As part of a $9 million research contract from the U.S. Department of Agriculture, UA plant sciences professor Judith Brown studies the molecular interactions between a tiny insect and a bacterium that causes citrus greening disease, a threat to commercial and residential citrus growing efforts.

Commercial and residential citrus growers dread this bacteria-caused disease because it kills every affected citrus tree. In heavily infested plants, only the leaf veins remain green while the remainder of the leaf turns yellow and eventually dies, earning the condition its alternative name, Huanglongbing disease – Chinese for "yellow dragon."

In the U.S., major citrus growing areas are in Arizona, Florida, California and Texas, with oranges, grapefruit, tangerines, tangelos and lemons accounting for the bulk of cultivated citrus fruit. The disease-causing pathogen is transmitted by the Asian Citrus Psyllid, Diaphorina citri, a tiny insect distantly related to aphids and whiteflies, all of which use a syringe-like mouth apparatus to pierce plant tissue and suck out the sugar-laden sap.

"Citrus greening disease affects the whole plant and interferes dramatically with growth and ability to produce quality fruit," said Judith Brown, a professor in the School of Plant Sciences in the UA’s College of Agriculture and Life Sciences, who serves as the principal investigator on the UA portion of the grant.

Brown's research group has been studying the molecular mechanisms used by the bacterium to get into the insect, multiply and then enter the host plant when the psyllid feeds on it, with the goal of finding ways to block the transmission process. It is the first project conceived and supported by the citrus growing industry as a novel approach to managing the disease.

Fruit from an infected tree typically is not marketable, Brown explained. For example, an orange might be green or splotchy, with some of its segments fused together or only half developed, and it is pithy in texture, rather than sweet and juicy.

"If you cut open an infected fruit, you'd think, 'They didn't leave this on the tree long enough to ripen,'" Brown said.

The story of citrus greening disease begins somewhere in China, India or Pakistan; the exact home range of the Asian Citrus Psyllid is still unclear. Carried by international commerce and travel, the insects have since spread across the world, taking with them the disease-causing bacterial pathogen, Candidatus Liberibacter asiaticus.