EVALUATION AND MODELING OF RATE-REDUCING RESISTANCE OF SOYBEAN SEEDLINGS TO PHYTOPHTHORA-SOJAE

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 (LSD0.05 = 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 (LSD0.05 = 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 (LSD0.05 = 0.67).