First steps towards rescuing Las-infected citrus germplasm

Authors: McCollum, Thomas; Stover, Eddie

Submitted to: International Research Conference on Huanglongbing
Publication Type: Proceedings
Publication Acceptance Date: 1/10/2011
Publication Date: 3/28/2011
Citation: McCollum, T.G., Stover, E.W. 2013. First steps towards rescuing Las-infected citrus germplasm. International Research Conference on Huanglongbing. In: Proceedings of the Internatinal Research Conference on Huanglongbing, January 11-14, 2011, Orlando, Florida. p. 5.

Interpretive Summary: Huanglongbing (HLB) disease is having significant impact on the USDA citrus breeding program as it has shown up in a number of trees which exist only in a virtually irreplaceable germplasm collection. It is critical that we rescue Las - budwood from elite germplasm that is Las+. We reasoned that by selecting budwood that tests Las-, albeit from Las+ trees, we would produce some propagations free of Las. At least 3, and as many as 6, branches from each of seven trees were tested for Las using standard qPCR methods. A total of 90 propagations (3 from each branch) were produced. Initially, 63% of the branches were Las+. Of all the propagations, 89% survived, with no apparent difference in survival between propagations made from Las+ or Las- branches. Among all propagations, 29% were Las+. Among propagations made from branches that were Las+, 55% (18/47) tested Las+ whereas among the propagations made from Las- branches, 12% (3/25) were Las+, with two of these propagations originating from the same original branch. Average Ct value for Las+ propagations from Las+ branches was 27.9 compared to 36.0 for the Las+ propagations from Las- branches. So far, our data support the notion that testing for Las prior to propagation is an important first step in the process of rescuing Las- tissue from Las+ trees. Propagated trees will continue to be monitored for the appearance of HLB symptoms and further development of Las as determined by qPCR, to determine whether uneven Las distribution and selected propagation from Las+ trees will permit rescue of critical germplasm.

Technical Abstract: Huanglongbing (HLB) disease is having significant impact on the USDA citrus breeding program as it has shown up in a number of trees which exist only in a virtually irreplaceable collection of diverse citrus types. It is critical that we rescue HLB-free budwood from elite germplasm that is HLB-pathogen-positive. We reasoned that by selecting budwood that tests free of HLB-pathogen, albeit from HLB-pathogen-positive trees, we would produce some propagations free of HLB-pathogen. At least 3, and as many as 6, branches from each of seven trees were tested for HLB-pathogen using standard lab methods. A total of 90 propagations (3 from each branch) were produced. Initially, 63% of the branches were HLB-pathogen-positive. Of all the propagations, 89% survived, with no apparent difference in survival between propagations made from HLB-pathogen-positive or HLB-free branches. Among all propagations, 29% were HLB-pathogen-positive. Among propagations made from branches that were HLB-pathogen-positive, 55% (18/47) tested HLB-pathogen-positive whereas among the propagations made from HLB-pathogen- branches, 12% (3/25) were HLB-pathogen-positive, with two of these propagations originating from the same original branch. So far, our data support the notion that testing for HLB-pathogen prior to propagation is an important first step in the process of rescuing HLB-free tissue from HLB-pathogen-positive trees. Propagated trees will continue to be monitored for the appearance of HLB symptoms and further development of HLB-pathogen as determined by lab tests, to determine whether uneven HLB-pathogen distribution and selected propagation from HLB-pathogen-positive trees will permit rescue of critical germplasm.