The species, density, and intra-plant distribution of mites on red raspberry (Rubus idaeus L.).

The adoption of the European Green Deal will limit acaricide use in high value crops like raspberry, to be replaced by biological control and other alternative strategies. More basic knowledge on mites in such crops is then necessary, like species, density, and their role as vectors of plant diseases. This study had four aims, focusing on raspberry leaves at northern altitude: (1) identify mite species; (2) study mite population densities; (3) investigate mite intra-plant distribution; (4) investigate co-occurrence of phytophagous mites, raspberry leaf blotch disorder and raspberry leaf blotch virus (RLBV). Four sites in south-eastern Norway were sampled five times. Floricanes from different parts of the sites were collected, taking one leaf from each of the upper, middle, and bottom zones of the cane. Mites were extracted with a washing technique and processed for species identification and RLBV detection. Mites and leaves were tested for RLBV by reverse transcription polymerase chain reaction (RT-PCR) with virus-specific primers. Phytophagous mites, Phyllocoptes gracilis, Tetranychus urticae, and Neotetranychus rubi, and predatory mites, Anystis baccarum and Typhlodromus (Typhlodromus) pyri were identified. All phytophagous mites in cultivated raspberry preferred the upper zone of floricanes, while in non-cultivated raspberry, they preferred the middle zone. The presence of phytophagous mites did not lead to raspberry leaf blotch disorder during this study. RLBV was detected in 1.3% of the sampled plants, none of them with leaf blotch symptoms, and in 4.3% of P. gracilis samples, and in some spider mite samples, implying that Tetranychids could also be vectors of RLBV.

First report of 'Candidatus Phytoplasma rubi' associated with Rubus stunt disease of raspberry and blackberry in the Czech Republic.

Raspberries (Rubus idaeus L.), occurring in the temperate zone of the northern hemisphere and blackberries (R. fruticosus L.), cultivated and growing all over the world, are plant species of the family Rosaceae. These species are susceptible to phytoplasma infections, which cause Rubus stunt disease. It spreads uncontrolled by vegetative propagation of plants (Linck and Reineke 2019a) and by phloem-sucking insect vectors, especially Macropsis fuscula (Hemiptera: Cicadellidae) (de Fluiter and van der Meer, 1953; Linck and Reineke 2019b). During a survey in commercial field in June 2021, over 200 raspberry bushes cv Enrosadira exhibiting typical symptoms of Rubus stunt were observed in Central Bohemia. Symptoms included dieback, leaf yellowing/reddening, stunted growth, severe phyllody and fruit malformations. Most diseased plants were growing in the edge rows of the field (about 80%). No symptomatic plants were observed in the middle of the field. Similar symptoms were observed in private gardens in South Bohemia on raspberry cv Rutrago and blackberry (unknown cultivar) in June 2018 and August 2022, respectively. DNA was extracted using the DNeasy Plant Mini Kit (Qiagen GmbH, Hilden, Germany) from flower stems and parts affected by phyllody of seven symptomatic plants as well as flower stems, leaf midribs, and petioles of five asymptomatic field plants. The DNA extracts were analyzed by a nested polymerase chain reaction assay using universal phytoplasma P1A/P7A primers followed by R16F2m/R1m and the group-specific R16(V)F1/R1 primers (Bertaccini et al. 2019). All samples from the symptomatic plants yielded an amplicon of expected size, while no product was amplified in asymptomatic plants. The P1A/P7A amplicons from three selected plants (two raspberries and one blackberry, each from different location) were cloned and bi-directionally Sanger sequenced (GenBank Accession Nos.OQ520100-2). The sequences spanned nearly full-length of 16S rRNA gene, 16S-23S rRNA intergenic spacer, tRNA-Ile gene, and a partial 23S rRNA gene. BLASTn search revealed the highest sequence identity (99.8-99.9%, query coverage 100%) to 'Candidatus Phytoplasma rubi' strain RS (GenBank Accession No. CP114006). To further characterize the 'Ca. P. rubi' strains, all these three samples were subjected to multigene sequence analysis. Sequences from a major portion of the tuf, rplV-rpsC, rpsH-rplR, uvrB-degV, and rplO-SecY-map genes (Acc. Nos. OQ506112-26) were obtained as described previously (Fránová et al. 2016). Comparison to GenBank sequences confirmed their highest identity (99.6-100%, query coverage 100%) with 'Ca. P. rubi' RS strain, regardless of their geographic location and host (raspberry or blackberry). Recently, Bertaccini et al. (2022) suggested the 98,65 % 'Ca. Phytoplasma' strain identity threshold within 16Sr RNA sequences. In this survey, all three strains sequenced shared ≥99.73% sequence identity of the analysed 16S rRNA gene sequences and the high identity in the other genes with the reference 'Ca. P. rubi' RS strain. To our knowledge, this is the first report of Rubus stunt disease in the Czech Republic as well as the first molecular identification and characterization of 'Ca. P. rubi' from raspberry and blackberry in our country. As Rubus stunt disease is of great economic importance (Linck and Reineke 2019a), the pathogen detection and prompt removal of the diseased shrubs are essential to mitigating the spread and impact of the disease.

Molecular Characterization of a Novel Rubodvirus Infecting Raspberries.

A novel negative-sense single-stranded RNA virus showing genetic similarity to viruses of the genus Rubodvirus has been found in raspberry plants in the Czech Republic and has tentatively been named raspberry rubodvirus 1 (RaRV1). Phylogenetic analysis confirmed its clustering within the group, albeit distantly related to other members. A screening of 679 plant and 168 arthropod samples from the Czech Republic and Norway revealed RaRV1 in 10 raspberry shrubs, one batch of Aphis idaei, and one individual of Orius minutus. Furthermore, a distinct isolate of this virus was found, sharing 95% amino acid identity in both the full nucleoprotein and partial sequence of the RNA-dependent RNA polymerase gene sequences, meeting the species demarcation criteria. This discovery marks the first reported instance of a rubodvirus infecting raspberry plants. Although transmission experiments under experimental conditions were unsuccessful, positive detection of the virus in some insects suggests their potential role as vectors for the virus.

Molecular Characterization of a Novel Enamovirus Infecting Raspberry.

Raspberry plants, valued for their fruits, are vulnerable to a range of viruses that adversely affect their yield and quality. Utilizing high-throughput sequencing (HTS), we identified a novel virus, tentatively named raspberry enamovirus 1 (RaEV1), in three distinct raspberry plants. This study provides a comprehensive characterization of RaEV1, focusing on its genomic structure, phylogeny, and possible transmission routes. Analysis of nearly complete genomes from 14 RaEV1 isolates highlighted regions of variance, particularly marked by indel events. The evidence from phylogenetic and sequence analyses supports the classification of RaEV1 as a distinct species within the Enamovirus genus. Among the 289 plant and 168 invertebrate samples analyzed, RaEV1 was detected in 10.4% and 0.4%, respectively. Most detections occurred in plants that were also infected with other common raspberry viruses. The virus was present in both commercial and wild raspberries, indicating the potential of wild plants to act as viral reservoirs. Experiments involving aphids as potential vectors demonstrated their ability to acquire RaEV1 but not to successfully transmit it to plants.

Known and Potential Invertebrate Vectors of Raspberry Viruses.

The estimated global production of raspberry from year 2016 to 2020 averaged 846,515 tons. The most common cultivated Rubus spp. is European red raspberry (Rubus idaeus L. subsp. idaeus). Often cultivated for its high nutritional value, the red raspberry (Rubus idaeus) is susceptible to multiple viruses that lead to yield loss. These viruses are transmitted through different mechanisms, of which one is invertebrate vectors. Aphids and nematodes are known to be vectors of specific raspberry viruses. However, there are still other potential raspberry virus vectors that are not well-studied. This review aimed to provide an overview of studies related to this topic. All the known invertebrates feeding on raspberry were summarized. Eight species of aphids and seven species of plant-parasitic nematodes were the only proven raspberry virus vectors. In addition, the eriophyid mite, Phyllocoptes gracilis, has been suggested as the natural vector of raspberry leaf blotch virus based on the current available evidence. Interactions between vector and non-vector herbivore may promote the spread of raspberry viruses. As a conclusion, there are still multiple aspects of this topic that require further studies to get a better understanding of the interactions among the viral pathogens, invertebrate vectors, and non-vectors in the raspberry agroecosystem. Eventually, this will assist in development of better pest management strategies.

Epigenetic and Genetic Integrity, Metabolic Stability, and Field Performance of Cryopreserved Plants.

Cryopreservation is considered an ideal strategy for the long-term preservation of plant genetic resources. Significant progress was achieved over the past several decades, resulting in the successful cryopreservation of the genetic resources of diverse plant species. Cryopreservation procedures often employ in vitro culture techniques and require the precise control of several steps, such as the excision of explants, preculture, osmo- and cryoprotection, dehydration, freeze-thaw cycle, unloading, and post-culture for the recovery of plants. These processes create a stressful environment and cause reactive oxygen species (ROS)-induced oxidative stress, which is detrimental to the growth and regeneration of tissues and plants from cryopreserved tissues. ROS-induced oxidative stresses were documented to induce (epi)genetic and somatic variations. Therefore, the development of true-to-type regenerants of the source germplasm is of primary concern in the application of plant cryopreservation technology. The present article provides a comprehensive assessment of epigenetic and genetic integrity, metabolic stability, and field performance of cryopreserved plants developed in the past decade. Potential areas and the directions of future research in plant cryopreservation are also proposed.