Texas A&M plant pathologist Erik Mirkov is fine-tuning a transgenic process which could help the citrus industry survive its worst scourge, the disease Huanglongbing (HLB).

Over the last six years, Mirkov, based at the Texas A&M AgriLife Research and Extension Center in Weslaco, has inserted multiple protein genes called definsins - found in spinach - into young citrus plants to create transgenic trees which act as a protective shield against HLB.

“Our greenhouse tests with spinach definsins in citrus trees reveal either complete immunity to HLB or extremely high resistance to the devastating disease,” Mirkov said.

Mirkov explained his research during the 2013 joint meeting of the Caribbean and Pacific divisions of the American Phytopathological Society held in Tucson, Ariz., in June.

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A definsin is a natural protein found in all plants, insects, and mammals, including humans.

In the human body, definsins are part of its pre-formed defense system to keep microorganisms (bacteria and fungi) from causing infection. Definsins are found in tears and mucous, and circulate through the blood stream.

HLB is a major problem in all citrus-growing regions in the world, including most recently the U.S. The disease was found in Florida in 2005 and has spread to almost all citrus-growing regions in the Sunshine State.

HLB has spread westward, found in several Texas commercial citrus groves early last year. A single case of HLB was found in California last year in a residential citrus tree in the Los Angeles area. HLB has not been identified in Arizona citrus.

HLB is caused by the bacteria Candidatus Liberibacter which is vectored to citrus trees by the Asian citrus psyllid.

While HLB research is ongoing worldwide, there is no known cure for the citrus malady. Every HLB-infected tree eventually dies. Fruit from infected trees become misshapen and the flavor turns sour leading to unmarketable fruit.

Field test findings 

Mirkov conducts his lab research in Weslaco. The transgenic tree greenhouse and field tests are underway at Southern Gardens Citrus, a citrus grower and processor in South Florida.

The tests include a screened psyllid greenhouse where large numbers of infected psyllids fly and land on the transgenic citrus plants, at much higher insect numbers and infection odds than a commercial grower would ever find in a grove.

The other test includes transgenic trees planted next to standard citrus trees in a commercial grove. The standard, unprotected trees are HLB infected with symptoms including yellowed shoots and leaves. The transgenic trees appear much healthier.

Laboratory tests confirm the transgenic trees have lower amounts of Liberibacter bacteria.

Mirkov is convinced that the transgenic tree could be a valuable resource to growers to help prevent the spread of HLB. Mirkov or his research sponsors make quarterly trips to Washington, DC to share his ongoing findings with the Environmental Protection Agency (EPA) and USDA.

“The EPA understands that HLB is a crisis,” Mirkov told the plant pathology crowd.

Mirkov is encouraged by the EPA’s overall positive response to the transgenic research. The agency has made it clear that, in general, transgenic trees must be safe for the environment and the fruit safe for human consumption.

Mirkov and Southern Gardens have studies underway seeking any adverse effects on non-target beneficial organisms, including honeybees.

Mirkov said, “In the long run, I believe EPA and USDA will give approval to proceed forward.”

If approved, Mirkov says the first commercially available transgenic trees could be on the market in two to three years.

Western Farm Press asked Mirkov about any possible consumer backlash over the idea of citrus consumption with a touch of spinach inside.

He responded, “Will my citrus taste like spinach or will my orange juice be green? Of course not. We are talking about a small piece of DNA from spinach placed in citrus.”

Better understanding HLB

All eyes of the U.S. citrus industry are focused on research by Mirkov and others at the university, government, and private sector levels in the U.S. and abroad. Citrus researchers have traveled to HLB-infested citrus regions to learn about this devastating disease.

The western U.S. is the last major citrus-producing region in the world to get HLB.

Florida is the nation’s largest citrus-growing area, mostly oranges for processing into juice. California ranks second where about two-thirds of the citrus is grown for the fresh market. Texas and Arizona rank third and fourth, respectively where most fruit is sold fresh.

The California, Arizona, and Texas commercial citrus industries have their eyes focused on the Florida experience to learn from its mistakes with the psyllid and disease. The first psyllid was found in Florida in 1997 and quickly spread like wildfire across the state.

Ron Brlansky, University of Florida plant pathologist, discussed the worsening ACP-HLB situation in Florida. Brlansky has 34 years with the Citrus Research and Education Center in Lake Alfred, working extensively on a wide array of citrus pests and diseases.

The first major mistake in Florida, Brlansky says, was failing to gain good psyllid control early on.

“We did not do a good job of keeping the psyllid contained in Florida, mainly due to the movement of plant materials,” Brlansky told the group.

He says the most important grower effort against the psyllid is insecticide sprays. Some growers spray once a month – 12 times a year – which is expensive and not financially sustainable over the long haul.

A major concern, Brlansky says, is the fear of insecticide resistance to the psyllid.

On young non-bearing citrus trees, growers can apply three soil-applied systemic insecticides including imidacloprid, thiamethoxam, and clothianidin. Aldicarb (Temik) is no longer allowed for use on bearing trees since the registration in citrus was cancelled.

“We’re trying to get growers to alternate insecticides to prevent resistance from occurring,” Brlansky said.

HLB shuts down the phloem in the tree, living tissue which carries photosynthetic products to roots and developing shoots and fruit. As a result, the tree basically starves to death.

Brlansky discussed a new procedure – a nutrient treatment - tried by one Florida citrus grower over the last several years in a 90-percent to 100-percent HLB-infected grove. The grower gives the trees a nutrient cocktail of sorts, including about 20 ingredients.

The nutrient kick start allegedly has helped build new phloem in diseased trees; ahead of the plugging and collapse of the tree’s original phloem tied to HLB. The grower also practices good psyllid control in the grove.

Remarkably, the HLB-infected trees are producing good fruit despite the HLB infection. The fruit is marketable.

A doubtful Brlansky inspected the grove and after close examination confirmed the trees were generating new phloem.

Suggestions for western citrus growers

Based on the lessons learned by the Florida-HLB experience, Brlansky offered these suggestions for citrus growers in California, Texas, and Arizona (when the disease is not widespread).

Keep the pathogen out. Regulate propagating materials to prevent introduction.

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