Agronomy

The mitochondrial genomes of two vector psyllids of the ‘Candidatus Phytoplasma mali’, Cacopsylla picta and C. melanoneura, were sequenced using high-throughput sequencing on the Illumina platform. The main objective of the study was to describe their mitogenome and characterize their genetic variability and the potential changes in the context of the observed global warming. The four complete sequences for C. picta, 14,801 bp and 14,802 bp in length, two complete and one partial sequence for C. melanoneura, ranging from 14,879 bp to 14,881 bp in length, were obtained for the first time for these European apple psyllids. The detected intraspecies mtDNA identity was highly similar (99.85–99.98%), the identity’s similarity with other Cacopsylla species varied between 79.79 and 86.64%. The mitogenomes showed a typical mitochondrial DNA structure with 13 protein-coding genes, 2 rRNA genes and 22 tRNA genes; the presence of CGGA motif in the ND1-trnS2 junction was detected in both species. Phylogenetic analysis placed both species in close relationship with C. burckhardti within the Cacopsylla clade-I O group. The analysis of complete mitogenomes and of partial COI sequences of fifty-two Cacopsylla individuals showed a high homogeneity of genotypes over 15 years and among the different localities in the Czech Republic.

‘Candidatus Liberibacter solanacearum’ (Lso) is a phloem-limited bacterial plant pathogen infecting solanaceous plants in the Americas and New Zealand and is associated with diseases of apiaceous crops in Europe, Northern Africa, and the Middle East. This pathogen is also related to other Liberibacter species that infect other crops. In the USA, two haplotypes of Lso, LsoA and LsoB, are predominant and responsible for diseases in potato and tomato. Tobacco, Nicotiana tabacum, a model species to study plant defenses, is a host for Lso; therefore, the interaction between Lso and this host plant could be used to study Liberibacter−plant interactions. In this study, we characterized the infection associated with LsoA and LsoB in tobacco. Under laboratory conditions, LsoB caused more severe symptoms than LsoA, and LsoA and LsoB titers were dynamic during the 7 weeks of the experiment. We also measured SA and other metabolites, including oxylipins, at an early point of infection and found that SA was accumulated in plants infected with LsoB but not with LsoA; whereas ABA levels were reduced in LsoA- but not in LsoB-infected plants.

‘Candidatus Phytoplasma prunorum’ is causing ever increasing economic losses through the decline of apricot trees in European countries, e.g., Hungary. In this study, the pathogen was identified from plant tissues and insects by nested-PCR. The insect species were identified via morphology and molecular methods. The incidence of the pathogen was 29.6% in randomly selected apricot trees. Most of the infected trees with symptoms died within a year. These results show that phytoplasma is significantly present and causes damage in the investigated plantations. The only known insect vector of this phytoplasma is the plum psyllid, Cacopsylla pruni, which was regularly encountered in the sampled apricot orchards and in their surroundings. In a two-year study, several adults among the sampled specimens were observed to be infected by the pathogen. This observation further confirms the role of the plum psyllid in vectoring the phytoplasma. All the sampled plum psyllid adults belonged to the ‘B’ biotype. Besides C. pruni, Cacopsylla crataegi was abundant in the samples. Several adults of the latter species were also infected by the pathogen ‘Ca. Phytoplasma prunorum’. The rates of occurrence of this phytoplasma in male and female adults of the two psyllid species appeared to be similar. The examined C. crataegi individuals showed genetic differences from each other and from specimens included in a previous investigation.

Huanglongbing (HLB) is an important citrus disease associated with the phloem-limited, uncultured bacterium ‘Candidatus Liberibacter asiaticus’(CLas). Effective treatments against CLas have to be validated in the field, however, methods for the field assessment of treatment effectiveness are time-consuming, in part because DNA-based assays, including quantitative PCR (qPCR), cannot differentiate between live and dead bacterial DNA. The aim of this study was to develop a method for rapid the evaluation of HLB therapies in field experiments. To this aim, a DNA extraction method from citrus leaf tissues with propidum monoazide (PMA), a dye that binds covalently to dsDNA making it unavailable for amplification in subsequent qPCR reactions, was optimized. The results indicated that the efficacy of PMA-qPCR was highly dependent on the primer set used. Primers targeting the 16S region of CLas showed a clear distinction between qPCR from PMA-treated and non-treated samples, while the RNR and LJ900 primers did not show significant differences between the DNA extraction methods. The PMA-qPCR viability analysis of CLas from citrus cuttings treated with different ampicillin (Amp) concentrations showed that all concentrations reduced CLas titers significantly starting 4 days after the initial treatment, unlike the water treatment, which did not show any change. This method was used for assessing the antibacterial activity of Amp, Streptomycin, Oxytetracycline (OTC), and a water control in field tests. The PMA-qPCR results indicated that Amp and OTC displayed significant antibacterial activity against CLas by 8 days post-injection, which was not detected in the non-PMA qPCR analysis. This method could allow the rapid validation of treatments against CLas in field experiments and facilitate the implementation of effective management strategies against HLB.

Background: Huanglongbing (HLB) is currently one of the most devasting diseases in citrus plants worldwide. Resistance against its causal agent, Candidatus Liberibacter asiaticus (CLas), in commercial Citrus species remains a challenge, even though they show differences in CLas multiplication. Methods: A total of 14 citrandarins and their parents (Sunki mandarin and Poncirus trifoliata cv. Rubidoux) were top-grafted onto the canopy of potted ‘Valencia’ sweet orange plants with high CLas titers. The grafted genotypes were tested for CLas infection and physiological effects of the disease (starch accumulation and callose deposition) for 12 months. Results: All tested genotypes were infected by CLas during the time frame of the experiment. However, a decrease in the infection rate ranging from 50% to 80% for the hybrids H109, H126, H157, and H222 was observed 360 days from the top-grafting. CLas was undetected by real-time PCR in H106 at the end of the experiment, which had low levels of starch and callose deposition. Conclusions: CLas infected all of the tested citrandarins, but a decrease in the infection rate over time was detected for some specific genotypes, which led to less starch accumulation and callose deposition.

Diaphorina citri is a serious insect pest of citrus and an insect vector of Candidatus Liberibacter asiaticus (CLas) that causes Huanglongbing disease (citrus greening). In this study, we investigated the effect of the CLas pathogen on the life history parameters of D. citri at different temperature regimes. Our results demonstrated that the survival rate of first to fifth instar CLas-positive and CLas-negative D. citri fluctuate with the change in temperature over the range of 16–35 °C. Meanwhile, the mean developmental time (52.5 d) (d = day(s)) and adult longevity (5.2 d) of the CLas-positive psyllids was longer as compared to CLas-negative psyllids mean developmental time (32.81 d) and adult longevity (3.50 d) at the low- and high-temperature regimes (16 and 35 °C). However, at high temperature regimes, the significant effect of CLas-bacteria on D. citri fecundity was higher than the corresponding non-significant effect on their survivorship when compared to non-vectored psyllids. These results indicate a long-term, stable evolutionary relationship among vector-pathogen and climate change.

Huanglongbing (HLB) is a devastating disease affecting citrus production worldwide. In China, the disease is associated with an unculturable alpha-proteobacterium, “Candidatus Liberibacter asiaticus” (CLas). Phages/prophages of CLas have recently been identified through intensive genomic research. The phage information has facilitated research on CLas biology such as population diversity and virulence gene identification. However, little is known about the roles of CLas phages in HLB symptom development. Such research is challenging due to the unculturable nature of CLas and the lack of laboratory strains that carry a single phage. In this study, CLas strains singly carrying Type 1 phage (Type 1 CLas) and Type 2 phage (Type 2 CLas) were identified and maintained in an experimental screenhouse in southern China. The strains were characterized through next-generation sequencing (NGS). Then, each CLas strain was inoculated into seedlings of three different citrus cultivars/species through graft transmission in a screenhouse in Guangdong, China. Symptom developments were recorded. All CLas-infected cultivars showed HLB symptoms in seven months. In cultivar Nianju (Citrus reticulata), Type 1 CLas caused pronounced yellowing symptoms and severe defoliation, whereas Type 2 CLas caused typical Zn-deficiency-like symptoms. In contrast, symptoms from the two CLas strains’ infections on cultivars Shatianyu (C. maxima), and Eureka lemon (Citrus limon) were more difficult to differentiate. Results from this study provide baseline information for future research to investigate the roles of CLas phages in HLB symptom development.

Health, growth, fruit production, and fruit quality of citrus crops are severely affected by tree infection with Candidatus Liberibacter asiaticus (CLas) and subsequent development of the disease huanglongbing (HLB). The use of HLB-tolerant rootstocks is one strategy that is used to ameliorate the effects of HLB in commercial production. Although there is a clear long-term rootstock effect to improve tree performance, such field trials take many years for establishment and evaluation, and this long time-period is very limiting to expeditious evaluation of new rootstocks. In this study, we have conducted a 50-week greenhouse experiment to evaluate rootstock influences on Valencia sweet orange tree response to CLas infection. The infection of trees with CLas reduced scion and rootstock growth, increased leaf yellowing, and reduced the number of leaves per tree and leaf area, regardless of rootstock. There were clear rootstock influences on some traits during the 50-week study. In general, infected trees on US-942 rootstock had lower CLas root titers, less reduction of the number of leaves, less reduction of leaf area, and less leaf yellowing, as compared with some of the other rootstocks. The 50-week greenhouse evaluation method provided results that corresponded well with results from long-term field testing, indicating this may be a useful tool to accelerate evaluation and selection of new rootstocks, as well as in testing other HLB management strategies.

Huanglongbing (HLB) reduces the growth and development of citrus and induces changes in secondary metabolites such as flavonoids, limonoids, and polyamines. Likewise, infected plants have a deficient absorption of nutrients such as zinc, potassium, manganese, and copper. Therefore, the objective of this study was to evaluate the influence of different rootstocks on morphology and mineral changes of Citrus limon and Citrus sinensis cv. Valencia plants inoculated with Candidatus Liberibacter asiaticus. In a greenhouse of the Experimental Station-Autonomous University of Tamaulipas, the Candidatus Liberibacter asiaticus bacteria were inoculated to Citrus limon plants (growing on Citrus volkameriana, Citrus macrophylla, and Citrus aurantium rootstocks) and Citrus sinensis cv. Valencia (growing on Citrus volkameriana and Citrus aurantium rootstocks). The experiment was established under a completely randomized design with 45 graft/rootstock repetitions. In each graft/rootstock combination, the plant height and stem diameter were determined using a tape measurer and a Vernier, respectively. In addition, the nutrient content of foliar samples was determined by an X-ray fluorescence spectrometer. In both citrus species, the C. aurantium rootstock promoted a higher concentration of the bacteria. On the other hand, the rootstock that showed the best agronomical results after inoculation with the bacteria was C. volkameriana, presenting the least variation in mineral content and conferring greater plant height (15%) and stem diameter (23%). In contrast, the presence of Ca. Liberibacter asiaticus decreased S content and increased Cu concentration in C. lemon plants. Similarly, plants infected with C. sinensis presented higher Fe content. Finally, in both species, no significant differences were observed for Mn, P, and Zn concentration.