Prunus spp. are affected by a large number of viruses, causing significant economic losses through either direct or indirect damage, which results in reduced yield and fruit quality. Among these viruses, members of the genus Ilarvirus (isometric labile ringspot viruses) occupy a significant position due to their distribution worldwide. Although symptoms caused by these types of viruses were reported early in the last century, their molecular characterization was not achieved until the 1990s, much later than for other agronomically relevant viruses. This was mainly due to the characteristic liability of virus particles in tissue extracts. In addition, ilarviruses, together with Alfalfa mosaic virus, are unique among plant viruses in that they require a few molecules of the coat protein in the inoculum in order to be infectious, a phenomenon known as genome activation. Another factor that has made the study of this group of viruses difficult is that infectious clones have been obtained only for the type member of the genus, Tobacco streak virus. Four ilarviruses, Prunus necrotic ringspot virus, Prune dwarf virus, Apple mosaic virus, and American plum line pattern virus, are pathogens of the main cultivated fruit trees. As stated in the 9th Report of the International Committee on Taxonomy of Viruses, virions of this genus are "unpromising subjects for the raising of good antisera." With the advent of molecular approaches for their detection and characterization, it has been possible to get a more precise view of their prevalence and genome organization. This review updates our knowledge on the incidence, genome organization and expression, genetic diversity, modes of transmission, and diagnosis, as well as control of this peculiar group of viruses affecting fruit trees.

Torrao or torrado is an emerging disease that is causing serious economic losses in tomato crops of southeastern Spain. The causal agent has been shown to be a new picorna-like plant virus, tentatively named Tomato torrado virus (ToTV) (4). By using trap tomato plants in a greenhouse affected by torrado located in the Murcia Region of Spain, we obtained a ToTV isolate (ToTV-CE) that we have biologically and molecularly characterized. Subtracted cDNA libraries (1) and expressed sequence tags sequencing were used to determine the partial nucleotide sequence of ToTV-CE. We covered ≈53% of the virus genome (GenBank Accession Nos. EU476181 and EU476182) and found that ToTV-CE RNAs 1 and 2 had a high nucleotide similarity (98 and 99%, respectively) with the ToTV published sequences (2,4). ToTV-CE sequences also showed a 70% nt similarity with those of Tomato apex necrosis virus, a newly identified virus in tomato crops of the Culiacan area (Sinaloa, Mexico) (3). To characterize the host range of ToTV-CE, 6 to 10 plants belonging to 14 species were mechanically inoculated with extracts from ToTV-CE-infected Nicotiana benthamiana plants. The presence of ToTV in these plants was analyzed at 3 and 6 weeks postinoculation (PI) by molecular hybridization in dot-blots. The determined host range was in agreement with that described earlier (2,4), but additional hosts and nonhosts were identified. Thus, the virus did not infect melon (Cucumis melo var. cantaloupe), cucumber (C. sativus cv. Marketmore), squash (Cucurbita pepo cv. Negro Belleza), Chenopodium album ssp. Amaranticolor, or Chenopodium quinoa. The virus infected systemically N. benthamiana, N. glutinosa, N. rustica, tobacco (N. tabacum cvs. Xanthi nc and Samsun), Physalis floridana, pepper (Capsicum annuum cv. Italian Long Sweet), tomato (Solanum lycopersicum cv. Boludo), and eggplant (S. melongena cv. Black Beauty). Pepper plants displayed severe symptoms of infection consisting of marked mosaics and stunting (but no necrosis), but eggplant remained asymptomatic for up to 6 weeks PI. A simple assay was devised to analyze whether ToTV can be transmitted by whiteflies. ToTV-CE-infected tomato plants were placed together with three to eight healthy tomato seedlings inside insect-proof glass boxes. Adult Bemisia tabaci (100 to 800 individuals in three replicates) or Trialeurodes vaporariorum (100 individuals in one replicate) were released into each box. For both treatments, symptoms typically induced by ToTV appeared in one to seven tomato seedlings by 1 week after the release of the whiteflies. ToTV infection was confirmed by molecular hybridization in tissue prints of petiole cross sections at 10 days PI. These data are in agreement with those reported by Pospieszny et al. (2) and strongly suggest that both B. tabaci and T. vaporariorum can transmit ToTV. References: (1) L. Diachenko et al. Proc. Natl. Acad. Sci. USA 93:6025, 1996. (2) H. Pospieszny et al. Plant Dis. 91:1364, 2007 (3) M. Turina et al. Plant Dis. 91:932, 2007. (4) M. Verbeek et al. Arch. Virol. 152:881, 2007.

The route of infection and the pattern of distribution of Prunus necrotic ringspot virus (PNRSV) in apricot pollen were studied. PNRSV was detected both within and on the surface of infected pollen grains. The virus invaded pollen during its early developmental stages, being detected in pollen mother cells. It was distributed uniformly within the cytoplasm of uni- and bicellular pollen grains and infected the generative cell. In mature pollen grains, characterized by their triangular shape, the virus was located mainly at the apertures, suggesting that PNRSV distribution follows the same pattern as the cellular components required for pollen tube germination and cell wall tube synthesis. PNRSV also was localized inside pollen tubes, especially in the growth zone. In vitro experiments demonstrated that infection with PNRSV decreases the germination percentage of pollen grains by more than half and delays the growth of pollen tubes by approximately 24 h. However, although PNRSV infection affected apricot pollen grain performance during germination, the presence of the virus did not completely prevent fertilization, because the infected apricot pollen tubes, once germinated, were able to reach the apricot embryo sacs, which, in the climatic conditions of southeastern Spain, mature later than in other climates. Thus, infected pollen still could play an important role in the vertical transmission of PNRSV in apricot.

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