Temporal Dynamics of Candidatus Liberibacter asiaticus Titer in Mature Leaves from Citrus sinensis cv Valencia Are Associated with Vegetative Growth

Phloem and Xylem Responses Are Both Implicated in Huanglongbing Tolerance of Sugar Belle

Although huanglongbing (HLB) is a devastating citrus disease, improved tolerant cultivars, such as Sugar Belle (SB) mandarin, have been identified. To understand the responses that HLB-affected SB undergoes, we compared 14CO2 fixation, carbohydrate export, phloem callose accumulation, relative expression of plant defense activators, and anatomical changes between healthy and infected SB trees versus susceptible Pineapple (PA) sweet orange. Eight- to ten-week-old leaves of infected SB showed a 2.5-fold increase in 14CO2 fixation and a 13% decrease in 14C-carbohydrate export, whereas HLB-affected PA presented a decrease of 33 and 50%, respectively. The mean distance of a callose deposit to its closest neighbor was 36% smaller in infected SB versus healthy, whereas in HLB-affected PA, it was 33% higher. Expression of papain-like cysteine proteases (PLCPs) was upregulated in SB but downregulated in PA. Infected SB showed minor alterations in the number of xylem vessels, a 16% larger xylem vessel lumen area, and a 14% increase in the proportional area of the xylem. In contrast, PA showed a 2.4-fold increase in the xylem vessel number and a 2% increase in the proportional xylem area. Three complementary mechanisms of tolerance in SB are hypothesized: (i) increased carbohydrate availability induced by greater CO2 fixation, mild effect in carbohydrate export, and local accumulation of callose in the phloem; (ii) activation of defense response via upregulation of PLCPs, and (iii) increased investment in the xylem structure. Thus, phloem and xylem modifications seem to be involved in SB tolerance.

Foliar Antibiotic Treatment Reduces Candidatus Liberibacter asiaticus Acquisition by the Asian Citrus Psyllid, Diaphorina citri (Hemiptera: Liviidae), but Does not Reduce Tree Infection Rate

Huanglongbing (HLB), or citrus greening, is the most destructive disease of cultivated citrus worldwide. Candidatus Liberibacter asiaticus (CLas), the putative causal agent of HLB, is transmitted by the Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Liviidae). In Florida, D. citri was first reported in 1998, and CLas was confirmed in 2005. Management of HLB relies on the use of insecticides to reduce vector populations. In 2016, antibiotics were approved to manage CLas infection in citrus. Diaphorina citri is host to several bacterial endosymbionts and reducing endosymbiont abundance is known to cause a corresponding reduction in host fitness. We hypothesized that applications of oxytetracycline and streptomycin would reduce: CLas populations in young and mature citrus trees, CLas acquisition by D. citri, and D. citri abundance. Our results indicate that treatment of citrus with oxytetracycline and streptomycin reduced acquisition of CLas by D. citri adults and emerging F1 nymphs as compared with that observed in trees treated only with insecticides, but not with antibiotics. However, under field conditions, neither antibiotic treatment frequency tested affected CLas infection of young or mature trees as compared with insecticide treatment alone (negative control); whereas trees enveloped with mesh screening that excluded vectors did prevent bacterial infection (positive control). Populations of D. citri were not consistently affected by antibiotic treatment under field conditions, as compared with an insecticide only comparison. Collectively, our results suggest that while foliar application of oxytetracycline and streptomycin to citrus reduces acquisition of CLas bacteria by the vector, even high frequency applications of these formulations under field conditions do not prevent or reduce tree infection.

Comparisons of economic thresholds for Asian citrus psyllid management suggest a revised approach to reduce management costs and improve yield

Huanglongbing (HLB) or citrus greening disease is transmitted by the Asian citrus psyllid, Diaphorina citri Kuwayama. Vector control is considered a basic component of HLB management even under high disease incidence scenarios. While vector management heavily relies on the application of synthetic chemical sprays, overuse of insecticides raises several concerns including insecticide resistance, environmental impacts, and secondary pest outbreaks. The present study aims to compare the effects of three different economic thresholds (ET-0.2, 0.5, 1.0) and one calendar-based application schedule on the incidence of D. citri and beneficial species in plots of commercially grown citrus, as well as end-of-season yield and overall management costs. The results suggest that reducing spray frequency from eight to as few as three sprays per year had little effect on counts of pest and beneficial insects in the field. The numbers of D. citri and that of a secondary weevil pest were similar between plots treated with the calendar-based spray plots and plots managed with the ET-1.0. Furthermore, spider numbers were higher in the ET-1.0 plots, while ant numbers were lower compared with calendar sprayed plots. Management input costs were lower under economic thresholds (ET-0.5–ET-1.0) than with monthly calendar-based sprays, while yield losses were only slightly greater in the lower threshold of 0.2 mean psyllids per tap than with calendar sprays. Overall, management savings of more than 100% made up for this difference. Together, these results suggest that implementing a spray program of rotated chemistries based on an economic threshold of 0.5–1.0 adult psyllids per stem tap could provide both economic and ecological benefits. We discuss the implications of such an approach in the context of a young citrus tree protection program and the greater goals of sustainable citrus production under HLB.

Citrus photosynthesis and morphology acclimate to phloem-affecting huanglongbing disease at the leaf and shoot levels

Huanglongbing (HLB) is a phloem-affecting disease in citrus that reduces growth and impacts global citrus production. HLB is caused by a phloem-limited bacterium (Candidatus Liberibacter asiaticus). By inhibiting phloem function, HLB stunts sink growth, including the production of new shoots and leaves, and induces hyperaccumulation of foliar starch. HLB induces feedback inhibition of photosynthesis by reducing foliar carbohydrate export. Here, we assessed the relationship of bacterial distribution within the foliage, foliar starch accumulation, and net CO₂ assimilation (A_net ). Because HLB impacts canopy morphology, we developed a chamber to measure whole-shoot A_net to test the effects of HLB at both the leaf and shoot level. Whole-shoot level A_net saturated at high irradiance, and green stems had high photosynthetic rates compared to leaves. Starch accumulation was correlated with bacterial population, and starch was negatively correlated with A_net at the leaf but not at the shoot level. Starch increased initially after infection, then decreased progressively with increasing length of infection. HLB infection reduced A_net at the leaf level but increased it at the whole-shoot level, in association with reduced leaf size and greater relative contribution of stems to the photosynthetic surface area. Although HLB-increased photosynthetic efficiency, total carbon fixed per shoot decreased because photosynthetic surface area was reduced. We conclude that the localized effects of infection on photosynthesis are mitigated by whole-shoot morphological acclimation over time. Stems contribute important proportions of whole-shoot A_net , and these contributions are likely increased by the morphological acclimation induced by HLB.

Salicylic acid mediated immune response of Citrus sinensis to varying frequencies of herbivory and pathogen inoculation

BACKGROUND

Plant immunity against pathogens and pests is comprised of complex mechanisms orchestrated by signaling pathways regulated by plant hormones [Salicylic acid (SA) and Jasmonic acid (JA)]. Investigations of plant immune response to phytopathogens and phloem-feeders have revealed that SA plays a critical role in reprogramming of the activity and/or localization of transcriptional regulators via post-translational modifications. We explored the contributing effects of herbivory by a phytopathogen vector [Asian citrus psyllid, Diaphorina citri] and pathogen [Candidatus Liberibacter asiaticus (CaLas)] infection on response of sweet orange [Citrus sinensis (L.) Osbeck] using manipulative treatments designed to mimic the types of infestations/infections that citrus growers experience when cultivating citrus in the face of Huanglongbing (HLB) disease.

RESULTS

A one-time (7 days) inoculation access period with CaLas-infected vectors caused SA-associated upregulation of PR-1, stimulating defense response after a long period of infection without herbivory (270 and 360 days). In contrast, while repeated (monthly) 'pulses' of 7 day feeding injury by psyllids stimulated immunity in CaLas-infected citrus by increasing SA in leaves initially (up to 120 days), long-term (270 and 360 days) repeated herbivory caused SA to decrease coincident with upregulation of genes associated with SA metabolism (BMST and DMR6). Similarly, transcriptional responses and metabolite (SA and its analytes) accumulation in citrus leaves exposed to a continuously reproducing population of D. citri exhibited a transitory upregulation of genes associated with SA signaling at 120 days and a posterior downregulation after long-term psyllid (adults and nymphs) feeding (270 and 360 days).

CONCLUSIONS

Herbivory played an important role in regulation of SA accumulation in mature leaves of C. sinensis, whether or not those trees were coincidentally infected with CaLas. Our results indicate that prevention of feeding injury inflicted by D. citri from the tritrophic interaction may allow citrus plants to better cope with the consequences of CaLas infection, highlighting the importance of vector suppression as a component of managing this cosmopolitan disease.

The Total Population Size of 'Candidatus Liberibacter asiaticus' Inside the Phloem of Citrus Trees and the Corresponding Metabolic Burden Related to Huanglongbing Disease Development

'Candidatus Liberibacter asiaticus' (CLas) is the predominant causal agent of citrus huanglongbing, the most devastating citrus disease worldwide. CLas colonizes phloem tissue and causes phloem dysfunction. The pathogen population size in local tissues and in the whole plant is critical for the development of disease symptoms by determining the load of pathogenicity factors and metabolic burden to the host. However, the total population size of CLas in a whole plant and the ratio of CLas to citrus cells in local tissues have not been addressed previously. The total CLas population size for 2.5-year-old 'Valencia' sweet orange on 'Kuharske' citrange rootstock trees was quantified using quantitative PCR to be approximately 1.74 × 10⁹ cells/tree, whereas 7- and 20-year-old sweet orange trees were estimated to be 4.3 × 10¹⁰ cells/tree, and 6.0 × 10¹⁰ cells/tree, respectively. The majority of CLas cells were distributed in leaf tissues (55.58%), followed by those in branch (36.78%), feeder root (4.75%), trunk (2.39%), and structural root (0.51%) tissues. The ratios of citrus cells to CLas cells for branch, leaf, trunk, feeder root, and structural root samples were within approximately 39 to 79, 44 to 124, 153 to 1,355, 191 to 1,054, and 561 to 3,760, respectively, representing the metabolic burden of CLas in different organs. It was estimated that the ratios of phloem cells to CLas cells for branch, leaf, trunk, feeder root, and structural root samples are approximately 0.39 to 0.79, 0.44 to 1.24, 1.53 to 13.55, 1.91 to 10.54, and 5.61 to 37.60, respectively. Approximately 0.01% of the total citrus phloem volume was estimated to be occupied by CLas, explaining the difficulty to observe CLas in most tissues under transmission electron microscopy. The CLas titer inside the leaf was estimated to be approximately 1.64 × 10⁶ cells/leaf or 9.2 × 10⁴ cells cm^-2 in leaves, approximately 10⁴ times less than that of typical apoplastic bacterial pathogens. This study provides quantitative estimates of phloem colonization by bacterial pathogens and furthers the understanding of the biology and virulence mechanisms of CLas.

A Multimodal Attract-and-Kill Device for the Asian Citrus Psyllid Diaphorina citri (Hemiptera: Liviidae)

Simple Summary

Control of Asian citrus psyllid (Diaphorina citri), a vector of Candidatus liberibacter asiaticus (CLas), contributes to management of citrus greening disease (huanglongbing). We developed two prototypes of a multimodal attract-and-kill (AK) device with specific elements of color, attractant, phagostimulant, ultraviolet (UV) reflectant, and toxicant. Key sensory stimuli comprising the AK ingredients were identified in our current and previous research studies and incorporated into a yellow, slow-release wax matrix (SPLAT). This formulation was applied directly to the surface of yellow cylinders, or to corrugated plastic cards housed within perforated cylinders. Psyllids landing on the devices attempted to feed from the wax matrix, became intoxicated, died, and fell from device surfaces. Our laboratory and field experiments showed that AK devices attracted and killed significantly more adult D. citri than ordinary yellow sticky cards and remained fully active over a period of 12 weeks. Effective use of attract-and-kill for management of D. citri could reduce need for broad-spectrum insecticide sprays and encourage biological control as part of an integrated approach to huanglongbing (HLB) management in citrus.

Abstract

Phytophagous insects, including Asian citrus psyllids (Diaphorina citri Kuwayama), use multiple sensory modalities (vision, olfaction, and gustation,) to locate and accept host plants. We explored incorporation of several sensory cues into a multi-modal attract-and-kill device (AK device) using a three-dimensional shape to increase visibility, as well as elements of color, attractant, phagostimulant, UV reflectant, and toxicant. Attraction of adult D. citri to the device was mediated by a combination of a highly reflective yellow cylinder, a UV reflectant compound (magnesium oxide), and an odorant blend as a short-range attractant. The device surface was coated with a slow-release wax matrix (SPLAT™) augmented with a phagostimulant consisting of a 3-component blend (formic acid, acetic acid, and para-cymene) and an insecticide (β-cyfluthrin). Psyllids landing on the device attempted to feed from the wax matrix, became intoxicated, died, and fell from the device. The device remained fully active over a period of 12 weeks partly because dead psyllids or nontargets did not adhere to the surface as occurs on adhesive yellow sticky cards, the industry standard. Laboratory and field assays showed that the device attracted and killed significantly more adult D. citri than ordinary yellow sticky cards. This device or a future iteration based on the design elements of this device is expected to contribute to sustainable and environmentally appropriate management of D. citri by exploiting the psyllid’s innate behavioral responses to visual, olfactory, and gustatory stimuli.