Tolerance to Phytophthora Rot in Soybean: I. Studies of the Cross ‘Ripley’ ✕ ‘Harper’

The pathogen Phytophthora megasperma Drechs. f. sp. glycinea T. Kuan & D.C. Erwin (Pmg) was shown to be the causative agent of a seed rot, damping off, and root and stem rot of susceptible cultivars of soybean [Glycine max (L.) Merr.]. Genetic control Ping has been partitioned into two classes: hypocotyl resistance and tolerance. The hypocotyl-susceptible ‘Ripley’ was observed to have a very tolerant reaction to Pmg in the field and in Race 4 slantboard tests in 1984. This study examined inheritance of tolerance in the cross Ripley ✕ ‘Harper’ (a caltivar with a low level of tolerance), evaluated Ripley as a donor parent of Pmg tolerance, and determined the value of the slant-beard test on single plants. Tolerance to Pmg (using the slant board with Race 4) was inherited quantitatively (h²= 54%). The slant-board test had single-plant escapes that reduced ability to select superior F₂ plants. It was a sensitive test if 20 or more plants were tested per line. Slant-beard and field tolerance data did not correlate as well as expected (r = 0.34, P < 0.01). Eighteen percent of the F₂ lines from this cross were no inter than Ripley and at least as tolerant as Harper based on field tolerance scores. Less than 1% of the F₂ lines were as early or earlier than Harper and as tolerant as Ripley. Ripley is not a good donor parent for Midwestadapted caltivars tolerant to all prevalent races. One line was identiffed which has earliness, indeterminate stem termination, and Riple's tolerance for Races 1,3, and 4. Salaries and research support provided by state and federal funds appropriated to the Ohio Agric. Res. and Development Ctr., the Ohio State Univ. Additional funding provided as a gift by the Ohio Seed Improvement Assoc. Part of thesis submitted by second author in partial fulfillment of M.S. degree.