Finally, Real Progress on Greening?

I'm still getting some good oranges off the mature trees left on our property by a previous owner. But last spring I had to put two Minneolas and a never-did-produce Valencia out of their misery, and as limbs die back, the remaining relics get smaller every year. They form far more fruit than they can possibly ripen in their weakened state. Most fall early or succumb to one assault or another, but a few make it through. And remind me that a good orange is among the finest of fruits.

The biggest, baddest of citrus diseases is, of course, the relentless HLB or citrus greening disease that continues to devastate the Florida industry. For years the news on greening has been dire, but Texas A&M researcher Erik Mirkov has developed greening-resistant trees by transferring spinach genes that produce antimicrobial proteins.

Now, scientists Jude Grosser and Manjul Dutt at Florida's Citrus Research and Education Center have used a gene from the mustard family to produce greening-resistant Hamlin and Valencia orange trees. Their September article in the prominent open-access scientific journal PLOS ONE details the work.

Test results of their trees appear similar to those reported by Mirkov. In field trials in groves heavily infested with greening, some lines tested positive for greening at some points in the trials, but greening-free later. Three lines tested free for the entire 36 months. Laboratory trees exposed to continuous attack by infected citrus psyllids exhibited similar resistance.

The work would have to be repeated for other varieties and citrus species, but this could be avoided if the method can be adapted to produce rootstocks that impart greening resistance even to non-genetically-modified scions. In addition, the authors speculate, this “could potentially be more acceptable to consumers than transgenic citrus scions.”

For both the Texas and Florida programs, considerable further development and testing are required, and it will still be years before these trees are available to the public.

Is victory at hand? Not yet. The trick now is to “stack” the mustard gene with another gene that works by a completely different mechanism. For such a doubly protected tree, it would be far more difficult for HLB to adapt to overcome the defenses. Then we win. At least until the next scourge spawned by the vast monocultures of industrial agriculture.