Andrew L. Stone

Plum Pox in North America: Identification of Aphid Vectors and a Potential Role for Fruit in Virus Spread

ABSTRACT Thirteen aphid species were tested for their ability to transmit Pennsylvania isolates of Plum pox virus (PPV) collected in Columbia (PENN-3), Franklin (PENN-4), and York (PENN-7) Counties, PA. Four species, Aphis fabae, A. spiraecola, Brachycaudus persicae, and Myzus persicae, consistently transmitted PPV in preliminary transmission tests. Two species, Metopolophium dirhodum and Rhopalosiphum padi, were occasional inefficient vectors. Toxoptera citricida, from Florida, also was an effective vector but it does not occur in major stone-fruit-growing states. Species not transmitting PPV in parallel tests included Acyrthosiphon pisum, Aphis glycines, Aulacorthum solani, Macrosiphum euphorbiae, Rhopalosiphum maidis, and Sitobion avenae. When given a 3-day probing access period simultaneously on PPV-infected peach seedlings and healthy peach seedlings, Myzus persicae, Aphis spiraecola, A. fabae, and B. persicae transmitted PPV to 63, 31, 38, and 32% of the healthy peach seedlings, respectively. When given a similar probing period on PPV-infected peach fruit and healthy peach seedlings, the same aphid species transmitted PPV to 50, 35, 0, and 0% of seedlings, respectively. Results support the hypothesis of secondary PPV spread by indigenous aphids in Pennsylvania, and suggest that PPV-infected fruit has the potential to function as a virus source for long-distance dispersal.

Genome Assembly of Citrus Leprosis Virus Nuclear Type Reveals a Close Association with Orchid Fleck Virus

ABSTRACT The complete genome of citrus leprosis virus nuclear type (CiLV-N) was identified by small RNA sequencing utilizing leprosis-affected citrus samples collected from the state of Querétaro, Mexico. The nucleotide identity and phylogenetic analysis indicate that CiLV-N is very closely related to orchid fleck virus, which typically infects Cymbidium species.

Identification, characterization, and relatedness of luteovirus isolates from forage legumes.

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.

Molecular, Ultrastructural, and Biological Characterization of Pennsylvania Isolates of Plum pox virus

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.

Molecular analysis of soybean dwarf virus isolates in the eastern United States confirms the presence of both D and Y strains and provides evidence of mixed infections and recombination.

Soybean dwarf virus (SbDV), first identified as an agricultural problem in Japan, has emerged as a growing problem in the Midwestern United States. The majority of research on SbDV had been limited to four lab maintained strains from Japan. SbDV had been found in clover in the eastern United States, but these isolates rarely emerged into soybeans. These isolates were analyzed by multiplex PCR and sequencing, revealing that some were infections of both Y and D components, including a recombinant subisolate. Phylogenetic analyses for the US isolates revealed a broad diversity of SbDV, with selection pressure greater on the movement protein than the coat protein. The field isolates from the Eastern United States showed differences in symptoms, aphid transmission and host range, demonstrating that a study of field isolates is an important complement to laboratory maintained strains in understanding the biology and evolution of plant viruses.

Complete Nucleotide Sequence of a Novel Hibiscus-Infecting Cilevirus from Florida and Its Relationship with Closely Associated Cileviruses

ABSTRACT The complete nucleotide sequence of a recently discovered Florida (FL) isolate of hibiscus-infecting cilevirus (HiCV) was determined by Sanger sequencing. The movement and coat protein gene sequences of the HiCV-FL isolate are more divergent than other genes of the previously sequenced HiCV-HI (Hawaii) isolate.

Host adaptation of soybean dwarf virus following serial passages on pea (Pisum sativum) and soybean (Glycine max)

Soybean Dwarf Virus (SbDV) is an important plant pathogen, causing economic losses in soybean. In North America, indigenous strains of SbDV mainly infect clover, with occasional outbreaks in soybean. To evaluate the risk of a US clover strain of SbDV adapting to other plant hosts, the clover isolate SbDV-MD6 was serially transmitted to pea and soybean by aphid vectors. Sequence analysis of SbDV-MD6 from pea and soybean passages identified 11 non-synonymous mutations in soybean, and six mutations in pea. Increasing virus titers with each sequential transmission indicated that SbDV-MD6 was able to adapt to the plant host. However, aphid transmission efficiency on soybean decreased until the virus was no longer transmissible. Our results clearly demonstrated that the clover strain of SbDV-MD6 is able to adapt to soybean crops. However, mutations that improve replication and/or movement may have trade-off effects resulting in decreased vector transmission.

First report of a Cilevirus associated with green ringspot on senescent hibiscus leaves in Tampa, Florida

A non-systemic cilevirus producing green ringspot symptoms in ornamental hibiscus (Hibiscus rosa-sinensis), was first reported from Hawaii, USA in 2013, and was named hibiscus infecting Cilevirus (HiCV) (Melzer et al., 2013). The false spider mite Brevipalpus yothersi is associated with symptomatic hibiscus but its role as a vector of HiCV has not yet been demonstrated. Positive sense bi-partite HiCV genome shared 86.2% and 80.0% nucleotide identity with the RNA1 and RNA2 of cytoplasmic citrus leprosis virus -2 (CiLV-C2), respectively (Roy et al., 2013a). Both CiLV-C and -C2 Cileviruses were reported from Colombia but the most prevalent one is CiLV-C2 (Roy et al., 2013a). The 92% amino acid identity between the full genome of these two viruses suggests that HiCV could be a strain of CiLV-C2, but it is not clear if HiCV is capable of infecting and causing symptoms in citrus (Melzer et al., 2013). Recently, we reported natural infection of Hibiscus by CiLV-C2 in Colombia that produced green ring lesions wit...