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Suppression of entomopathogens of soybean aphid by foliar fungicides.

Presenter: James E. Kurle, Department of Plant Pathology, University of Minnesota
Coauthor(s): K. Koch1, D. Ragsdale1, B. Potter2. 1Dept. of Entomology, University of Minnesota, St. Paul, MN 55108. 2University of Minnesota, Southwest Research and Outreach Center, Lamberton, MN 56152.

Entomopathogenic fungi (EF) are important natural enemies suppressing soybean aphid, Aphis glycines, populations. Effects of EF may include direct effects on aphid density on soybean or effects on aphid survival on overwintering hosts. Effects of Bravo, Quilt, Stratego, Headline, or Headline+Folicur, and Headline+Folicur+Bravo on mycoses of the soybean aphid were investigated in 2005 at two locations in Minnesota. Initial fungicide applications were made at growth stage R1-R2 and subsequent application at 14-day intervals. Aphid density, prevalence of mycoses, and species composition of EF infecting soybean aphid were sampled during plant growth stages R3 through R5. Environment and aphid density strongly influenced the prevalence of EF. Peak prevalence never exceeded 5% at Rosemount. At Lamberton an epizootic developed which reached its maximum level on 30 August with 31% prevalence in the untreated plots, 6% prevalence in the plots treated with Headline, Folicur, and Bravo, 5% in the plots treated with Quilt, 2% prevalence in the plots treated with Stratego, and 1% in plots treated with Headline and Folicur. The prevalence of EF-infected aphids was reduced by all fungicide applications at Lamberton. The relationship between EF prevalence and aphid numbers was inconsistent; i.e. the greatest reduction in prevalence was not associated with the highest aphid populations or greatest frequency of fungicide application. When an epizootic occurred, all fungicide application delayed onset and suppressed overall prevalence of EF. If fungicide application reduce prevalence of EF-infected aphids in late fall, overwintering populations may be larger, and have higher survival on overwintering hosts.

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