Date of Award
Master of Science
Plant Pathology and Microbiology
Gary P. Munkvold
Bean pod mottle virus (BPMV), Soybean mosaic virus (SMV), bean leaf beetles (Cerotoma trifurcata), soybean aphids (Aphis glycines) and Phomopsis spp. all affect soybean seed quality in addition to causing yield losses. However, interactions among these pests and pathogens, and the effects of combined management practices, are not well understood. Infection of soybean plants by BPMV and SMV has been reported to increase their susceptibility to seed infection by Phomopsis spp., but the mechanism of predisposition is unclear. The overall goal of this research was to better understand the interactions between these soybean viruses and Phomopsis spp., and to assess the impact of virus vector management practices on infection of soybean plants by Phomopsis spp. Effects of SMV and BPMV were studied in separate greenhouse experiments. Two soybean cultivars, Colfax (tolerant to SMV but not to BPMV) and Spansoy 201 (tolerant to BPMV but not to SMV) were mechanically inoculated with either SMV or BPMV. Cultivar 92M02 was inoculated with BPMV. Virus inoculations were followed by inoculation with P. longicolla at stages R3 or R5. Neither virus significantly increased susceptibility to stem infection by P. longicolla. In the BPMV-Phomopsis experiments, inoculation with BPMV significantly increased susceptibility of Spansoy 201 to seed infection by P. longicolla at growth stage R5, without affecting plant maturity. Susceptibility of 92M02 to P. longicolla at growth stages R3 and R5 was increased by BPMV. Plants of 92M02 displayed typical BPMV foliar symptoms, seed coat mottling and a delay in maturity. In the SMV-Phomopsis experiments, inoculation with the SMV-G2 strain did not increase the incidence of P. longicolla seed infection in either of the soybean cultivars (Colfax and Spansoy 201). These results confirm BPMV-induced predisposition to P. longicolla seed infection and indicate that the mechanism of predisposition is not due solely to prolonging seed maturation. The previously reported SMV- P. longicolla relationship was not confirmed, but this affect may be cultivar- and strain-dependent. To evaluate the effects of management strategies, four experiments were established in 6 locations in Iowa during 2008 and 2009. The impacts of bean leaf beetle management strategies on infection of seedborne BPMV and Phomopsis spp. infection were evaluated in field trials. In 2008, treatments included two soybean cultivars (BPMV tolerant and BPMV susceptible) and two insecticide treatments (treated and untreated). In 2009 treatments consisted of insecticide applications towards different bean leaf beetle generations combined with fungicide applications at growth stage R5 to control Phomopsis spp. infection. Insecticide applications reduced beetle feeding injury of leaves and pods in both years, and Phomopsis spp. stem infection in 2008. BPMV incidence was significantly reduced when a virus-tolerant genotype and insecticide applications were combined. To assess the impact of soybean aphid management tactics on seedborne SMV and Phomopsis spp. infection, stems and seeds were collected from a soybean aphid management study conducted in 2008 and 2009. None of the insecticide treatments reduced Phomopsis spp. incidence. There was no evidence of a relationship between aphid attack and Phomopsis infection. Fungicides pyraclostrobin (strobilurin) and tebuconazole (triazole) were applied at growth stages R3, R5 or R3+R5, to evaluate the effect on stem and seed infection by Phomopsis spp. Late applications of pyraclostrobin were more effective for reducing Phomopsis spp. infection of stems. In 2009, treatments including a late application of pyraclostrobin or two applications of tebuconazole (R3 and R5) were more effective for reducing Phomopsis spp. infection of seed. However, none of the treatments had a significant effect on yield, or seed quality determined by warm and cold germination tests. To assess the effects of foliar applications of fungicides and insecticides on infection by Phomopsis spp., BPMV and SMV, soybean stems and seeds were collected from field trials conducted over two years in five regions of Iowa. Treatments consisted of an untreated control and foliar applications of fungicides, insecticides or combinations at growth stage R3. In some locations fungicide applications reduced stem and seed infection, but none of the treatments reduced both stem and seed infection. Insecticide applications reduced aphid populations, and infection of seeds by SMV, Phomopsis spp. and BPMV, but in an inconsistent manner. Only the combination treatments increased yield in some locations. Results suggested that R3 applications targeted against soybean aphid and foliar diseases can have an added benefit by reducing SMV and Phomopsis spp. infection. Overall, virus incidence and beetle populations were very low in both years, and seed mottling was not observed. Although Phomopsis spp. infection of seeds was also low, it affected seed germination in some experiments. This project was the first to evaluate the effect of SMV and BPMV on susceptibility to P. longicolla infection on soybean plants under controlled conditions. This research was also the first to investigate benefits of insect management tactics for reduction of Phomopsis infection. In addition, It was shown that R3 and R5 fungicide applications targeted to control foliar and stem diseases can have some benefits on seed quality by reducing Phomopsis infection.
Jose Pablo Soto-arias
Soto-arias, Jose Pablo, "Interactions among viruses, insect vectors and the Phomopsis complex in soybean, and effects of integrated management strategies" (2010). Graduate Theses and Dissertations. 11651.