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Development of soybean rust on indeterminate vs. determinate soybean

Presenter: Gary L. Cloud, Syngenta Crop Protection
Coauthor(s): Marty Wiglesworth, Agnelo Vitti and W.A. Harrell, Syngenta Crop Protection, Greensboro, NC

Extension research has been conducted in understanding the management of Asian soybean rust (SBR), Phakopsora pachyrhizi worldwide. However, little research has been conducted evaluating the development of SBR on indeterminate soybean types. In 2005, research trials were conducted in Brooks Co. GA evaluating the development patterns of SBR on indeterminate vs. determinate soybean varieties. Soybean varieties chosen for the experiments in the U.S. included were Pioneer 95B32 and 94M90; both being considered within the maturity group 5.4. The experimental design for all trials were a split-plot design with 4 replicants per treatment. Assessments of SBR were done weekly by sampling 8 leaves per plot from the mid-canopy and upper-canopy of the soybean plant. Results indicate within 4 weeks after SBR was first found SBR infections were readily noticeable on the upper leaves (9.77% severity) of the determinate variety while upper leaves of the indeterminate variety had much less (3.97% severity). Age of leaf possibly is the determinant for susceptible or symptom expression of SBR. Although the upper canopy of the indeterminate variety showed little disease, significant disease and defoliation was occurring in the mid and lower-canopy (17.69% severity and 20.66%, respectively). Based on these findings and the fact that a majority of the soybean production in the U.S. is indeterminate soybeans, scouting of individual fields to assess SBR levels for determination of fungicide application timings will be challenging.

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