The objectives of this research were to understand interactions between Phomopsis spp. and seedborne viruses of soybean and study the role that insect vectors may have increasing
Phomopsis infection. In Iowa, Phomopsis spp., BPMV and SMV have been prevalent in some
years, depending on weather conditions during key periods. Recently the frequent detection of
Phomopsis spp. in stems coincided with a resurgence in bean leaf beetle (Cerotoma trifurcata
Förster), populations and BPMV symptoms. Results included in this thesis correspond to different studies designed to evaluate the effect of Bean pod mottle virus and Soybean mosaic
virus on susceptibility of soybean plants to infection by Phomopsis spp., and the impacts of
combined management practices currently used on soybean production.
Data from greenhouse experiments show that BPMV can increase susceptibility to seed infection by P. longicolla in plants of two different cultivars, in which virus infection did not induce the same response. Inoculated plants of the cultivar 92M02 displayed typical BPMV foliar symptoms, seed coat mottling and a delay in maturity, while in Spansoy 201 only foliar symptoms were observed. However, BPMV infection enhanced P. longicolla seed infection in both cultivars. Therefore, we concluded that BPMV induced predisposition to P. longicolla seed infection and this effect is not due solely to prolonging seed maturation. Unlike previous studies, the effect of BPMV on incidence of P. longicolla seed infection observed in this study was completely independent from the effects that beetle vectors of BPMV can have in pod and seed infection by Phomopsis spp. 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 tested (Colfax and Spansoy 201), which suggested that SMV- P. longicolla relationship may be cultivar- and strain-dependent.
During the years when this research was conducted, Iowa experienced extremely severe winters; and predictive models have suggested the occurrence of very high bean leaf beetle winter mortality and low risk for BPMV incidence. Moreover, in 2008 a low incidence of seed infection by Phomopsis spp. was observed in some of the experiments. This low disease pressure could have been a consequence of the late planting due to the heavy early season rain, which caused the period of maximum susceptibility of seeds to occur under dry and cool conditions. In general, these situations may have obscured treatment effects.
Results from field experiments show that applications of fungicides and insecticides reduced disease incidence, insect populations and their negative effects on soybean plants. In studies aimed to evaluate the impact of insect vector management strategies on infection of seedborne viruses and Phomopsis spp., insecticide applications reduced beetle feeding injury of leaves and pods and plant exposure to soybean aphids. These treatments in combination with other management strategies also reduced seed and stem infection by BPMV and Phomopsis spp., respectively. In addition to the known effect that feeding injury of pods has on reducing seed quality, our results suggest that bean leaf beetle may also increase secondary stem infection by Phomopsis spp. However, this study found no relationship between insect pod injury and seed infection by Phomopsis spp. Low beetle populations associated with the severe winters experienced lately may have limited the impact of insect management strategies on interactions with Phomopsis spp. On the other hand, although populations of C. trifurcata were low in both years and seedcoat mottling was not observed, samples from all treatments tested positive for BPMV at a low incidence level. The use of virus resistance or insecticide treatments alone was
ineffective for reducing BPMV incidence of seeds compared with controls. However, when these strategies were combined, BPMV incidence was significantly reduced, suggesting that resistance mechanisms should be combined with chemical treatments in vector-virus management programs to enhance individual control effects. The BPMV-tolerant cultivar used in 2008 had reduced BPMV incidence, but its agronomic performance was poor. This emphasizes the need for incorporating virus resistance traits into high-yielding adapted cultivars. In general, management techniques aimed to control soybean aphids did not affect Phomopsis spp. infection, and we did not observe any evidence that aphid colonization of soybeans increases susceptibility to Phomopsis infection, or that A. glycines management strategies have any added benefit related to Phomopsis spp.
Even though timing was earlier than that previously recommended for Phomopsis control, current practices involving fungicide applications at growth stage R3 or R5 targeted to control foliar and stem diseases, can have some benefits on seed quality by reducing Phomopsis infection of stems or seeds. In addition, when this approach was complemented with insecticide applications, yield enhancement was observed. This effect might be related to suppression of aphid populations and infection by Phomopsis spp. or other fungal diseases. Therefore, it appears that applications targeted against soybean aphid and foliar diseases can have an added benefit by reducing Phomopsis spp. infection.
However, caution should be taken when implementing these results. Compared with untreated controls, applications of fungicides and insecticides reduced disease incidence and insect injury. These effects did not always result in an increase in yield, so the economic justification for these applications may not have been sufficient. The need and effectiveness of these strategies depends on environmental conditions, disease pressure and insect population
dynamics; therefore, they should be taken into consideration while designing management programs.