Perennial crops, such as fruit trees, are infected by many viruses, which are transmitted through vegetative propagation and grafting of infected plant material. Some of these pathogens cause severe crop losses and often reduce the productive life of the orchards. Detection and characterization of these agents in fruit trees is challenging, however, during the last years, the wide application of high-throughput sequencing (HTS) technologies has significantly facilitated this task. In this review, we present recent advances in the discovery, detection, and characterization of fruit tree viruses and virus-like agents accomplished by HTS approaches. A high number of new viruses have been described in the last 5 years, some of them exhibiting novel genomic features that have led to the proposal of the creation of new genera, and the revision of the current virus taxonomy status. Interestingly, several of the newly identified viruses belong to virus genera previously unknown to infect fruit tree species (e.g., Fabavirus, Luteovirus) a fact that challenges our perspective of plant viruses in general. Finally, applied methodologies, including the use of different molecules as templates, as well as advantages and disadvantages and future directions of HTS in fruit tree virology are discussed.

Plant viruses infecting crop species are causing long-lasting economic losses and are endangering food security worldwide. Ongoing events, such as climate change, changes in agricultural practices, globalization of markets or changes in plant virus vector populations, are affecting plant virus life cycles. Because farmer’s fields are part of the larger environment, the role of wild plant species in plant virus life cycles can provide information about underlying processes during virus transmission and spread. This review focuses on the Solanaceae family, which contains thousands of species growing all around the world, including crop species, wild flora and model plants for genetic research. In a first part, we analyze various viruses infecting Solanaceae plants across the agro-ecological interface, emphasizing the important role of virus interactions between the cultivated and wild zones as global changes affect these environments on both local and global scales. To cope with these changes, it is necessary to adjust prophylactic protection measures and diagnostic methods. As illustrated in the second part, a complex virus research at the landscape level is necessary to obtain relevant data, which could be overwhelming. Based on evidence from previous studies we conclude that Solanaceae plant communities can be targeted to address complete life cycles of viruses with different life strategies within the agro-ecological interface. Data obtained from such research could then be used to improve plant protection methods by taking into consideration environmental factors that are impacting the life cycles of plant viruses.

Plum pox virus (PPV) is the causal agent of sharka, the most detrimental virus disease of stone fruit trees worldwide. PPV isolates have been assigned into seven distinct strains, of which PPV-C regroups the genetically distinct isolates detected in several European countries on cherry hosts. Here, three complete and several partial genomic sequences of PPV isolates from sour cherry trees in the Volga River basin of Russia have been determined. The comparison of complete genome sequences has shown that the nucleotide identity values with other PPV isolates reached only 77.5 to 83.5%. Phylogenetic analyses clearly assigned the RU-17sc, RU-18sc, and RU-30sc isolates from cherry to a distinct cluster, most closely related to PPV-C and, to a lesser extent, PPV-W. Based on their natural infection of sour cherry trees and genomic characterization, the PPV isolates reported here represent a new strain of PPV, for which the name PPV-CR (Cherry Russia) is proposed. The unique amino acids conserved among PPV-CR and PPV-C cherry-infecting isolates (75 in total) are mostly distributed within the central part of P1, NIa, and the N terminus of the coat protein (CP), making them potential candidates for genetic determinants of the ability to infect cherry species or of adaptation to these hosts. The variability observed within 14 PPV-CR isolates analyzed in this study (0 to 2.6% nucleotide divergence in partial CP sequences) and the identification of these isolates in different localities and cultivation conditions suggest the efficient establishment and competitiveness of the PPV-CR in the environment. A specific primer pair has been developed, allowing the specific reverse-transcription polymerase chain reaction detection of PPV-CR isolates.