GENETIC CROSSES BETWEEN PHYSIOLOGICAL RACES OF PHYTOPHTHORA-SOJAE

Bhat, R. G., and Schmitthenner, A. F. 1993. Genetic crosses between physiologic races of Phytophthora sojae. Experimental Mycology 17, 122-129. It was not known if cross-fertilization occurred between races of Phytophthora sojae, a homothallic, diploid fungal pathogen or soybean (Glycinea max). In this study, we developed a technique to outcross P. sojae races and to select those cultures of hybrid single-oospore origin from among cultures of selfed origin. Metalaxyl- or p -fluorophenylalanine-resistant races 1, 3, and 4 and a universally avirulent culture were crossed in nine different virulence-gone combinations with a mixed inoculum technique. One hundred thirty-eight intraracial and interracial hybrid single-oospore cultures were selected directly on a double-inhibitor oospore germination medium containing both metalaxyl (MEX) and p -fluorophenylalanine (FPA). The percentage of hybrids ranged from 6 to 76 when the progenies from crosses were isolated on an inhibitor-free germination medium and screened on lima bean agar medium amended with MEX or FPA. This study indicated that P. sojae races can cross-fertilize with one another and hybrids could be separated from a mixture of inbred and hybrid oospores. © 1993 Academic Press. All rights reserved.