Effects of fungicide timing and active ingredient on soybean aphid fungal epizootics

Soybean aphid, Aphis glycines Matsumura, and Asian soybean rust, Phakopsora pachyrhizi Sydow, are invasive soybean pests causing economic losses in soybean production in North America. Understanding how foliar applied fungicides targeting Asian soybean rust will impact fungal pathogens of soybean aphid is important in developing effective integrated pest management strategies. In 2007, at Becker, MN two independent trials were conducted: one during early reproductive (R2) stage soybean and a second during late reproductive (R5) stage. Effects of fungicide application timing with respect to release of an aphid pathogen, Pandora neoaphidis, and how either a two active ingredient (AI) or single AI treatment regime affected aphid infection was explored using single-plant microplots. Single plants were caged to exclude predators and aphids were released and allowed to increase for two weeks prior to adding inoculum. Aphids infected with P. neoaphidis were released into cages as infected cadavers and disease prevalence in apparently healthy aphids was measured at intervals following fungicide treatment compared to an untreated control. When treatments consisted of two AIs (pyraclostrobin + tebuconazole), aphid disease prevalence was significantly depressed compared to a single AI (pyraclostrobin) or an untreated control in both trials. During the early reproductive stage experiment, plots treated with fungicide before the release of P. neoaphidis significantly reduced cumulative disease prevalence. Thus, fungicide applications with two AIs made prior to the arrival of P. neoaphidis are likely to lead to the largest depression in aphid disease prevalence. No aphid population response was observed in this study.