VIEW ARTICLE

Resistance

Evaluation and Modeling of Rate-Reducing Resistance of Soybean Seedlings to Phytophthora sojae. R. E. Wagner,Department of Plant Pathology, University of Illinois, Urbana, IL 61801; S. G. Carmer(2), and H. T. Wilkinson(3). (2)Department of Agronomy, University of Illinois, Urbana 61801; (3)Department of Plant Pathology, University of Illinois, Urbana, IL 61801. Phytopathology 83:187-192. Accepted for publication 26 October 1992. Copyright 1993 The American Phytopathological Society. DOI: 10.1094/Phyto-83-187.

Lesion length and rate of lesion expansion on taproots of aeroponically grown soybeans infected with Phytophthora sojae provided an accurate assessment of rate-reducing resistance. Four days after inoculation with race 3 of P. sojae (1,000 zoospores/plant), mean lesion lengths for the cultivars Corsoy, Sloan, Cumberland, Williams, Asgrow 3127, Agripro 26, and Asgrow 2575 were 8.7, 8.6, 7.0, 7.0, 7.7, 5.2, and 4.0 cm, respectively (LSD_0.05 = 0.84). Linear spline regression models consisting of two intersecting straight lines with slopes B₁ and B₂ were used to describe lesion expansion over a 14-day period for each cultivar. Coefficients of determination (R²) for the selected models ranged from 0.98 to 0.99. Estimates of B₁ among the cultivars did not differ significantly. Estimates of B₂ for the seven cultivars were 1.2, 1.1, 0.9, 0.8, 0.6, 0.4, and 0.1 cm/day, respectively (LSD_0.05 = 0.29). Estimates of B₂ provided an accurate description of rate-reducing resistance. The points where the two lines intersect (join points) for the seven cultivars occurred at 2.1, 2.0, 1.8, 1.7, 2.2, 1.1, and 1.3 days after inoculation, respectively (LSD_0.05 = 0.67).