Pictured above, oranges with HLB are afflicted with a green color. Courtesy of Wikimedia Commons

Riverside County agricultural officials report the first instances this July of a deadly new plant disease in county limits. A tree affected by citrus greening disease was found in the vicinity of the junction of the 215 and 91 freeways, marking the disease’s first beachhead in this part of the Inland Empire. This finding comes four months after UCR researchers were awarded $5.1 million to combat the disease.

These reports are particularly alarming to a region traditionally dependent on citrus farming. The disease, caused by a bacterium known as Huanglongbing (HLB), a Mandarin term meaning “yellow dragon disease,” results in the fruit of citrus plants turning green and dying. HLB is carried and transmitted by the Asian citrus psyllid, an insect which thrives among susceptible trees.

Upon transmission, HLB often remains undetected as it spreads from leaf to leaf around the tree. It can take up to five years for an affected tree to develop clear and identifiable signs of the disease, by which time it is often too late to limit the damage and spread of the infection. At the time of detection, the tree is usually unsalvageable and will die. There is no known treatment for HLB as of yet, and the most common way of managing the disease is containment.

County officials have now quickly acted to secure the affected area and treat all citrus trees within an 800-foot radius. It is hoped that taking action against the carrier psyllids could help mitigate the potential spread of the disease. Unfortunately, according to Dr. Peggy Mauk, UCR Director of Agricultural Operations, the long gestation period of the disease in citrus tree means that the infection has been present for some years, and has likely been transmitted elsewhere by now.

Agricultural officials’ concern at the appearance of this disease is based on the bacterium’s destructive history. “HLB is the most serious threat to citrus worldwide,” Mauk told the Highlander in an email. She cited statistics from Florida, another citrus-growing area of the United States in which up to 70 percent of citrus growing acres have been lost. Underscoring the severity of the situation, Mauk added via email, “They (Florida farmers) cannot replant and have a tree survive long enough to make it economical to grow citrus.” Here in the Inland Empire, she says, “Riverside citrus is seriously threatened and there is no cure. We can only control the pest, Asian Citrus Psyllid (ACP) that carries the bacterial pathogen. Without 100% control of the ACP, the pathogen is spreading.”

When questioned regarding the response to this blight, Mauk outlined UCR researchers’ strategies to limit the damage. The most important tool in this arsenal is early detection, with significant efforts in this field being researched by scientists both at UCR and other institutions across the country. Among these methods is the use of dogs to detect and “sniff out” affected trees, much like the use of canine bomb detection units in public security.

Another avenue researchers are turning to is breeding and gene editing for resistance to the disease. Progress is slow, but researchers, among them UCR’s own Dr. Mikeal Roose, see these breeding methods as a long-term solution once implemented.

Obstacles to this work are myriad, but among these is the inability to inoculate plants with a dormant HLB pathogen due to fear of spreading. Mauk has emphasized that UCR is committed to building a new greenhouse facility near campus with special conditions that prevent pathogen escape. It is hoped that inoculation research can occur within this facility to better help scientists combat the disease. “When the facility is finished,” Mauk says, “it will enhance UCR’s ability to develop and test new technologies to control the disease.”

For now, agricultural officials and UCR researchers emphasize vigilance and proactiveness among citrus tree owners. Further information on citrus greening disease, HLB and ACP is available at the California Department of Food and Agriculture (CDFA) website. Any suspicion of infection should be promptly reported to the CDFA at (800) 491-1899.