Impacts of foliar fungicides on infection of soybean by Phomopsis spp. in Iowa, USA

Fungicides pyraclostrobin (strobilurin) and tebuconazole (triazole) were applied to soybean [ Glycine max  (L.) Merr.] at growth stages R3, R5 or R3 + R5, in 2008 and 2009 at two locations in Iowa. Incidence of infection of stems and seeds by  Phomopsis  spp. was evaluated, along with yield and seed quality. Stem infection by  Phomopsis  spp. was reduced in both years by pyraclostrobin applied at R3 + R5, and in 2008 by pyraclostrobin at R5, by approximately three-fold compared to the untreated control. In 2009, treatments including applications of tebuconazole at R3 and pyraclostrobin at R5 significantly reduced infection of seed by  Phomopsis  spp., by approximately two-fold compared to the untreated control. Only the application of pyraclostrobin at R3 + R5 reduced both stem and seed infection by  Phomopsis  spp. in 2009. None of the treatments had a significant effect on yield, or seed quality, evaluated by warm and cold germination tests. Seed infection by  Phomopsis  spp. was negatively correlated with seed quality. Fungicides applied at these growth stages can have an impact on infection by  Phomopsis  spp., but their effectiveness varies with environment and disease intensity.