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Severity assessment for soybean rust.

Presenter: Monte R. Miles, USDA-ARS
Coauthor(s): M. R. Miles1, I. Rosenblatt2, P. Traynor2 and G. L. Hartman1,2. 1USDA-ARS, National Soybean Research Center, University of Illinois, Urbana, IL 61801; 2Dept. of Crop Sciences, National Soybean Research Center, University of Illinois, Urbana, IL 61801

The accurate assessment of soybean rust, Phakopsora pachyrhizi, is needed by both fungicide efficacy and germplasm evaluation programs. The small size of soybean rust pustules makes them difficult to count or measure in field evaluations. An evaluation of individual leaflets should provide an accurate assessment of the difference among treatments or genotypes. To ensure repeatable evaluation, a set of images representing the range of soybean rust severity was developed with a combination of APS Assess and Adobe® Photoshop®. A hand-drawn leaf outline with points the size of soybean rust lesions was scanned and imported into Adobe® Photoshop®, where the color and number of points were edited. These images were then evaluated in APS Assess to obtain the percent of the leaf area covered and the number of points present. Images were reworked in Photoshop® to add or decrease the number of lesions to obtain the desired percent coverage and pustule number. Several cycles of evaluation and editing were needed to develop an accurate representation of each severity level. Images were developed for 30 lesions (0.1% surface coverage), 75 lesions (0.25% surface coverage), 150 lesions (0.5% surface coverage), 300 lesions (1.0% surface coverage), 750 lesions (2.5% surface coverage), 1,500 lesions (5% surface coverage) and 3,000 lesions (10% surface coverage). Images with 20, 30, 40 and 50% of the surface covered were also produced, but lesions were not counted.

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