First Report of Transmission of Little cherry virus 2 to Sweet Cherry by Pseudococcus maritimus (Ehrhorn) (Hemiptera: Pseudococcidae)

Improved Detection of Little Cherry Virus-2 Using a Hydrolysis Probe to Manage the Pacific Northwest Little Cherry Disease Epidemic

Little cherry virus-2 (LChV-2) is a viral pathogen that is reaching epidemic levels in Washington State. This virus is insect vectored and has significant impacts on sweet cherry production. To aid growers in making informed management decisions, we sought to develop a diagnostic assay to better detect isolates of LChV-2 currently found in Washington, allowing more accurate estimations of disease occurrence. This study showed that there were two distinct genotypes of LChV-2 present in Washington State. This information was used to develop an up-to-date reverse transcription real-time quantitative PCR assay, which was then optimized, validated, and compared with four previously published assays of a panel of field samples. This comparison demonstrated that the newly developed assay provided greater sensitivity, accurately detecting <10 copies per reaction and could detect both LChV-2 genotypes. Finally, we examined the effect of potential inhibitors in various tissue types from cherry, finding that young leaf tissue affected sensitivity of detection less than root tissues.

Towards the identification of the scale insects (Hemiptera: Coccomorpha) of continental Africa: 1. Identification of the families

Scale insects (Hemiptera: Sternorrhyncha: Coccomorpha) are obligate plant parasites feeding on plant sap; some are damaging pests in agriculture, horticulture and forestry. Despite their economic importance, the scale insects found in continental Africa have not been extensively studied and the keys for identifying them are incomplete and scattered through the literature in several languages. The aim of this study is to improve our understanding of the African scale insect fauna. As a first step towards their identification, we provide a key to the 23 families currently known from continental Africa, based on slide-mounted adult females, covering Aclerdidae, Asterolecaniidae, Cerococcidae, Coccidae, Conchaspididae, Dactylopiidae, Diaspididae, Eriococcidae, Halimococcidae, Kermesidae, Kerriidae, Kuwaniidae, Lecanodiaspididae, Margarodidae, Matsucoccidae, Micrococcidae, Monophlebidae, Ortheziidae, Phoenicococcidae, Pseudococcidae, Putoidae, Rhizoecidae and Stictococcidae.

Titer and distribution of little cherry virus 2 in Prunus avium

Little cherry virus 2 (LChV-2) is a causal agent of little cherry disease, which produces small, misshapen fruit with poor color and taste. As LChV-2 symptoms are only present near harvest, molecular detection is essential for effective control. Therefore, we determined the titer and distribution of this virus in infected trees over time. While initial infections were found to be basipetal, in field trees, early-stage infection was characterized by uneven distribution and low titer, concentrated in woody stems. In contrast, established infections were systemic, and detection was consistent across tissues. These data provide improved sampling recommendations for the detection of LChV-2.

Updating the Quarantine Status of Prunus Infecting Viruses in Australia

One hundred Prunus trees, including almond (P. dulcis), apricot (P. armeniaca), nectarine (P. persica var. nucipersica), peach (P. persica), plum (P. domestica), purple leaf plum (P. cerasifera) and sweet cherry (P. avium), were selected from growing regions Australia-wide and tested for the presence of 34 viruses and three viroids using species-specific reverse transcription-polymerase chain reaction (RT-PCR) or polymerase chain reaction (PCR) tests. In addition, the samples were tested using some virus family or genus-based RT-PCR tests. The following viruses were detected: Apple chlorotic leaf spot virus (ACLSV) (13/100), Apple mosaic virus (ApMV) (1/100), Cherry green ring mottle virus (CGRMV) (4/100), Cherry necrotic rusty mottle virus (CNRMV) (2/100), Cherry virus A (CVA) (14/100), Little cherry virus 2 (LChV2) (3/100), Plum bark necrosis stem pitting associated virus (PBNSPaV) (4/100), Prune dwarf virus (PDV) (3/100), Prunus necrotic ringspot virus (PNRSV) (52/100), Hop stunt viroid (HSVd) (9/100) and Peach latent mosaic viroid (PLMVd) (6/100). The results showed that PNRSV is widespread in Prunus trees in Australia. Metagenomic high-throughput sequencing (HTS) and bioinformatics analysis were used to characterise the genomes of some viruses that were detected by RT-PCR tests and Apricot latent virus (ApLV), Apricot vein clearing associated virus (AVCaV), Asian Prunus Virus 2 (APV2) and Nectarine stem pitting-associated virus (NSPaV) were also detected. This is the first report of ApLV, APV2, CGRMV, CNRNV, LChV1, LChV2, NSPaV and PBNSPaV occurring in Australia. It is also the first report of ASGV infecting Prunus species in Australia, although it is known to infect other plant species including pome fruit and citrus.

Developing a Management Strategy for Little Cherry Disease: The Case of Washington State

Little cherry disease (LCD) threatens the long-term economic sustainability of the Pacific Northwest sweet cherry (Prunus avium) industry. Results from a series of partial budget analyses indicate that additional investments in monitoring, testing, spraying to control for insect vectors, and removing infected trees are lower than the reduced profit losses compared with the do-nothing scenario. Also, management can prevent or lessen the negative impacts of higher little cherry virus (Velarivirus little cherry virus 1, Ampelovirus little cherry virus 2) spread rates. Our findings illustrate the importance of prevention, correct identification, and controlling for insect vectors in preventing the dissemination of LCD, for which the only known treatment is tree removal.

High-Throughput Sequencing Assists Studies in Genomic Variability and Epidemiology of Little Cherry Virus 1 and 2 infecting Prunus spp. in Belgium

Little cherry disease, caused by little cherry virus 1 (LChV-1) and little cherry virus 2 (LChV-2), which are both members of the family Closteroviridae, severely affects sweet (Prunus avium L.) and sour cherry (P. cerasus L.) orchards lifelong production worldwide. An intensive survey was conducted across different geographic regions of Belgium to study the disease presence on these perennial woody plants and related species. Symptomatic as well as non-symptomatic Prunus spp. trees tested positive via RT-PCR for LChV-1 and -2 in single or mixed infections, with a slightly higher incidence for LChV-1. Both viruses were widespread and highly prevalent in nearly all Prunus production areas as well as in private gardens and urban lane trees. The genetic diversity of Belgian LChV-1 and -2 isolates was assessed by Sanger sequencing of partial genomic regions. A total RNA High-Throughput Sequencing (HTS) approach confirmed the presence of both viruses, and revealed the occurrence of other Prunus-associated viruses, namely cherry virus A (CVA), prune dwarf virus (PDV) and prunus virus F (PrVF). The phylogenetic inference from full-length genomes revealed well-defined evolutionary phylogroups with high genetic variability and diversity for LChV-1 and LChV-2 Belgian isolates, yet with little or no correlation with planting area or cultivated varieties. The global diversity and the prevalence in horticultural areas of LChV-1 and -2 variants, in association with other recently described fruit tree viruses, are of particular concern. Future epidemiological implications as well as new investigation avenues are exhaustively discussed.

Genomic, Morphological and Biological Traits of the Viruses Infecting Major Fruit Trees

Banana trees, citrus fruit trees, pome fruit trees, grapevines, mango trees, and stone fruit trees are major fruit trees cultured worldwide and correspond to nearly 90% of the global production of woody fruit trees. In light of the above, the present manuscript summarizes the viruses that infect the major fruit trees, including their taxonomy and morphology, and highlights selected viruses that significantly affect fruit production, including their genomic and biological features. The results showed that a total of 163 viruses, belonging to 45 genera classified into 23 families have been reported to infect the major woody fruit trees. It is clear that there is higher accumulation of viruses in grapevine (80/163) compared to the other fruit trees (each corresponding to less than 35/163), while only one virus species has been reported infecting mango. Most of the viruses (over 70%) infecting woody fruit trees are positive-sense single-stranded RNA (+ssRNA), and the remainder belong to the -ssRNA, ssRNA-RT, dsRNA, ssDNA and dsDNA-RT groups (each corresponding to less than 8%). Most of the viruses are icosahedral or isometric (79/163), and their diameter ranges from 16 to 80 nm with the majority being 25-30 nm. Cross-infection has occurred in a high frequency among pome and stone fruit trees, whereas no or little cross-infection has occurred among banana, citrus and grapevine. The viruses infecting woody fruit trees are mostly transmitted by vegetative propagation, grafting, and root grafting in orchards and are usually vectored by mealybug, soft scale, aphids, mites or thrips. These viruses cause adverse effects in their fruit tree hosts, inducing a wide range of symptoms and significant damage, such as reduced yield, quality, vigor and longevity.

Pest categorisation of Little cherry pathogen (non-EU isolates)

The EFSA Panel on Plant Health performed a pest categorisation of non‐EU isolates of the Little cherry pathogen (LCP) for the European Union (EU) territory. LCP is now known to be in fact two distinct, well characterised viruses, Little cherry virus 1 (LChV1) and Little cherry virus 2 (LChV2) collectively referred to here as LChV. Efficient molecular detection assays are available for both viruses but not to discriminate EU and non‐EU isolates. LChV are transmitted by vegetative multiplication of infected hosts and, for LChV2, by mealybug vectors. LChV are reported from a range of countries, both outside and within the EU. Non‐EU isolates are not known to occur in the EU and therefore do not meet one of the criteria for being a Union regulated non‐quarantine pest. The host ranges of LChV are restricted to Prunus species, in particular cultivated and ornamental cherries. LChV non‐EU isolates are listed for some, but not all hosts, in Annex IIAI of Directive 2000/29/EC. LChV isolates are expected to be able to enter and establish in the EU. They have the potential to subsequently spread through plants for planting and, for LChV2, through the action of the Phenacoccus aceris vector, which is present in many EU MS. LChV are able to cause severe symptoms in some cherry varieties while others are less affected. Overall, non‐EU LChV isolates meet all the criteria evaluated by EFSA to qualify as Union quarantine pests. However, given the currently limited impact of EU LChV isolates, the impact of non‐EU isolates, if introduced, could be similarly limited. The main knowledge gaps and uncertainties concern (1) whether EU and non‐EU isolates of LChV might differ in their biology, epidemiology or symptomatology, (2) efficiency of natural spread by vectors under EU conditions and (3) extent of symptoms caused on many EU‐grown varieties.

Control of pome and stone fruit virus diseases

Many different systemic pathogens, including viruses, affect pome and stone fruits causing diseases with adverse effects in orchards worldwide. The significance of diseases caused by these pathogens on tree health and fruit shape and quality has resulted in the imposition of control measures both nationally and internationally. Control measures depend on the identification of diseases and their etiological agents. Diagnosis is the most important aspect of controlling fruit plant viruses. Early detection of viruses in fruit trees or in the propagative material is a prerequisite for their control and to guarantee a sustainable agriculture. Many quarantine programs are in place to reduce spread of viruses among countries during international exchange of germplasm. All these phytosanitary measures are overseen by governments based on agreements produced by international organizations. Also certification schemes applied to fruit trees allow the production of planting material of known variety and plant health status for local growers by controlling the propagation of pathogen-tested mother plants. They ensure to obtain propagative material not only free of "quarantine" organisms under the national legislation but also of important "nonquarantine" pathogens. The control of insect vectors plays an important role in the systemic diseases management, but it must be used together with other control measures as eradication of infected plants and use of certified propagation material. Apart from the control of the virus vector and the use of virus-free material, the development of virus-resistant cultivars appears to be the most effective approach to achieve control of plant viruses, especially for perennial crops that are more exposed to infection during their long life span. The use of resistant or tolerant cultivars and/or rootstocks could be potentially the most important aspect of virus disease management, especially in areas in which virus infections are endemic. The conventional breeding for virus-tolerant or resistant fruit tree cultivars using available germplasm is a long-term strategy, and the development and production of these cultivars may take decades, if successful. Genetic engineering allows the introduction of specific DNA sequences offering the opportunity to obtain existing fruit tree cultivars improved for the desired resistance trait. Unfortunately, genetic transformation of pome and stone fruits is still limited to few commercial genotypes. Research carried out and the new emerging biotechnological approaches to obtain fruit tree plants resistant or tolerant to viruses are discussed.

Rapid and sensitive detection of Little cherry virus 2 using isothermal reverse transcription-recombinase polymerase amplification

Little cherry virus 2 (LChV2) (genus Ampelovirus) is the primary causal agent of little cherry disease (LCD) in sweet cherry (Prunus avium) in North America and other parts of the world. This mealybug-transmitted virus does not induce significant foliar symptoms in most sweet cherry cultivars, but does cause virus-infected trees to yield unevenly ripened small fruits with poor flavor. Most fruits from infected trees are unmarketable. In the present study, an isothermal reverse transcription-recombinase polymerase amplification (RT-RPA) technique was developed using LChV2 coat protein specific primers and probe. Detection of terminally labeled amplicons was achieved with a high affinity lateral flow strip. The RT-RPA is confirmed to be simple, fast, and specific. In comparison, although it retains the sensitivity of RT-PCR, it is a more cost-effective procedure. RT-RPA will be a very useful tool for detecting LChV2 from crude extracts in any growth stage of sweet cherry from field samples.