Skip to main content
ARS Home » Southeast Area » Fort Pierce, Florida » U.S. Horticultural Research Laboratory » Citrus and Other Subtropical Products Research » Research » Publications at this Location » Publication #330788
Title: Developing resistance to HLB

Author
item RAMADUGU, CHANDRIKA - University Of California
item Keremane, Manjunath
item McCollum, Thomas
item Hall, David
item ROOSE, MIKEAL - University Of California

Submitted to: Citrograph
Publication Type: Trade Journal
Publication Acceptance Date: 1/5/2016
Publication Date: 4/15/2016
Citation: Ramadugu, C., Keremane, M.L., Mccollum, T.G., Hall, D.G., Roose, M. 2016. Developing resistance to HLB. Citrograph. 7(2):46-51.

Interpretive Summary: Huanglongbing (HLB) disease of citrus is caused by Candidatus Liberibacter asiaticus (CLas) a bacterium that is transmitted among citrus trees by its insect vector, the Asian citrus psyllid (ACP). HLB has had negative impact on citrus production in the far east for over one hundred years, but was not confirmed present in the western hemisphere until 2004 in Brazil. In 2005, HLB was confirmed in Florida and since then has devastated the Florida citrus industry. In addition to Florida, HLB has been confirmed in Texas and California and poses significant threat to those industries. There is no effective cure or therapy for CLas infected trees, following infection the trees begin to decline and eventually become economically unsustainable. Unfortunately, all cultivated citrus types are susceptible to HLB. Resistance to HLB represents the ultimate solution to the problem, but is a daunting task. Breeding citrus is a long term proposition and hindered by many nuances of citrus biology. In this work we have attempted to integrate HLB resistance from citrus relatives into commercially import citrus types. Wide hybridizations with citrus and citrus relatives were made in an effort to create hybrids with improved HLB resistance. Plants generated from hybrid seeds are being screened by exposing them to CLas-infected ACP and following the incidence of infection, bacterial titer and HLB disease symptoms. Based on early results it appaears that some of the hybrids are in fact less susceptible to HLB than are others.

Technical Abstract: Huanglongbing (HLB) disease of citrus is caused by Candidatus Liberibacter asiaticus (CLas) a bacterium that is transmitted among citrus trees by its insect vector, the Asian citrus psyllid (ACP). HLB has had negative impact on citrus production in the far east for over one hundred years, but not confirmed present in the western hemisphere until 2004 in Brazil. In 2005 HLB was confirmed in Florida and since then has devastated the Florida citrus industry. In addition to Florida, HLB has been confirmed in Texas and California and poses significant threat to those industries. There is no effective cure or therapy for CLas infected trees, following infection the trees begin to decline and eventually become economically unsustainable. Unfortunately, all cultivated citrus types are susceptible to HLB. Because important citrus cultivars have been clonally propagated for centuries, there is essentially no genetic diverstiy, making the search for HLB resistance especially challenging. Although no cultivated citrus types are immune to HLB, there are differences among citrus types in the rate at which they decline following infection by CLas. However, reduced susceptibility to HLB is not sufficient to maintain sustainable production. Within subfamily Aurantioideae and tribe Citreae, there are genera that are sexually compatible with cultivated members of the citrus family. It is possible that hybridization of these citrus relatives with cultivated citrus types may result in hybrids with reduced susceptibility to HLB. Therefore, the objective of this project was to create wide hybrids between standard citrus cultivars and citrus relatives to create novel germplasm with resistance to HLB. Citrus hybridizations were conducted amongst trees in the UC Riverside citrus variety collection. Hybrid seeds were sent to the USDA US Horticultural Laboratory in Fort Pierce, Florida where they were germinated, and resulting seedlings propagated. The propagations from these trees have been exposed to ACP infected with Liberibacter to determine the incidence of infection, subsequent CLas titer and HLB disease development. To date, several hundred plants have been exposed to ACP and tissue sampled for CLas diagnostics. Some evidence suggests that some of the hybrids show lower incidence of infection and are slower to develop HLB than are others. These results show promise for subsequent breeding efforts to decrease susceptibility to HLB.