The efficacy of fungicides in managing soybean rust was evaluated in 12 environments in South America and southern Africa over three growing seasons from 2002 to 2005. There were differences in final soybean rust severity, defoliation, and yield among the treatments at most locations. In locations where soybean rust was not severe, all the fungicides evaluated reduced severity. In locations where soybean rust was severe, applications of triazole and triazole + strobilurin fungicides resulted in lower severity and higher yields compared with other fungicides. The strobilurin fungicides provided the highest yields in many locations; however, severity tended to be higher than that of the triazole fungicides. There also were differences in yield and severity between the trials with two and three applications of several fungicides, with three applications resulting in less severe soybean rust and higher yields. However, the third application of tebuconazole, tetraconazole, and the mixtures containing azoxystrobin and pyraclostrobin was not needed to maintain yield. These fungicides were among the most effective for managing soybean rust and maintaining yield over most locations.

The variability of North American and Asian strains and isolates of Soybean mosaic virus was investigated. First, polymerase chain reaction (PCR) products representing the coat protein (CP)-coding regions of 38 SMVs were analyzed for restriction fragment length polymorphisms (RFLP). Second, the nucleotide and predicted amino acid sequence variability of the P1-coding region of 18 SMVs and the helper component/protease (HC/Pro) and CP-coding regions of 25 SMVs were assessed. The CP nucleotide and predicted amino acid sequences were the most similar and predicted phylogenetic relationships similar to those obtained from RFLP analysis. Neither RFLP nor sequence analyses of the CP-coding regions grouped the SMVs by geographical origin. The P1 and HC/Pro sequences were more variable and separated the North American and Asian SMV isolates into two groups similar to previously reported differences in pathogenic diversity of the two sets of SMV isolates. The P1 region was the most informative of the three regions analyzed. To assess the biological relevance of the sequence differences in the HC/Pro and CP coding regions, the transmissibility of 14 SMV isolates by Aphis glycines was tested. All field isolates of SMV were transmitted efficiently by A. glycines, but the laboratory isolates analyzed were transmitted poorly. The amino acid sequences from most, but not all, of the poorly transmitted isolates contained mutations in the aphid transmission-associated DAG and/or KLSC amino acid sequence motifs of CP and HC/Pro, respectively.

Soybean mosaic virus (SMV) is an aphid- and seed-transmitted virus that infects soybean (Glycine max) plants and causes significant yield losses. Seed-borne infections are the primary sources of inoculum for SMV infections. The strain specificity of SMV transmission through seed and SMV-induced seed-coat mottling were investigated in field experiments. Six soybean plant introductions (PIs) were inoculated with eight SMV strains and isolates. Transmission of SMV through seed ranged from 0 to 43%, and isolate-by-soybean line interactions occurred in both transmission rates and percentages of mottled seeds. For example, SMV 746 was transmitted through 43% of seed in PI 229324, but was not transmitted through seed of PIs 68522, 68671, or 86449. In contrast, SMV 413 was transmitted through seed from all PIs. SMVs that were transmitted poorly by the Asian soybean aphid, Aphis glycines, also were transmitted poorly through seed. No predicted amino acid sequences within the helper-component protease or coat protein coding regions differentiated the two groups of SMV strains. The loss of aphid and seed transmissibility by repeated mechanical transmission suggests that constant selection pressure is needed to maintain the regions of the SMV genome controlling the two phenotypes from genetic drift and loss of function.

Soybean lines transformed with the coat protein (CP) gene of Soybean mosaic virus (SMV) were evaluated for SMV resistance by quantifying the temporal and spatial spread of SMV strain AL-5 released from a point source in the field. The temporal spread of SMV within field plots during 1999 and 2000 was quantified by enzyme-linked immunosorbent assay. The Gompertz model most appropriately described temporal spread. Two SMV CP transformed lines (genotypes) had significantly lower infection rates and significantly lower final SMV incidence values (P

Bean leaf beetles (BLB; Cerotoma trifurcata) were collected in soybean (Glycine max) fields in 58 and 99 Illinois counties surveyed during the 2000 and 2001 growing seasons, respectively. In 2000, BLB counts were highest in the central portion of the state. BLB counts were lower the following year, but were more uniformly distributed throughout the state. BLB tested positive for Bean pod mottle virus (BPMV) in 37 of 41 counties assayed in 2000. In 2001, BLB tested positive for BPMV in 86 of 99 counties sampled. In 2000 and 2001, western corn rootworm (WCR; Diabrotica virgifera virgifera) adults were abundant in soybean fields only in east central Illinois. WCR adults tested positive for BPMV in 21 of 21 east central Illinois counties in 2000 and 20 of 24 sampled in 2001. BPMV was detected in soybean plants in 38 of 46 counties sampled in 2000. Field-collected WCR adults transmitted BPMV to potted soybean plants at low rates either directly from BPMV-infected soybean fields or with prior feeding on BPMV-infected plants. This is the first report of the distribution of BLB, WCR adults, and BPMV in Illinois and of BPMV transmission by adult WCR.

Infection of soybean plants with Soybean mosaic virus (SMV), which is transmitted by aphids and through seed, can cause significant reductions in seed production and quality. Because seedborne infections are the primary sources of inoculum for SMV infections in North America, host-plant resistance to seed transmission can limit the pool of plants that can serve as sources of inoculum. To examine the inheritance of SMV seed transmission in soybean, crosses were made between plant introductions (PIs) with high (PI88799), moderate (PI60279), and low (PI548391) rates of transmission of SMV through seed. In four F(2) populations, SMV seed transmission segregated as if conditioned by two or more genes. Consequently, a recombinant inbred line population was derived from a cross between PIs 88799 and 548391 and evaluated for segregation of SMV seed transmission, seed coat mottling, and simple sequence repeat markers. Chromosomal regions on linkage groups C1 and C2 were significantly associated with both transmission of isolate SMV 413 through seed and SMV-induced seed coat mottling, and explained ≈42.8 and 46.4% of the variability in these two traits, respectively. Chromosomal regions associated with seed transmission and seed coat mottling contained homologues of Arabidopsis genes DCL3 and RDR6, which encode enzymes involved in RNA-mediated transcriptional and posttranscriptional gene silencing.

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.

Materials and Methods Results 32 Discussion 39 Acknowledgments 45 Literature cited 46