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.”