HLB Resistance and Tolerance in Citrus Scion Breeding

HLB Resistance and Tolerance in Citrus Scion Breeding

Report Date: 07/14/2017
Project: 15-025   Year: 2017
Category: Horticultural & Management
Author: Ed Stover
Sponsor: Citrus Research and Development Foundation

1) Trees have been in the ground for 3.5 years in a trial of 50 selections and cultivars on US-802 following no-choice ACP inoculation and several months in an ACP house. Standard growth measurements and disease ratings were initiated in July 2014 and will continue on a semi-annual basis. HLB is now widespread and the trees looking healthiest include a full sib of our best mandarin selection, and several of our best grapefruit-like hybrids. The one true grapefruit is the least healthy selection in the trial. There are eleven selections with a canopy volume 50% greater than Valencia and 28 with canopy volume >2X that of Flame. The best performers include hybrids containing Poncirus, and conventional hybrids which are predominately mandarin or pummelo. It may take 2-3 more years to clearly distinguish tolerant material. These trees are cropping this year and fruit will be used in a complementary project exploring synthesis of orange-like juice from HLB-tolerant types. 2) In June 2015 a field planting was established of: seedling trees of 133 Fortune x Fairchild hybrids from an earlier mapping study, seedlings of 27 Ponkan-like accessions, budded trees of 10 advanced ARS selections that are predominately mandarin, and budded trees of Fortune, Fairchild and Valencia. Data collection is underway. A NIFA grant is in preparation to map genes associated with tolerance. 3) Replicated trials in multiple locations are established of our best sweet-orange-like cultivars and mandarin-types. Volatiles from sweet-orange-like hybrids are so similar to sweet orange that likely can be legally named sweet orange. 4) RNA-seq compared transcriptome responses in HLB moderately tolerant �Sun Chu Sha� mandarin and susceptible �Duncan� grapefruit, to Xcc-flg22 and CLas-flg22 (most active epitope from the pathogen flagella; project initiated with Gloria Moore at University of Florida). Differential expression of a number of genes occurred between tolerant and susceptible citrus infected with CLas, suggesting their involvement in HLB tolerance. In addition, several genes were similarly regulated by CLas-flg22 and CLas treatments. Genes identified are valuable for studying HLB tolerance mechanisms and potential for screening for HLB-tolerant citrus using CLas-flg22 as a pathogen proxy. Using these genes as markers, expression analysis from a group of mandarin hybrid in their responses to CLas-flg22 is underway. Highly and lowly responsive plants will be marked for long term observation of field tolerance. 5) Seedlings with a range of pedigree contributions from Microcitrus have been received in a collaboration with M. Smith, Queensland Aus. citrus breeder, are being grown, and will be planted soon for field testing of HLB resistance. 6) Evaluation of existing cultivar/rootstock combinations for HLB resistance/tolerance is completed, has revealed potentially valuable tolerance and indicates that early HLB symptoms and earlier CLas titer are unrelated to growth and cropping. In August 2010, the plants were established at Pico�s farm in Ft. Pierce FL. Despite the high incidence of mottle in �SugarBelle�/ SourOrange, it had the greatest overall increase in diameter. ‘SugarBelle’ and ‘Tango’ (which were not on the same stock as ‘Hamlin’ and so results should be viewed as comparing cultivar/rootstock combinations) were the healthiest in overall appearance in 10/15 and had the most fruit (88 per tree). 7) Our putative chimeras have not proven to be successful. We identified a chimera (Satsuma and Poncirus) from the Citrus genebank, arranged its importation, and we finally got permission to accept this material and maintain it in a quarantine death house. Cuttings of the chimera and each separate component (Owari and Poncirus) have been rooted and will be challenged by hot ACP feeding in the next quarters.


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