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Plum pox virus (PPV) was identified in Pennsylvania in 1999. The outbreak was limited to a four-county region in southern Pennsylvania. Initial serological and molecular characterization indicated that the isolates in Pennsylvania belong to the D strain of PPV. The Pennsylvania isolates were characterized by sequence analysis, electron microscopy, host range, and vector transmission to determine how these isolates related to their previously studied European counterparts. Genetically, Pennsylvania (PPV-Penn) isolates were more closely related to each other than to any other PPV-D strains, and isolates from the United States, Canada, and Chile were more closely related to each other than to European isolates. The PPV-Penn isolates exist as two clades, suggesting the possibility of multiple introductions. Electron microscopy analysis of PPV-Penn isolates, including cytopathological studies, indicated that the virions were similar to other Potyvirus spp. PPV-Penn isolates had a herbaceous host range similar to that of European D isolates. There were distinct differences in the transmission efficiencies of the two PPV-Penn isolates using Myzus persicae and Aphis spiraecola as vectors; however, both PPV-Penn isolates were transmitted by M. persicae more efficiently than a European D isolate but less efficiently than a European M isolate.

Cucumber mosaic virus (CMV) is a major component of the virus complex that has become more pronounced in snap bean in the midwestern and northeastern United States since 2001. Multiple-vector-transfer tests were done to estimate the CMV transmission efficiencies (p) of the main aphid species identified in commercial snap bean fields in New York and Pennsylvania. The four most efficient vectors (p > 0.05) were Aphis gossypii, A. glycines, Acyrthosiphon pisum, and Therioaphis trifolii, which were all significant species in the migratory aphid populations in snap bean. Moderately efficient vectors (0.01 < p < 0.04) were A. spiraecola, A. craccivora, Macrosiphum euphorbiae, and Rhopalosiphum maidis. Poor vectors (p < 0.01) included A. fabae, Nearctaphis bakeri, and Myzus persicae. Only one species, Sitobion avenae, failed to transmit CMV in replicated tests. Estimates of p were consistent between different clones of the same aphid species and among three different field isolates of CMV tested. Single-vector-transfer test results for a subset of the species supported those obtained via the multiple-vector-transfer approach. Our results are consistent with the notion that A. glycines is a major vector of recent CMV epidemics in snap bean, but that species is only one of several that are involved.

ABSTRACT Sexual forms of two genotypes of the aphid Schizaphis graminum, one a vector, the other a nonvector of two viruses that cause barley yellow dwarf disease (Barley yellow dwarf virus [BYDV]-SGV, luteovirus and Cereal yellow dwarf virus-RPV, polerovirus), were mated to generate F1 and F2 populations. Segregation of the transmission phenotype for both viruses in the F1 and F2 populations indicated that the transmission phenotype is under genetic control and that the parents are heterozygous for genes involved in transmission. The ability to transmit both viruses was correlated within the F1 and F2 populations, suggesting that a major gene or linked genes regulate the transmission. However, individual hybrid genotypes differed significantly in their ability to transmit each virus, indicating that in addition to a major gene, minor genes can affect the transmission of each virus independently. Gut and salivary gland associated transmission barriers were identified in the nonvector parent and some progeny, while other progeny possessed only a gut barrier or a salivary gland barrier. Hemolymph factors do not appear to be involved in determining the transmission phenotype. These results provide direct evidence that aphid transmission of luteoviruses is genetically regulated in the insect and that the tissue-specific barriers to virus transmission are not genetically linked.

ABSTRACT Lepidopteran cells (Spodoptera frugiperda) produced isometric virus-like particles (VLP) when infected with a recombinant baculovirus Ac61 that contained the Potato leafroll virus (PLRV) coat protein gene modified with an N-terminal histidine tag (P3-6H). Cells infected with AcFL, a recombinant baculovirus that expressed cDNA copies of the PLRV genome RNA, did not produce virus-like particles (VLP). In cell lines doubly infected with Ac61 and AcFL, VLP were formed that contained PLRV-RNA packaged in P3-6H coat protein (FL). Both the P3-6H and the FL particles were morphologically indistinguishable from particles of PLRV despite the fact that they lacked the P5 readthrough protein present in wild-type PLRV. When aphids (Myzus persicae) were fed on, or injected with, purified PLRV, or VLP of either type (FL or P3-6H) and examined by electron microscopy, no differences were observed among treatments for particle endocytosis, transcellular transport, or exocytosis at the aphid midgut or accessory salivary glands. Particles were observed in the salivary canals and in the salivary duct leading out of the aphid. These results suggest that P5 readthrough protein of PLRV may not be essential for cellular transport of virus through aphid vectors.

ABSTRACT Virus isolates from forage legumes collected from eight different states were identified as luteoviruses closely related to soybean dwarf luteovirus dwarfing (SbDV-D) and yellowing (SbDV-Y) described in Japan. All isolates produced reddened leaf margins in subterranean clover and were transmitted in a persistent manner by Acrythosiphon pisum, but not by Aulacorthum solani. Specific monoclonal antibodies raised against SbDV-Y were differentially reactive with endemic isolates. Immunoblots probed with a SbDV-D polyclonal antiserum showed single 26-kDa coat protein bands, confirming close serological relatedness to SbDV. Analyses of genomic and subgenomic double-stranded RNAs and northern blot analyses confirmed genomic relatedness to SbDV. Based on our results, we conclude that the U.S. luteovirus isolates studied comprise a strain or strains of the soybean dwarf virus that have clovers as common hosts and the pea aphid as a common vector.