register on the PCR test.

Simply put, Ma says the bacteria, which spread through the phloem, do not uniformly spread through the tree.

One of the more promising aspects of her work is how the evidence of certain proteins in the plant material can pick up the HLB-causing bacterium well before a PCR test might reflect a diseased tree.

These proteins appear as a result of the bacterium attacking the tree, and can be detected fairly early in the infection process, she said.

One issue with the protein work Ma is performing is the relative difference in the secreted proteins between citrus trees in various parts of the United States and the world, making a single test for the proteins unlikely to work universally.

Message to growers

California citrus growers were told in no uncertain term that holding the psyllid at bay as long as possible is the best possible defense against the disease. Beyond that, finding and removing infected trees continues to be the best-known defense against creating a reservoir of disease for psyllids to spread.

Part of Florida’s problem is the estimated 130,000 acres of abandoned groves sitting with the inoculum in them as a refuge for psyllids to feed upon and spread the disease. California officials are concerned of similar situations, particularly in southern California, where the psyllid is well-established.

To meet the objective of reducing potential HLB inoculum, CCM has once-again partnered with Bayer CropScience in an effort to remove citrus trees that are not being cared for. A web site is under development wherein growers can report trees they suspect are not being managed or cared for.

Through those reports, officials can investigate and determine if the trees are problematic, and with landowner permission, remove the trees to reduce the reservoir of potentially infected trees.

This is similar to the California program, which will remove non-commercial citrus trees that are found to be infected with the bacteria that cause HLB.

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