A nutrition expert at UC Davis has discovered important clues to the deadly attack strategy of a puzzling plant pathogen that has destroyed hundreds of thousands acres of citrus across the world. The novel research by Carolyn Slupsky, associate professor and nutritionist with the UC Davis Department of Food Science and Technology and Agricultural Experiment Station, and her team may pave the way for safe, effective ways to treat and prevent the spread of citrus greening disease, also known as huanglongbing or HLB.

“HLB is not just bad for growers and for the economy,” said Slupsky. “The loss of fresh oranges and other citrus could seriously impact our health.”

HLB is a disease caused by a microbe called Candidatus Liberibacter asiaticus and spread by the Asian citrus psyllid, a tiny insect that feeds on the leaves and stems of citrus trees. There is no cure yet for HLB, so once a tree is infected, it will slowly die. The disease has decimated citrus groves in Asia, Brazil and the Dominican Republic. Florida has lost one-third of its citrus to the disease. Both HLB and the Asian citrus psyllid have recently been spotted in California.

HLB is a silent killer – an infected tree can live for years without symptoms, allowing the pathogen to spread undetected to other trees. Symptoms emerge over time, as a tree’s fruit starts to turn green and misshapen, with a bitter, metallic taste.

Is there a way to spot HLB before visual symptoms occur? The microbe that causes HLB can sometimes be found in a leaf sample, but since the pathogen isn’t evenly distributed throughout the tree, results can be misleading.

(See related, California citrus growers schooling on Florida for coming HLB war)

“Just because the pathogen doesn’t show up in one leaf, that doesn’t guarantee the tree isn’t infected,” said MaryLou Polek, vice president of science and technology for the California Citrus Research Board. “So when you sample a leaf, there’s a high probability of a false negative result.”

Slupsky and Andrew Breksa, research chemist with the USDA Agricultural Research Service based in Albany, Calif., tried a different tack, searching for clues in a tree’s chemical fingerprint. They used nuclear magnetic resonance spectroscopy to study the amino acid composition of juice from three types of citrus: fruit from healthy tress, symptom-free (asymptomatic) fruit from HLB-positive trees, and fruit with symptoms from HLB-positive trees.