Tolerance to Phytophthora Rot in Soybean: II. Evaluation of Three Tolerance Screening Methods
- B. A. McBlain *,
- M. M. Zimmerly and
- A. F. Schmitthenner
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.
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