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United States Department of Agriculture

Agricultural Research Service

Research Project: Screening Citrus Germplasm for Tolerance to HLB and Diaphorina Citri

Location: National Clonal Germplasm Repository for Citrus

Project Number: 2036-21000-010-14-T
Project Type: Trust Fund Cooperative Agreement

Start Date: Oct 1, 2011
End Date: Sep 30, 2014

This is a project to screen a second generation germplasm block in Florida to further pursue identification of huanglongbing (HLB) tolerance identified in the first screening block. In a previous project funded by CRB in 2007, we established a field planting in Florida which contained eight replications of 87 genotypes of citrus and citrus relatives. This planting has been evaluated for field tolerance to Diaphorina citri and citrus leaf miner, and assayed bi-yearly for incidence of HLB using qPCR. Two additional assays for HLB (Fall 2011 and Spring 2012) will be made in this present planting. Data available from the germplasm screening block at Ft. Pierce, the arboretum in Winter Haven, FL and trees in the landscape in South Florida, indicate that Poncirus trifoliata and hybrids with P. trifoliata, sour orange types, and kumquats are generally showing some level of tolerance to HLB. With this proposed screening, we would establish an additional field planting emphasizing P. trifoliata and its hybrids, kumquats, and sour orange genotypes, with seed provided from California. Some of the replicates would be graft challenged with HLB positive plant material prior to planting in the field to determine if apparent tolerance is due to psyllid feeding behavior or other factors genetically inherent in the plant.

While the evaluation of the block for huanglongbing (HLB) will continue for another year, the preliminary findings suggest that P. trifoliata and its hybrids are largely still negative based on testing results for three years. Additionally, most of the sour orange type and kumquat genotypes have tested negative for HLB. We have received an evaluation grant from the USDA ARS National Germplasm Programs to fund for testing the block for HLB in Oct 2011, so additional information will be obtained. Similar observations were made from testing plants in the arboretum in Winter Haven, although the challenge pressure is not as severe in Central Florida as it is in the Ft. Pierce area. Barkley, et al (2006) summarized her research on the genetic diversity and population structure of 370 citrus hybrids maintained at the citrus variety collection, UCR, which she analyzed using SSR markers. Using her data, we have a total of 80 genotypes in the citrus variety collection which have either sour orange, P. trifoliata, or kumquat as part of their population structure. In addition from holdings at the Repository and in the citrus variety collection, we have selected 8 kumquat accessions, 17 sour orange types, and 40 P. trifoliata and P. trifoliata hybrids not included in the 80 genotypes identified from Barkley’s work mentioned earlier. From the targeted list of about 145 genotypes with potential tolerance to HLB, only five have been tested in the first germplasm screening experiment. Seed will be collected, sent to Florida where seedlings will be grown out, half of the seedlings to be planted will be inoculated with HLB before planting so differentiation may be made between free choice by the psyllid and inherent genetic tolerance to HLB from the plant. Replicated, randomized, single tree plots will be planted in the field, and evaluations made twice a year for growth, colonization by psyllids, incidence of citrus canker, and samples collected for testing for presence of Candidatus Liberibacter asiaticus.

Last Modified: 10/16/2017
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