Ilarviruses of Prunus spp.: a continued concern for fruit trees.

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.

Low genetic variability in the coat and movement proteins of American plum line pattern virus isolates from different geographic origins.

Until very recently isolates of American plum line pattern virus (APLPV) had not been reported from outside North America. The nucleotide sequences corresponding to the movement (MP) and coat (CP) proteins of eight APLPV isolates from five Mediterranean countries were determined. Sequence analysis showed that both MP and CP genes are highly conserved irrespective of geographic origin. The study of the distribution of synonymous and nonsynonymous changes along both open reading frames revealed that these proteins are under the effect of negative purifying selection. The MP and CP of APLPV possess most of the functional motifs described for other members of the genus Ilarvirus.

First Report of Peach latent mosaic viroid on Peach in Uruguay.

Peach latent mosaic viroid (PLMVd) (2) is widely distributed and causes yellow, chlorotic mosaics and delayed foliation, flowering, and ripening. Infected fruits display a cracked suture and are often dented, misshapen, frequently flattened, and discolored. In the greenhouse, PLMVd natural isolates are divided into severe or latent strains depending on whether they induce leaf symptoms on seedlings of the peach indicator GF-305. PLMVd was detected in 2001 during a survey in three locations in the Canelones Department, the main peach producing area in Uruguay. Fifty samples were tested for the presence of five viruses: Prunus necrotic ringspot virus (PNRSV), Prune dwarf virus (PDV), American plum line pattern virus (APLPV), Plum pox virus (PPV) and Apple chlorotic leaf spot virus (ACLSV); samples were also tested for the viroids affecting stone fruits, Hop stunt viroid (HSVd) and PLMVd. The analyses were completed with molecular hybridization using specific nonisotopic riboprobes for each virus (4). PLMVd, undescribed in Uruguay, was detected in 9 of 50 samples in three peach cultivars, Scarlet Pearl, EarliGrande, and Barcelo. The PLMVd-positive sample for 'Scarlet Pearl' showed mild mosaic symptoms on leaves whereas the two PLMVd-positives of 'EarliGrande' showed clear calico type symptoms. The remaining PLMVd-positive samples belonged to 'Barcelo' and showed no symptoms or mild chlorosis. The first two cultivars were imported from the United States, a source with a high percentage of PLMVd infections in peach germ plasm (1). In five of nine PLMVd-positive samples, the viroid occurred with PNRSV and in one with PDV and PNRSV. PLMVd has previously been reported in Brazil (3), but to our knowledge, this is the first report of PLMVd in Uruguay. These results reveal the importance of following strict sanitary practices with plant material used for propagation. Molecular tools are available to prescreen scion and rootstock sources for PLMVd. References: (1) M. L. Badenes and G. Llácer. Acta Hortic. 309:565, 1998. (2) R. Flores et al. Res. Virol. 141:109, 1990. (3) A. Hadidi et al. Plant Dis. 81:154, 1997. (4) V. Pallás et al. Detection of plant RNA viruses by non-isotopic dot-blot hybridization. Pages 461-468 in: Plant Virus Protocols: From Virus Isolation to Transgenic Resistance. G. Foster and S. Taylor, eds. Humana Press, Totowa, NJ. 1998.

Simultaneous detection of the three ilarviruses affecting stone fruit trees by nonisotopic molecular hybridization and multiplex reverse-transcription polymerase chain reaction.

ABSTRACT The three most economically damaging ilarviruses affecting stone fruit trees on a worldwide scale are the related Prunus necrotic ringspot virus (PNRSV), Prune dwarf virus (PDV), and Apple mosaic virus (ApMV). Nonisotopic molecular hybridization and multiplex reverse-transcription polymerase chain reaction (RT-PCR) methodologies were developed that could detect all these viruses simultaneously. The latter technique was advantageous because it was discriminatory. For RT-PCR, a degenerate antisense primer was designed which was used in conjunction with three virus-specific sense primers. The amplification efficiencies for the detection of the three viruses in the multiplex RT-PCR reaction were identical to those obtained in the single RT-PCR reactions for individual viruses. This cocktail of primers was able to amplify sequences from all of the PNRSV, ApMV, and PDV isolates tested in five Prunus spp. hosts (almond, apricot, cherry, peach, and plum) occurring naturally in single or multiple infections. For ApMV isolates, differences in the electrophoretic mobilities of the PCR products were observed. The nucleotide sequence of the amplified products of two representative ApMV isolates was determined, and comparative analysis revealed the existence of a 28-nucleotide deletion in the sequence of isolates showing the faster electrophoretic mobility. To our knowledge, this is the first report on the simultaneous detection of three plant viruses by multiplex RT-PCR in woody hosts. This multiplex RT-PCR could be a useful time and cost saving method for indexing these three ilarviruses, which damage stone fruit tree yields, and for the analysis of mother plants in certification programs.