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United States Department of Agriculture

Agricultural Research Service

Research Project: GENETIC IMPROVEMENT OF CITRUS

Location: Subtropical Insects and Horticulture Research

2011 Annual Report


4. Accomplishments
1. Characterized pollination requirements of ‘US Early Pride’ seedless mandarin. ‘US Early Pride’ is a new seedless mandarin released by the USDA in 2009. Although ‘US Early Pride’ consistently produces good yields of seedless fruit in the presence of alternate pollenizers, it was not known if ‘US Early Pride’ requires a pollenizer for fruit production nor if ‘US Early Pride’ can serve as a source of pollen for other mandarins that require cross pollination. We determined that ‘US Early Pride’ pollen is limited in abundance and of low viability compared to other mandarin types. In addition, we found that when bees were prevented from visiting ‘US Early Pride’ flowers, no fruit were produced. Taken together these results indicate that it is likely that ‘US Early Pride’ will require an alternate source of pollen to provide acceptable cropping. This information is of critical importance to citrus growers who are interested in producing ‘US Early Pride’.

2. Determined susceptibility to Huanglongbing pathogen Candidatus Liberibacter asiaticus (CLas). CLas infection among sixteen citrus scion genotypes. To screen citrus germplasm for resistance to CLas, an effective method to quantify differences in pathogen abundance among genotypes is essential. In a greenhouse study, we exposed sixteen citrus genotypes to Asian citrus psyllids that were known to be infected with CLas and followed the incidence of infection over one year. We found that detectable levels of the pathogen did not develop until about five months of exposure to the citrus psyllids. Among the genotypes, we were able to detect consistent differences in abundance of the pathogen. Results of this work will be important in the design of experiments to compare resistance to CLas infection among new citrus germplasm that is being created.

3. Identified breeding sources of resistance and tolerance to Huanglongbing (HLB) pathogen Candidatus Liberibacter asiaticus (CLas). We characterized resistance to CLas and Asian citrus psyllid in diverse genetic material in multiple experiments in the greenhouse and the field. Among extensive material tested, Poncirus trifoliata and some of its hybrids display tolerance to CLas infection, resistance to proliferation of CLas, and/or resistance to colonization by Asian citrus psyllid. Poncirus trifoliata has been utilized extensively in USDA breeding efforts, and it is expected that some advanced hybrid selections approaching commercial fruit quality may already have useful resistance or tolerance to CLas and its insect vector. We have identified genes associated with this resistance, through comparison of gene expression in tolerant hybrid and susceptible selections. These genes are being incorporated into new desirable cultivars through hybridization and transformation to help sustain the U.S. citrus industry in the face of HLB.


Review Publications
Maul, D.P., Mccollum, T.G., Guy, C.L., Porat, R. 2011. Temperature conditioning alters transcript abundance of genes related to chilling stress in 'Marsh' grapefruit flavedo. Postharvest Biology and Technology. 60:177-185.

Castle, W.S., Bowman, K.D., Baldwin, J.C., Grosser, J.W., Gmitter, Jr., F.G. 2011. Rootstocks affect tree growth, yield, and juice quality of 'Marsh' grapefruit. HortScience. 46(6):841-848.

Hert, M.M., Evens, T.J., McCollum, T.G., Niedz, R.P. 2011. Bud emergence and shoot growth from mature citrus nodal stem segments. Plant Cell Tissue And Organ Culture. 106(1):81-91.

Niedz, R.P., Evens, T.J. 2011. The effects of benzyladenine and meta-topolin on in vitro shoot regeneration of sweet orange. ARPN Journal of Agricultural and Biological Science. 6:64-73.

Stover, E.W., McCollum, T.G. 2011. Levels of Candidatus Liberibacter asiaticus and Xanthomonas citri in diverse citrus genotypes and relevance to potential transmission from pollinations. HortScience. 46(6):854-857.

Westbrook, C.J., Hall, D.G., Stover, E., Duan, Y.P., Lee, R.F. 2011. Colonization of citrus and citrus-related germplasm by Diaphorina citri (Hemiptera: Psyllidae). HortScience. 46(7):997-1005.

Stover, E., Portier, K. 2011. Design of field experiments: Influence of treatment response relative to standard deviation and blocking factor characteristics on efficient blocking strategy. Journal of American Pomological Society. 65:2-16.

Last Modified: 08/16/2017
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