Tolerance to Phytophthora Rot in Soybean: II. Evaluation of Three Tolerance Screening Methods

Five screening methods for tolerance to Phytophthora megasperma f. sp. g1ycinea (Drechs.) T. Kuan & D.C. Erwin ( Pmg ) in soybean [ Glycine max (L.) Merr.] have been published. This study compares three of these methods. The 45 soybean strains used were reported susceptible to Race 4 of Pmg . Field tests were grown in 1985 and 1986 near Vickery, OH, on Fulton silty‐clay (fine, illitic, mesic Aeric Ochraqualt) known to be infested with Race 4 and other races of Pmg . Inoculum‐layer and slant‐board tests were conducted in a greenhouse and growth chamber, respectively, in 1986 and 1987. Height and maturity data were obtained from replicated yield tests with low Pmg pressure. The inoculum‐layer test gives tolerant lines high scores, whereas the slant board and field tests give low values to tolerant lines. The inoculum‐layer and slant board data correlated well ( r = ‐0.77, P < 0.01) but neither correlated as well with the field data ( r = ‐0.41, P < 0.05; and r = 0.49, P < 0.01, respectively). Field tolerance scores were correlated with maturity ( r 0.46, P < 0.01); however, the field scores of the 15 latest maturing strains were not correlated with maturity and correlated well with the inoculum‐layer and slant board scores ( r = ‐0.74, P < 0.01; and r = 0.76, P < 0.01, respectively). Apparently, field scores must be taken prior to onset of pod yellowing (R7). Both laboratory methods are suitable substitutes for the field test, but they evaluate seedling tolerance only. The inoculum‐layer method is easier, less expensive, and allows both root rot ratings and tolerance scores based on shoot growth.