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Disease Note.

Phytophthora Rot of Soybean Caused by Phytophthora sojae Found in Quebec, Canada. T. R. Anderson, Agriculture and Agri-Food Canada, Harrow, ON, Canada NOR 1G0. J.-M. Montpetit, Pioneer Hi-Bred Ltd., 104 Evangeline, Vaudreuil, PQ, Canada, J7V 8A8. Plant Dis. 78:1218. Accepted for publication 3 October 1994. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/PD-78-1218B.

In 1992, root rot of soybean (Glycine max L. Merr.) caused by Phytophthora sojae M. J. Kaufmann & J. W. Gerdemann was identified in a 25-ha field in Brome-Missisquoi County in Quebec. Other soybean fields in the area did not have symptoms of the disease. The field soil was a fine-textured clay loam that was planted with soybeans in 1987, 1988, and 1989, followed by 2 yr of corn. Above-average rainfall between April and October 1992 (65 cm) probably contributed to the disease severity. The soybean cultivar grown in 1992 contained the [Rps 1-a] gene for resistance and yielded approximately 500 kg/ha. A test strip of another cultivar containing the [Rps 1-c] gene for resistance yielded 2,100 kg/ha. Thirty-three isolates of P. sojae were obtained by baiting field soil with a soybean line without resistance [HARO (1-7)1], When tested by the hypo-cotyl inoculation technique on a set of 15 differential lines (1), 32 isolates were virulent on [Rps 1-a] and only 1 isolate was virulent on [Rps 1-c]. The observations suggest that genes for resistance to Phytophthora rot are required in short-season cultivars to reduce yield loss.

Reference: (1) T. R. Anderson and R. I. Buzzell. Plant Dis. 76:587, 1992.