APS Abstracts of Presentations

The impact of foliar diseases of soybean in Iowa during the 2005 growing season
A. E. ROBERTSON (1), F. W. Nutter (1), P. D. Esker (1), J. M. Shriver (1), S. S. Navi (1)
(1) Iowa State University, Ames, IA, USA
Phytopathology 96:S98

Foliar diseases of soybean are not considered economically important in Iowa. Consequently, local soybean growers do not currently make management decisions regarding the use of foliar fungicides. The arrival of Asian soybean rust in the United States has resulted in increased awareness and greater interest in the use of foliar fungicides. The seasonality, incidence and severity of foliar disease during the 2005 growing season in Iowa were determined in a soybean disease survey. At each of 2 to 5 fields per county, 30 plants were sampled at four growth stages. The presence and severity of each disease for each leaf on the main stem was assessed. The most prevalent fungal diseases (pathogens) were brown spot (Septoria glycines), frogeye leaf spot (Cercospora sojina), Cercospora leaf blight (Cercospora kikuchii) and downy mildew (Peronospora manshurica). The mean disease severity within fields over the season, ranged from 0.1% to 0.5% for brown spot, 0.1% to 12.8% for frogeye leaf spot, and 0.1% to 3.6% for downy mildew. The mean disease incidence (within fields) for Cercospora leaf blight ranged from 3–83%. Thus, foliar disease management in Iowa was probably unnecessary during the 2005 growing season. This statement is justified by additional data from fungicide efficacy trials in which fungicides did not result in significantly (P < 0.05) greater yields compared with unsprayed controls.

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