Plant Losses and Yield Responses to Monoculture of Soybean Cultivars Susceptible, Tolerant, and Resistant to Phytophthora megasperma f. sp. glycinea. T. R. Anderson, Agriculture Canada, Research Station, Harrow, Ont., Canada N0R 1G0. Plant Dis. 70:468-471. Accepted for publication 1 November 1985. Copyright 1986 The American Phytopathological Society. DOI: 10.1094/PD-70-468.

Three soybean cultivars differing in reactions to Phytophthora megasperma f. sp. glycinea (P. m. glycinea) were grown on the same plots for five consecutive years to study the effect of monoculture on the severity of disease. Average emergence, plant losses, and yields differed significantly among soybean cultivars susceptible, tolerant, and resistant to P. m. glycinea. The resistant cultivar sustained significantly less plant loss and higher yield than the tolerant and susceptible cultivars. Interplanting the monocultured plots with all three cultivars during the sixth year of the experiment resulted in severe disease of the susceptible cultivar on plots previously planted with susceptible and tolerant cultivars and moderate disease on plots planted with the resistant cultivar. Plant losses of the tolerant cultivar were similar in plots previously planted with susceptible and tolerant cultivars and less in plots planted with the resistant cultivar. Plant losses of the resistant cultivar were similar on all plots regardless of previous cultivar. Oospore numbers in roots of soybean seedlings grown hydroponically in the laboratory were greater in the tolerant cultivar than in the susceptible cultivar. Oospore numbers in roots of the resistant cultivar inoculated with an incompatible race of P. m. glycinea were low. Disease severity can be similar after monoculture of tolerant and susceptible soybean cultivars, and this may result from similar increases in soilborne inoculum with repeated cropping.