Influence of Citrus Source and Test Genotypes on Inoculations with Candidatus Liberibacter asiaticus

Abstract

<jats:p>Assessments of the resistance of citrus germplasm to huanglongbing (HLB) can be expedited by inoculating plants under laboratory or greenhouse settings with the HLB bacterium, <jats:italic>Candidatus</jats:italic> 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 bud grafted 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. Trunk cross-sectional area increase for the year following infection was 3× higher in ‘Kuharske’ and rough lemon, compared with ‘Valencia’. ‘Kuharske’ had very low levels of CLas (30 CLas/µg DNA), whereas ‘Valencia’ (43,000 CLas/µg DNA) and rough lemon (6700 CLas/ 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 are greater when the ACP have been feeding on the same host genotype. 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% 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. Because of the rapidity of the detached leaf assay, efforts are merited to improve consistency of this inoculation method.</jats:p>

Incidence of “Candidatus Liberibacter asiaticus” in a Florida population of Asian citrus psyllid

“…Methods reported in the literature for assessing incidence of “ Ca . L. asiaticus” in D. citri and plants vary but most over the past 10–15 years have involved standard, real‐time or nested PCR assays with various procedures including different PCR primer targets (e.g., Coy et al., ; Li et al., ; Morgan et al., ; Stover, Hall, Shatters, & Moore, ; Ukuda‐Hosokawa et al., ). Some PCR procedures may be less sensitive for “ Ca .…”

“…Pelz-Stelinski, Brlansky, Ebert, and Rogers (2010) PCR primer targets (e.g., Coy et al, 2014;Li et al, 2006;Morgan et al, 2012;Stover, Hall, Shatters, & Moore, 2016;Ukuda-Hosokawa et al, 2015). Some PCR procedures may be less sensitive for "Ca.…”

Construction of high-density genetic maps and detection of QTLs associated with Huanglongbing infection in citrus

“…Moreover, movement of C Las occurs slowly from an inoculated position to other branches and finally systemically throughout the whole tree (Albrecht et al , 2014). Other reports where trees were inoculated only at a single time point documented great variability in C Las titer over time, with negligible differences between sweet orange and Carrizo (an HLB-tolerant or somewhat-resistant citrange) titers when C Las is initially inoculated into leaves, and a tendency for overall decline from initial titer within Carrizo (Stover et al , 2016a; Stover et al , 2016b). These observations suggest that C Las titer would be quite dynamic in a field planting where psyllid inoculation is ongoing and sporadic, especially in trees where high titers do not develop.…”

Influence of Photoperiod Duration and Phloem Disruption through Scoring on Growth, Disease Symptoms, and Bacterial Titer in Citrus Graft Inoculated with Candidatus Liberibacter asiaticus

“…Trees of rough lemon (Citrus jambhiri, nucellar seedlings) and citron (Citus medica, cultivar Mhac Nao from rooted cuttings) were graft inoculated on 20 Oct. 2010 with CLas using infected 'Valencia' (Citrus sinensis) budwood at three buds per tree. These rough lemon and citron trees developed distinctive HLB mottling and high CLas titer and provided materials for graft inoculations for the experiment reported here (Stover et al, 2016a). Sixty trees each of 'Valencia' (budded onto rough lemon nucellar seedlings), nucellar seedlings of rough lemon, and nucellar seedlings of 'Kuharske' (C. sinensis • P. trifoliata) were graft inoculated on 26 Mar.…”