First report of the occurrence of Asian citrus psyllid Diaphorina citri (Hemiptera: Liviidae), an invasive species in Nigeria, West Africa

Bacterial plant pathogens affect the locomotor behavior of the insect vector: a case study of Citrus volkameriana-Triozae erytreae-Candidatus Liberibacter asiaticus system

Plant pathogens can alter the behavior of their insect vectors as well as their survival and reproduction. The African psyllid, Trioza erytreae, is one of the vectors of Huanglongbing, a citrus disease caused mainly by "Candidatus Liberibacter asiaticus" (CLas). The purpose of this study was to characterize the effects of CLas on the psyllid, T. erytreae using Citrus volkamerina plants as the study system. The study focused more specifically on the CLas effects prior to and after its acquisition by the psyllid T. erytreae. Our results did not support the hypothesis that CLas effects psyllid probing behavior prior to acquisition; few differences were observed between uninfected T. erytrea feeding on CLas-infected versus control plants. On the other hand, compared to psyllids that had completed their development on control plants, the ones that had completed their development on a CLas-infected plant exhibited changes in their behavior (greater velocity), physiology (smaller mass) and biochemistry (lower water and lipid content). Altogether, our results confirm the existence of a marked postacquisition effect on the vector locomotor behavior and a minor preacquisition effect of CLas on the vector behavior, which can be partially explained by physiological and biochemical changes.

First report on the presence of huanglongbing vectors (Diaphorina citri and Trioza erytreae) in Ghana

As significant threats to global citrus production, Diaphorina citri (Kuwayama; Hemiptera: Psyllidae) and Trioza erytreae (Del Guercio; Hemiptera: Triozidae) have caused considerable losses to citrus trees globally. Diaphorina citri vectors "Candidatus Liberibacter asiaticus" and "Ca. L. americanus", whereas T. erytreae transmits "Ca. L. africanus" and "Ca. L. asiaticus", the pathogens responsible for citrus greening disease or Huanglongbing (HLB). Though HLB is a destructive disease of citrus wherever it occurs, information on the occurrence and geographical distribution of its vectors in Africa is limited. In recent surveys to determine if HLB vectors are present in Ghana, we observed eggs, nymphs, and adults of insects suspected to be D. citri and T. erytreae. Using morphological traits and DNA analyses, the identity of the suspected insects was confirmed to be D. citri and T. erytreae. Individuals of D. citri and T. erytreae were examined using qPCR for CLaf, CLam, and CLas, but none of them tested positive for any of the Liberibacter species. Herein we report, for the first time, the presence of D. citri and T. erytreae in Ghana (West Africa). We discuss the implications of this new threat to the citrus industry to formulate appropriate management strategies.

Report on the first detection of Asian citrus psyllid Diaphorina citri Kuwayama (Hemiptera: Liviidae) in the Republic of Benin, West Africa

The Asian citrus psyllid (ACP), Diaphorina citri, was detected for the first time in the Republic of Benin, West Africa. The ACP is a known vector of Candidatus Liberibacter asiaticus (CLas), the putative causal agent of the devastating Huanglongbing (HLB; citrus greening disease). During visual surveys, ACP was only observed on residential citrus trees in southern Benin, but not in residential areas or commercial groves in the central and northern parts of the country. Its identity was confirmed morphologically and molecularly via DNA barcoding with published primers. Analysis of the obtained sequences showed that the ACP recorded in Benin clustered with the ones previously reported from Nigeria, suggesting a common origin of both populations. The ACP samples from Benin also carried Ca. Carsonella ruddii and Ca. Profftella armatura, two commonly found ACP endosymbionts. However, all the sampled ACP individuals tested negative for Ca. Liberibacter africanus, Ca. Liberibacter americanus, and CLas by quantitative polymerase chain reaction. This is the second report of the ACP in West Africa after Nigeria, the eastern bordering country of the Republic of Benin. Benin has an expanding commercial citrus industry, especially in the southern part of the country. Although the ACP samples tested negative for the HLB associated bacteria, the detection of ACP in the country requires swift actions including area-wide surveys to determine the extent of spread of this pest and the implementation of eradication or control efforts to prevent its establishment and spread of HLB in the country.

Genetic diversity of Diaphorina citri (Hemiptera: Liviidae) unravels phylogeographic structure and invasion history of eastern African populations

The Asian citrus psyllid (Diaphorina citri Kuwayama) is a key pest of Citrus sp. worldwide, as it acts as a vector for Candidatus Liberibacter asiaticus, the bacterial pathogen that causes citrus Huanglongbing. Diaphorina citri has been reported in Kenya, Tanzania, and more recently in Ethiopia. This study assessed the genetic diversity and phylogeographic structure of the pest to gain insights into the potential sources of its introduction into Africa. Population structure and differentiation of D. citri populations from China, Ethiopia, Kenya, Tanzania, and the USA were assessed using 10 microsatellite loci. Additionally, five new complete mitogenomes of D. citri collected in China, Ethiopia, Kenya, Tanzania, and the USA were analyzed in the context of publicly available sequences. Genotype data grouped the D. citri populations from Kenya and Tanzania in one cluster, and those from Ethiopia formed a separate cluster. The two genetic clusters inferred from genotype data were congruent with mitochondrial sequence data. The mitogenomes from Kenya/Tanzania/China had 99.0% similarity, and the Ethiopia/USA had 99.9% similarity. In conclusion, D. citri populations in eastern Africa have different sources, as the Kenyan and Tanzanian populations probably originated from southeastern Asia, while the Ethiopian population most probably originated from the Americas.

Climate-induced range shifts of invasive species (Diaphorina citri Kuwayama)

BACKGROUND

The Asian citrus psyllid (ACP) Diaphorina citri Kuwayama (Hemiptera: Liviidae) is a destructive, invasive species that poses a serious threat to the citrus industry wherever it occurs. The psyllid vectors the phloem-limited bacteria 'Candidatus Liberibacter americanus' and 'Ca. L. asiaticus', causal agents of the incurable citrus greening disease or huanglongbing (HLB). It is essential to understand which regions and areas are suitable for colonization by ACP to formulate appropriate policy and preventive measures. Considering its biology and ecology, we used a machine learning algorithm based on the MaxEnt (Maximum Entropy) principle, to predict the potential global distribution of ACP using bioclimatic variables and elevation.

RESULTS

The model predictions are consistent with the known distribution of ACP and also highlight the potential occurrence outside its current ecological range, that is, primarily in Africa, Asia and the Americas. The most important abiotic variables driving the global distribution of ACP were annual mean temperature, seasonality of temperature and annual precipitation.

CONCLUSION

Our findings highlight the need for international collaboration in slowing the spread of invasive pests like D. citri. © 2022 Society of Chemical Industry.

Cultural Management of Huanglongbing: Current Status and Ongoing Research

Huanglongbing (HLB), formerly known as greening, is a bacterial disease restricted to some Asian and African regions until two decades ago. Nowadays, associated bacteria and their vectors have spread to almost all citrus-producing regions, and it is currently considered the most devastating citrus disease. HLB management can be approached in terms of prevention, limiting or avoiding pathogen and associated vectors to reach an area, or in terms of control, trying to reduce the impact of the disease by adopting different cultural strategies depending on infestation/infection levels. In both cases, control of psyllid populations is currently the best way to stop HLB spread. Best cultural actions (CHMAs, TPS system) to attain this goal and, thus, able to limit HLB spread, and ongoing research in this regard is summarized in this review.

A Review of the 'Candidatus Liberibacter africanus' Citrus Pathosystem in Africa

It has been nearly 100 years since citrus growers in two distinct regions in the northern provinces of South Africa noticed unusual symptoms in their citrus trees, causing significant crop losses. They had no idea that these symptoms would later become part of an almost global pandemic of a disease called greening or huanglongbing (HLB). The rapid spread of the disease indicated that it might be caused by a transmissible pathogen, but it took >50 years to identify the causative agent as 'Candidatus Liberibacter africanus'. Recently, the disease appeared in more African countries, spreading by both infected planting material and Trioza erytreae. To date, five 'Ca. L. africanus' subspecies have been identified in various rutaceous species, with 'Ca. L. africanus subsp. clausenae' the only subspecies for which a biovar was detected in citrus. Efforts to detect and differentiate HLB-causing Liberibacter species are ongoing, and recent developments are discussed here. This review focuses on aspects of the African form of HLB, including its specific bacterial species and subspecies, its main insect vector, its geographic distribution, and current management strategies.

High Temperatures Decrease the Flight Capacity of Diaphorina citri Kuwayama (Hemiptera: Liviidae)

Diaphorina citri Kuwayama (Hemiptera: Liviidae), commonly known as Asian citrus psyllid (ACP), is an invasive insect pest and the vector of the bacterium causing Huanglongbing (HLB), a lethal disease of citrus. In the United States, ACP has been established in all citrus-producing zones, all of which have different environmental conditions. The spread of ACP and, more importantly, HLB, has progressed differently depending on the state, with more rapid spread in Florida and Texas than in California. Climatic variations between the regions are likely a strong factor in the difference in the rate of spread. Despite this, it is unknown how the flight capacity of D. citri is influenced by high temperatures (>30 °C) and subsequently, low humidity experienced in California but not in Texas or Florida. In this study, by using a custom-made, temperature-controlled flight mill arena, we assessed the effect of high temperatures on the flight capacity and flight propensity of D. citri under low (20-40%) and high (76-90%) relative humidity conditions. We found that temperature and humidity influence the propensity to engage in short or long-distance flight events. Psyllids exposed to temperatures above 43 °C only performed short flights (˂60 s), and a high relative humidity significantly decrease the proportion of long flights (≥60 s) at 26 and 40 °C. The flight capacity for insects who engaged in short and long flights was significantly affected by temperature but not by humidity. For long flyers, temperature (in the 26-43 °C range) was negatively correlated with distance flown and flight duration. The most favorable temperature for long dispersion was 26 °C, with suboptimal temperatures in the range of 32-37 °C and the least favorable temperatures at 40 and 43 °C. In conclusion, D. citri is able to fly in a broad range of temperatures and efficiently fly in high and low humidity. However, temperatures above 40 °C, similar to those experienced in semi-arid environments like Southern California or Arizona, are detrimental for its flight capacity.

First report of Cucurbit chlorotic yellows virus infecting cantaloupe (Cucumis melo L.) in Texas

Texas is a major producer of cucurbits such as cantaloupe (Cucumis melo L.), but outbreaks of virus-like diseases often adversely affect yields. Little is known about the identity of the causal or associated viruses. During studies conducted in fall 2020 to explore the virome of cucurbit fields in Texas, a commercial cantaloupe field (~4.1 ha) in Cameron County was observed with virus-like symptoms of interveinal chlorotic mottle and foliar chlorosis and disease incidence was estimated at 100%. Virus-like symptoms including mosaic and leaf curl were also observed in six additional fields across five south and central Texas counties of Atascosa, Hidalgo, Fort Bend, Frio, and Wharton. Forty-six plants, which included 32 symptomatic and 14 non-symptomatic, were sampled from these fields for virus diagnosis and each sample was subjected to total nucleic acid extraction according to Dellaporta et. al. (1983). Initially, equal amounts of nucleic acids from 14 symptomatic plants (two/field) were pooled into one composite sample for preliminary diagnosis by high throughput sequencing (HTS). The cDNA library obtained from the composite sample with a TruSeq Stranded Total RNA with Ribo-Zero Plant Kit (Illumina) was sequenced on the Illumina NextSeq 500 platform, generating ~26.3 M single-end HTS reads (75 nucleotides [nt] each). Analyses of the reads according to Al Rwahnih et al. (2018) revealed several virus-like contigs; among them 23 contigs (206 to 741 nt) shared 98 to 100% nt identities to isolates of cucurbit chlorotic yellows virus (CCYV), genus Crinivirus, family Closteroviridae. Three pairs of CCYV-specific primers were designed from the HTS contigs with primers CCYV-v1330: 5'-AGTCCCTTACCCTGAGATGAA/CCYV-c2369: 5'-CGGAGCATTCGACAACTGAATA targeting ~1 kb fragment of the ORF1a (RNA1), primers CCYV-v4881: 5'-ATAAGGCGGCGACCTAATC/CCYV-c5736: 5'-GATCACTTGACCATCTCCTTCT targeting a ~0.9 kb fragment encompassing the coat protein (CP) cistron of CCYV (RNA2), and primers CCYV-v6362: 5'-CACCTCTTCCAGAACCAGTTAAA/CCYV-c7423: 5'-TGGGAACAACTTATTTCTCCTAGC targeting ~1 kb spanning partial minor coat protein (CPm) and p26 sequences (RNA2). Total nucleic acid extracts of each of the 46 samples from the seven fields were tested by two-step reverse transcription polymerase chain reaction using all three CCYV-specific primer pairs and they yielded amplicons of expected sizes from all five symptomatic cantaloupe samples from the Cameron County field and one additional symptomatic butternut squash sample from a field in Hidalgo County. The DNA bands from three randomly chosen cantaloupe samples were cloned and sequenced as previously described (Oke et al. 2020). In pairwise comparisons, the obtained 1,040 nt ORF1a (MW584332-334), 753 nt complete CP (MW584335-337), and 1,062 nt CPm/p26 (MW584338-340) gene specific sequences from the three samples shared 100% nt identity with each other, and 99-100% nt identities with corresponding RNA1 (AB523788) and RNA2 (AB523788) sequences of the exemplar isolate of CCYV. This is the first report of CCYV in Texas, thus expanding the current geographical range of the virus in the U.S. that includes California (Wintermantel et al. 2019) and Georgia (Kavalappara et al. 2021). The abundance of whiteflies of the Bemisia tabaci species complex in south Texas and other major U.S. cucurbit production areas presents additional challenges to virus disease management.

Pest categorisation of Diaphorina citri

The EFSA Panel on Plant Health performed a pest categorisation of Diaphorina citri (Hemiptera: Liviidae) (Asian citrus psyllid) for the EU. D. citri is a key pest of citrus in several countries as it is a vector of serious bacterial pathogens, the putative causal agents of Huanglongbing (HLB) also known as citrus greening. Eggs are laid on tips of growing shoots on and between unfurling leaves. Females may lay more than 800 eggs during their lives. Nymphs pass through five instars. The life cycle requires from 14 to 49 days, depending upon the season. There is no diapause, but populations are low in winter. It overwinters as an adult which may live for several months. The species completes 9-10 generations/year; however, under protected conditions, up to 16 generations have been recorded. Commission Implementing Regulation (EU) 2019/2072 (Annex IIA) regulates D. citri, as a quarantine pest not known to occur in the EU territory. Fruits and plants for planting provide potential pathways for entry into the EU. Climatic conditions and the availability of host plants provide conditions to support establishment in the EU. The introduction of D. citri would have an economic impact in the EU through direct but mainly indirect effects due to potential transmission of HLB. Phytosanitary measures are available to reduce the likelihood of entry. D. citri satisfies the criteria that are within the remit of EFSA to assess for it to be regarded as a potential Union quarantine pest. D. citri does not meet the criteria of occurring in the EU, nor plants for planting being the principal means of spread, for it to be regarded as a potential Union regulated non-quarantine pest.