Attacking HLB From All Angles

Golden Nugget is a new mandarin variety released recently by the UCR Citrus Variety Collection. It was developed as a seedless fruit, which eliminates the need to net the orchard.

University of California Riverside (UCR) is committed to a major offensive against huanglongbing (HLB, or citrus greening disease), as the threat is marching ever closer to commercial citrus orchards in California. The only confirmed cases in Southern California have been in residential trees in Los Angeles and Orange counties, including near the UCR campus. But some experts believe it is only a matter of time until the disease shows up in a commercial grove.

The bacteria, which is carried by the tiny Asian citrus psyllid insect, has already decimated citrus in Florida and elsewhere. Building on many years of research efforts, experts in the UCR’s College of Natural and Agricultural Sciences (CNAS) have been working with government and industry officials to address the threat and keep the disease from damaging California’s citrus industry.

Ongoing research efforts include breeding plants for resistance, developing molecular tools to combat the disease, new treatment programs using insecticides and bactericides, and early disease-detection techniques. In the short term, UCR is working to increase citrus production and improve overall plant health to give the industry a boost.

“Now that HLB is in Riverside, there is fear of devastation,” Peggy Mauk, UCR’s director of agricultural operations, said, adding that Florida’s orange juice production has dropped over 70 percent as a result of the disease.

Mauk researches abiotic or biotic stresses to subtropical fruit crops, such as avocado, citrus, dates and mangos. As director, she oversees field research in support of the CNAS mission.

“UCR continues to provide research results that support the California citrus industry and beyond,” she said.

PROTECTION OF THE COLLECTION
Tracy Kahn is the curator of UCR’s Citrus Variety Collection. This is one of the world’s most diverse collections of live citrus and related types with approximately 1,000 different varieties. The list of varieties includes mandarins, blood oranges, navel oranges, citrons, clementines, tangors, grapefruit, Valencia oranges and pummelos. As part of managing the collection, Kahn researches the commercial potential of new citrus varieties and provides various Extension activities for the citrus industry and the public.

Recognized as one of the most vital UCR initiatives is the Citrus Clonal Protection Program (CCPP). This program was established to provide clean budwood for all citrus growers in the state.

CCPP’s director is Georgios Vidalakis. He is charged with the responsibility to ensure HLB and other exotic citrus diseases do not enter California via propagative plant materials. The CCPP has been the gatekeeper in excluding lethal citrus diseases, such as HLB, from entering and spreading in California since 1956.

“The CCPP online budwood distribution system has over 1,365 registered users and we are inviting every citrus enthusiast in California and the U.S.A. to join our HLB-free budwood community,” said Vidalakis.

MULTIPLE APPROACHES
UCR scientists are working on all levels to find ways to fight the disease. Experts in plant pathology are studying the disease at a molecular level. Understanding how HLB occurs will identify ways to stop it from killing citrus trees and develop varieties that are resistant to the disease. Their research has also led to the identification of molecular markers that can be used for HLB detection.

Another approach being studied is generating HLB-resistance/tolerance by boosting citrus immune systems. Researchers are also identifying and applying antimicrobial molecules on citrus to kill and inhibit the bacterium that causes HLB.

Researchers have found that some citrus types originating in Australia are resistant or tolerant to HLB. Mikeal Roose, UCR professor of genetics, is working to develop commercially viable varieties with resistance or tolerance to the disease.

When a plant is resistant, little or no disease is present. When it is tolerant, disease is present, but the tree remains reasonably healthy. Resistance is preferable to tolerance, but varieties with resistance are more difficult to develop. Researchers are working to identify the genes responsible for resistance. The research includes determining DNA sequences of resistant types and studies of citrus and bacteria gene expression during infection.

“Developing HLB-resistant citrus is the greatest challenge of my career,” Roose stated in a recent UCR news release, “because the disease is difficult to detect at early stages of infection, and because sources of resistance are very different from most citrus that we grow and consume. But we can be hopeful because we have powerful new genomics tools that make breeding more precise and predictable.”

While scientists are working to modify citrus trees to resist HLB, another approach is to prevent exposure to the disease. A citrus under protective screen (CUPS) system is being evaluated both in Florida and at UCR to exclude the Asian citrus psyllids that transmit the bacteria causing HLB. The CUPS is a large screened building in which the citrus is grown. Tests are planned which will evaluate the impact of the reduced amount of sunshine the trees will receive under the CUPS.

The resources dedicated to these multiple avenues of research demonstrate the high level of commitment that the University of California has made to the battle against HLB. Coupled with efforts at the University of Florida and other institutions around the world, there is hope this destructive disease can be brought under control.

Information for this article was obtained from UCR Today.