Incidence of Soybean dwarf virus and Identification of Potential Vectors in Illinois.

Soybean dwarf virus (SbDV), which causes an important disease of soybeans in Japan, is persistently transmitted by aphids and is endemic in forage legumes in the United States. To determine the incidence of SbDV in Illinois, we collected clovers and forage legumes in a total of 49 Illinois counties in 2001 and 2002 and tested them for the presence of SbDV by reversetranscription-polymerase chain reaction. SbDV was detected in 43% of red clover (Trifolium pratense), 10% of white clover (T. repens), and 3% of yellow sweet clover (Melilotus officinalis) plant samples. The dwarfing strain (SbDV-D) was the predominant strain detected in Illinois. In 2000, Aphis glycines, an aphid species that colonizes soybeans, was reported for the first time in North America. To determine whether A. glycines or aphid species found colonizing clover were vectors of SbDV, transmission studies were conducted. Aphids of the species Nearctaphis bakeri reproducibly vectored SbDV among red clovers, and from red clover to soybean. A. glycines did not transmit SbDV; neither did two other clover-infesting aphid species, Acyrthosiphon pisum and Therioaphis trifolii.

Nucleotide sequence shows that Bean leafroll virus has a Luteovirus-like genome organization.

The complete nucleotide sequence of the Bean leafroll virus (BLRV) genomic RNA and the termini of its smallest subgenomic RNAs were determined to better understand its mechanisms of gene expression and replication and its phylogenetic position within the Luteoviridae: The number and placement of open reading frames (ORFs) within the BLRV genome was Luteovirus-like. The nucleotide and predicted amino acid sequences of BLRV were most similar to those of Soybean dwarf virus (SbDV). Phylogenetic analyses employing the neighbour-joining method and sister-scanning analysis indicated that the BLRV nonstructural proteins were closely related to those of Barley yellow dwarf virus-PAV (BYDV-PAV), a luteovirus: The region surrounding the frameshift at the junction between ORFs 1 and 2 also contained sequences very similar to those of BYDV-PAV and a Dianthovirus, Red clover necrotic mosaic virus. Similar analyses showed that the structural proteins were most similar to those of the Polerovirus genus. The 3'-noncoding regions downstream of ORF5 contained sequences similar to translational control elements identified in the BYDV-PAV genome. These data suggest that BLRV, like SbDV, is derived either through selection from a common ancestor with BYDV-PAV or that BLRV is the product of two recombination events between luteovirus-like and polerovirus-like ancestors where the 5' 2900 nt and 3' 700 nt of the BLRV genome are from a Luteovirus and the intervening sequences are derived from a Polerovirus:

Symptomatology of Barley Yellow Dwarf Virus-RMV Infection in Sweet Corn.

Infection of sweet corn (Zea mays L.) by barley yellow dwarf viruses (BYDVs) caused different symptoms on hybrids with shrunken-2 (sh-2) when compared with hybrids with sugary-1 (su-1) endosperm mutations. Sweet corn hybrids inoculated with BYDV-RMV in Urbana, IL, developed either yellow or red-purple leaf symptoms similar to those caused by phosphorus and/or potash deficiency (1). In a field trial near Rochelle, IL, 329 of 339 sweet corn hybrids naturally infected by BYDV exhibited one or the other of these symptoms. Eighteen of the hybrids with severe symptoms were tested for two strains of BYDVs in enzyme-linked immunosorbent assays (1). All of the hybrids tested were positive for BYDV-RMV; none were positive for BYDV-PAV. Symptoms on su-1 hybrids were typically characterized by dark red to purple coloration of leaf margins from the tip down to the mid-leaf. Leaves of sh-2 hybrids exhibited chlorosis in a similar pattern. A single sh-2 hybrid exhibited red-purple leaf symptoms, probably as a result of a crossover between the anthocyanin production (a1) and sh-2 genes, which are linked about 0.25 map units apart on chromosome 3 (2). Infection of sh-2 sweet corn hybrids by BYDV-RMV is most likely to induce chlorosis, whereas infection of su-1 sweet corn hybrids can result in either chlorosis or red-purple discoloration of leaves. Sweet corn hybrids with these symptoms should be tested for BYDVs, particularly the RMV strain. References: (1) R. L. C. Itnyre et al. Plant Dis. 83:566, 1999. (2) E. B. Mains. J. Hered. 40:21, 1948.

Reaction of Sweet Corn to Inoculation with Barley Yellow Dwarf Virus RMV-IL.

The reaction of five sweet corn hybrids to barley yellow dwarf virus (BYDV-RMV-IL) was determined in 1992 and 1993. In 1992, symptoms were observed in three of the five hybrids planted 20 May and four of the five hybrids planted 20 June. No symptoms were observed in hybrids planted June or July 1993. The mean virus incidences of RMV-IL determined by enzyme-linked immunosorbent assay (ELISA) in all plots for the May and June 1992 planting dates were 3.5 and 21%, respectively. The mean virus incidence for the inoculated plots for the June 1992 planting date was significantly higher than incidence for the control plots (29 versus 13%). Ear weights were significantly lower for inoculated plots than for the control plots (1.2 kg versus 1.4 k/10 ears). The mean virus incidences of RMV-IL in all plots for the June and July 1993 planting dates were 31 and 23%, respectively. The mean virus incidence for the inoculated plots for the June 1993 planting date was significantly higher than incidence for the control plots (49 versus 14%). Plots inoculated in June 1993 also had a significantly lower ear weight than the control plots (1.8 kg versus 2.0 kg/10 ears). A range of symptoms, infection levels, and yield responses of sweet corn hybrids to BYD-RMV-IL was demonstrated in this study. Hybrid susceptibility to this strain of BYDV should be considered when planting sweet corn from middle to late June in order to limit potential yield losses.

Comparison of Techniques for Detection of Barley Yellow Dwarf Virus-PAV-IL.

Detection of barley yellow dwarf virus (BYDV)-PAV-IL by an improved nucleic acid hybridization technique, using a nonradioactive probe with chromogenic and chemiluminescent substrates, was compared with detection by polymerase chain reaction (PCR), double antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA) with polyclonal antibodies, and triple antibody sandwich ELISA with polyclonal and monoclonal antibodies. Each method was used to detect purified virus and virus in sap extracts from infected oat leaves. The detection limits for both ELISA procedures were 1 ng of purified BYDV-PAV-IL and the equivalent of 78 ng of infected tissue. Nucleic acid hybridization with either chemiluminescent or chromogenic substrates also detected as little as 1 ng of purified BYDV-PAV-IL, but it was slightly more sensitive at detecting virus in tissue extracts (25 ng of infected tissue). The most sensitive detection technique was PCR amplification, which could detect as little as 0.1 pg of RNA extracted from purified virus and detected viral RNA in the equivalent of 0.5 pg of infected leaf tissue.