Are GMO Orange Trees The Answer To A Disease Effecting Orange Groves?

The curtain is slowly parting on the Huanglongbing (HLB) citrus drama in California.

Three Asian citrus psyllids have been discovered this year in Tulare County, the heart of the San Joaquin Valley citrus belt the third only recently. They join the cast of 43,000 similar psyllids detected in Southern California since 2008, mostly in ornamental or backyard trees. Two of the three SJV cousins, possible hitchhikers from the southland, were too beat up to evaluate if they were carrying the Huanglongbing or citrus greening disease. No word if the third carried the disease. County agricultural officials are looking at the boundaries for quarantine, which will regulate and restrict the movement of fruit and plant material.

Asian citrus psyllid injects a bacterium which causes greening that spreads when other psyllids feed on infected trees. Only one tree so far has been confirmed infected in a Southern California backyard. However, grower Dan Dreyer of Exeter, Calif., and most others like him agree it is only a matter of time until many more trees are infected and a full-scale war breaks out to contain the spread.

California cannot escape the disease, says Dreyer. “It will eventually get here, if it’s not here already, since we have found the psyllid.” 

California growers have a pretty good idea what the future will hold because the story has featured in Florida for seven years. The psyllid and disease cost Florida growers $331 million annually from 2006 through 2011. That’s a $1.66 billion hit to the industry for five years, according to a recent study released by University of Florida agricultural economists Thomas Spreen and Alan Hodges.

Total economic impact of the disease, including indirect effects, was $4.54 billion, or $908 million annually, over those five years. An estimated 8,257 jobs were lost in Florida due to greening during that period. California wants to win skirmishes and eventually the war by going to school on what Florida and perhaps Brazilian growers have done to save their citrus industries.

However, the general consensus is that it will take a big bazooka to end it all and that means trees resistant to the disease. This could lead to uncharted waters for a food crop like citrus. It may take a genetically modified tree to turn back perhaps the biggest threat to the U.S. citrus industry. California producers were given a closer look at Florida through the eyes of Rick Cress, president of Southern Gardens Citrus in south Florida. Cress spoke at the California Citrus Conference at the Porterville Fairgrounds sponsored by the Citrus Research Board.

Southern Gardens Citrus’ processing plant in Clewiston, Fla., processes more juice in a day, 25,000 tons, than all of California in a year. It produces more than 50 percent of the private label not-from-concentrate orange juice in America. It processes 900,000 tons of oranges annually into juice. Florida’s citrus industry is primarily a juice business, while California is mostly fresh market. Owned by U.S. Sugar, Southern Gardens owns and/or manages 16,500 net acres of citrus groves in Southern Hendry County. These groves contain 1.8 million trees.

HLB is the “worst challenge we have ever faced,” greater than the citrus canker that took out 4,500 acres of the 32,000 acres Southern Gardens was farming before the canker came in.

Cress said four things are grown in southern Florida: “Citrus, sugar cane, cattle and alligators.” Southern Gardens must continue to grow citrus. It has no other farming options.

“We had to find a solution,” after HLB was found on the property in 2005.

The first step was to identify diseased trees and destroy them quickly. That has been a $1 million per year expense. Infected groves quickly disappeared to be replanted. Older trees take longer to die. “I have seen four-year-old trees literally die before my eyes,” he said. Typically, it takes two to two and a half years for an HLB-infected tree to die. “It looks like a Charlie Brown Christmas tree,” he added.

One of the approaches used to battle back has been nutritional. Growers heavily fertilized to keep diseased trees alive. This is expensive and does not save trees in the long run, said Cress. It just prolongs productivity of infected fruit.

Cress said this approach to counter HLB has improved the understanding of the nutritional needs of citrus. “No nutritional program has ever cured a disease.” Besides, “There is no hard data to support,” that the nutritional approach is winning the battle, even temporarily.

The yield per tree is less than healthy trees, even with the nutritional approach. This is a disease effect, not inadequate fertility. These trees also do not handle stress well. He adds this may keep the tree green, but the fruit is tainted. Fruit from an HLB tree is very bad tasting, he says.

Overall, the psyllid and the disease have increased Florida growing costs by 50 percent. Typically, most Florida growers aggressively treat to control the psyllid to protect the new plant after taking out an infected tree. Pesticides to control the disease vector are not economical in the long run, and there are environmental challenges as well as spraying to kill the psyllid. Resistance is another issue after continually treating with a limited number of registered pesticides.

Another approach is to take out infected groves and replant with high density groves, hoping to make money quickly before trees die. A normal planting is about 145 trees per acre. Southern Garden has one trial of 600 trees per acre. In Brazil growers are moving far away from infected areas and planting densities of 200 to 300 trees per acre, figuring to get 15 to 16 years of high production before infections kill trees.

It will take an integrated solution with plant resistance as the cornerstone. That could come down to genetically modified (GMO) citrus trees, which would be a challenge to educate consumers about the safety of the fruit and juice from trees in the highly charged anti-GMO atmosphere.

Dr. Erik Mirkov, a Texas AgriLife Research plant pathologist at the Texas AgriLife Research and Extension Center at Weslaco, Texas, has transferred two genes from spinach into citrus trees, apparently providing resistance to HLB. The transgenic trees have shown resistance in greenhouse trials and will soon be planted in Florida for field testing, he said. The research is funded by Southern Gardens Citrus.

“This project started with a three-year grant from the U.S. Department of Agriculture when the interest was to find resistance to citrus canker,” Mirkov said. “But then citrus greening moved into Florida. Both are bacterial diseases, but citrus greening devastated the industry far worse than canker did.”

Mirkov knew that spinach proteins had broad-spectrum resistance against multiple bacteria and fungi, and started testing his transgenic trees against greening.

“We injected canker into the leaves of transgenic plants with one spinach gene and found that the bacterial lesions didn’t spread,” he said. “But we also showed that transgenic plants infected in the rootstock with citrus greening disease flourished and produced lots of leaves, while the non-transgenic trees produced just one leaf.”

With good greenhouse results, those first-generation transgenic trees were taken to the field in 2009, Mirkov said. After 25 months of growth, some of the transgenic trees never showed infection, while 70 percent of the non-transgenic control trees did. In the meantime, Mirkov developed improved second-, third- and fourth-generation transgenic trees by adding a second spinach gene and improving how and where the genes expressed themselves.

“Citrus greening basically shuts off the tree’s ability to take up and use water and nutrients, causing the tree to die,” Mirkov says. ”We were able to improve the transgenic trees by having the genes express themselves in the vascular system.”

Mirkov also found that while one spinach gene is more effective than the other, they work better together than they do alone.

“The first field trial involved transgenic trees using only the weaker of the two genes, but it worked; it gave us encouragement” he said. “By using both genes, we’re hoping to get immunity so that trees are never infected in the field.” - Harry Cline, West Farm Press