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38

Urediniospore release and escape from rust-infected soybean fields

Presenter: L. F. Beck

All authors and affiliations: L. F. BECK (1), M. R. Miles (1), T. A. Steinlage (2), and G. L. Hartman (1,2). (1) USDA-ARS, National Soybean Research Center, and (2) Department of Crop Sciences, National Soybean Research Center, University of Illinois

Phakopsora pachyrhizi, soybean rust, has been infecting soybeans in the continental United States from 2004 to 2006. This study monitored the release, dispersal, and germination rates of soybean rust spores produced from epidemics at various stages of development using rotorods, passive traps, and water agar plates, respectively, at three locations for 610 days each in 2005. Rotorod samples were collected daily at 2-h intervals from 08:00 to 18:00, with each sampling interval lasting 30 min. Passive trap samples were collected at 24-h intervals. Spores were collected onto water agar plates to estimate germination rates in the lower, mid, and upper canopy at 10:00, 14:00, and 18:00. Enumerating spores on rods indicated a typical pattern of spore release and escape over the course of a day. In the morning, before dew dried, spore release was minimal. However, spore release increased after leaves dried, peaked during midday, and then tapered off toward the evening. Rainfall events drastically reduced spore release for a 24- to 48-h period. Spores enumerated on passive trap slides indicated that spores dispersed in the direction of prevailing winds at lower-than-expected rates (3 of 22 observation days). Spores collected on water agar plates germinated at rates ranging from 0.8 to 99%, with an average of 52%. Though varying widely with epidemic severity, crop development, and environmental factors, daily spore release patterns, escape, and dispersal of spores from the canopy and germination rates are important in developing and improving forecasting models that predict soybean rust outbreaks.

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