Skip to main content
ARS Home » Research » Publications at this Location » Publication #325466
Title: Influence of citrus source and test genotypes on inoculations with Candidatus Liberibacter asiaticus

Author
item Stover, Eddie
item Hall, David
item Shatters, Robert - Bob
item MOORE, GLORIA - University Of Florida

Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/17/2016
Publication Date: 7/1/2016
Citation: Stover, E.W., Hall, D.G., Shatters, R.G., Moore, G.A. 2016. Influence of citrus source and test genotypes on inoculations with Candidatus Liberibacter asiaticus. HortScience. 51:805-809.

Interpretive Summary: Resistance to the citrus disease huanglongbing (HLB) can be assessed by inoculating plants under laboratory or greenhouse settings with the Huanglongbing bacterium (Candidatus Liberibacter asiaticus (CLas)). Two experiments were conducted to assess factors that might influence efficiency in screening for HLB resistance, one in which different citrus genotypes were inoculated using CLas-infected budwood from different source genotypes, and one in which different citrus genotypes were inoculated using CLas-infected Asian citrus psyllids (ACP) from a colony on CLas-infected lemon. The results of the first experiment indicated that the source of CLas-infected buds used to graft-inoculate some genotypes will influence disease development. Results of the second experiment indicated that inoculations by ACP from CLas-infected lemon resulted in similar disease development among different citrus types. Inoculations using CLas-infected ACP may therefore provide more consistent results among different genotypes than inoculations using CLas-infected budwood.

Technical Abstract: Assessments of the resistance of citrus germplasm to huanglongbing (HLB) can be expedited by inoculating plants under laboratory or greenhouse settings with the Huanglonbing bacterium, Candidatus Liberibacter asiaticus (CLas). Consistent rapid screening is critical to efficiently assess disease resistance among plant materials; however, a number of factors may govern the efficacy of such inoculations. Despite the rapidity at which HLB can spread in a grove, it often takes 8 to 10 months for high levels of CLas and HLB symptoms to develop even in highly susceptible sweet orange. Therefore, two experiments were conducted to assess factors that might influence efficiency in screening for HLB resistance. In one experiment, three test citrus genotypes (‘Kuharske’-previously shown to be HLB-resistant, rough lemon-previously shown to be HLB tolerant and ‘Valencia’-HLB susceptible) were graft-inoculated using CLas-infected buds from four different source genotypes. All bud source genotypes had similar levels of CLas titer, but citron, rough lemon, and Volkamer lemon were hypothesized to be better bud inoculum sources as they are more tolerant of HLB than ‘Valencia’. Among the three test genotypes over all sources of infected buds, inoculations of ‘Kuharske’ resulted in lower CLas titers and fewer HLB symptoms than inoculations of rough lemon or ‘Valencia’. Inoculations of rough lemon resulted in higher CLas titers and more pronounced HLB symptoms when it was inoculated using infected buds from rough lemon or ‘Valencia’. Grafting ‘Valencia’ with infected buds from Volkamer lemon resulted in less disease than when ‘Valencia’ was grafted with infected citron, rough lemon or ‘Valencia’ buds. Overall these results suggest that the source of CLas-infected buds used to graft-inoculate some genotypes will influence disease development. This suggests that different genotypes might vary in their influence on systemic movement of CLas. Trunk cross sectional area increase for the year following infection was 3X higher in ‘Kuharske’ and rough lemon, compared to ‘Valencia’. ‘Kuharske’ had very low levels of CLas (30 / µg DNA) while ‘Valencia’ (43,000 / µg DNA) and rough lemon (6700 / µg DNA) had relatively high levels. As an alternative to graft-inoculating plants with CLas-infected buds, plants can be subjected to infestations of CLas-infected Asian citrus psyllid (ACP) as occurs naturally. Of interest is if transmission rates of CLas and the development of HLB in a genotype is influenced by the source plant genotype of infected ACP. An experiment was therefore conducted to assess transmission of CLas by ACP reared on CLas-infected rough lemon to five different genotypes (‘Carrizo’, ‘Flame’ grapefruit, rough lemon, ‘Temple’ and ‘Valencia’). These assessments were made using a detached leaf assay recognized as a faster method of gauging transmission rates of CLas than using whole plants. Higher percentages of ACP died when they were transferred from infected rough lemon to healthy ‘Carrizo’, and lower percentages died when they were transferred to rough lemon or ‘Flame’. However, CLas transmission by infected ACP occurred to at least some leaves of each genotype in each of the five different assays, with 70 percent or more leaves of each genotype becoming infected in at least one assay. Over all assays, there was relatively little variation among genotypes in the percentage of leaves becoming CLas-infected and in the titer of CLas developing in infected leaves. However, there were relatively large differences in transmission rates among individual assays unrelated to differences among test genotypes. Due to the rapidity of the detached leaf assay, efforts are merited to improve consistency of this inoculation method.