ARS | Publication request: Levels of Candidatus Liberibacter asiaticus and Xanthomonas citri in diverse Citrus genotypes and relevance to potential transmission from pollinations

Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 19, 2011
Publication Date: June 20, 2011
Citation: 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.

Interpretive Summary: The diseases Huanglongbing (HLB) and Asian citrus canker (ACC) are widespread in Florida and many other citrus-growing areas, presenting new challenges for citrus breeding. Since HLB and ACC weaken trees, breeding is less efficient using mother trees which have been exposed to these diseases. Therefore it may be best to use unique disease-exposed trees only as male (pollen-source) parents, with pollen applied to disease-free trees. However, there may be a risk of introducing these diseases using such pollen sources. To assess this, levels of bacteria which cause these diseases were assessed in anthers and flowers by measuring their distinctive DNA. Since the HLB bacterium moves throughout the tree, bacterial levels on mature leaves from the flower source trees were assessed to see if presence of HLB bacterium in flowers was associated with leaf levels. Disease exposed trees were tested in ten different types from each of three broad citrus groups, with different levels of susceptibility to the diseases studied: trifoliate orange hybrids (most resistant to HLB), pummelo and hybrids (susceptible to both diseases), and mandarin orange and hybrids (considerable resistance to ACC). Of the 90 samples of each tissue type analyzed for the HLB bacterium, 88% of mature leaves, 69% of flowers, and 88% of anthers had =1 HLB bacterium per sample. The trifoliate group had significantly lower levels of HLB bacterium than the pummelo and mandarin groups in mature leaf samples, but bacterial levels were more similar between groups in anther and flower samples. Mean numbers of HLB bacteria per sample were 100-800 X higher in mature leaf samples, compared to anther samples. ACC bacterial DNA was detected in 30% of flower samples, and 23% of anther samples. No significant differences in ACC bacterium levels were found between tissue types or broad citrus groups. The biology of HLB is consistent with the bacterium being in anthers from unopened flowers, while the ACC bacterium detected inside flowers was likely due to contamination, despite great care in sample collection and handling. These results suggest that it may be possible to accidentally introduce these diseases through use of pollen from trees on infected farms.

Technical Abstract: The diseases huanglongbing (HLB, associated with Candidatus Liberibacter asiaticus, CLas) and Asian citrus canker (ACC, caused by Xanthomonas citri, Xcc) are widespread in Florida and many other citrus-growing areas, presenting unprecedented challenges for citrus breeding. Since HLB and ACC weaken trees and compromise cropping, breeding is much less efficient using seed parents which have been exposed to these diseases. Therefore it would be highly desirable to use unique disease-exposed selections only as pollen parents, with pollen applied to disease-free trees. However, there may be a risk of introducing these diseases using such pollen sources. To assess this potential, abundance of the pathogens associated with these diseases was assessed in anthers and flowers using qPCR. Since CLas is systemic, levels on mature leaves from the flower source trees were assessed to see if presence of CLas in flowers was associated with leaf levels. Disease exposed trees were tested in ten genotypes from each of three broad genotypic categories, which reflect different levels of susceptibility to the diseases associated with the pathogens studied: Poncirus trifoliata hybrids (most resistant to HLB), Citrus maxima and hybrids (susceptible to both diseases), and C. reticulata and hybrids (considerable resistance to ACC). Of the 90 samples of each tissue type analyzed for CLas, 88% of mature leaves, 69% of flowers, and 88% of anthers had =1 CLas bacterium per sample. The trifoliate genotypic group had significantly lower levels of CLas than the pummelo and mandarin groups in mature leaf samples, but CLas levels were more similar between groups in anther and flower samples, and the pathogen was present in most of the trifoliate hybrids tested. Mean numbers of CLas detected per ng nucleic acid were 100-800 X higher in mature leaf samples, most characteristic of HLB symptoms, compared to anther samples. Xcc DNA was detected in 30% of flower samples, and 23% of anther samples. No significant differences in Xcc levels were found between tissue type or genotypic group. However, regressions between Xcc levels in flowers and % of plant pedigree derived from mandarin had a negative correlation and an r2 of 0.159 (p=0.029). The biology of CLas is consistent with the pathogen being present in anthers from unopened flowers, while the ACC pathogen detected inside flowers was likely due to contamination, despite great care in sample collection and handling. Where exceptional diligence to exclude HLB and ACC is appropriate, results suggest that there may be a risk of spreading these pathogens through use of pollen from trees on infected farms.