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Comparison of Field and Greenhouse Evaluations for Tolerance to Phytophthora Rot in Soybean¹

Tolerance to phytophthora rot caused by Phytophthora megasperma Drechs. f. sp. glycinea Kuan and Erwin (Pmg) is an alternative control method to race.specific resistance. Simple, effective, and consistent methods of evaluating tolerance would improve the etficiency of screening genotypes for tolerance. Therefore, the effectiveness and reliability of a field, hill-plot method and a greenhouse, mycelium-inoculum-layer method for evaluating soybean [Glycine max (L.) Merr.] for tolerance to phytophthora rot was studied. A visually rated hill-plot evaluation and a yield test of 22 genotypes were conducted at Viekery, Ohio in 1977, 1978, and 1979 on soil infested with races 1, 3, 4, and 7 of Pmg. Hill plots were planted on 76-cm centers with 12 seeds per hill. Hill plots were rated for root-rot severity in late August by visually estimating the percentage of dead plants and amount of stunting. The yield test was conducted in an area adjacent to the hill plots. The greenhouse evaluations were conducted at Wooster, Ohio by placing 14-day-old, V-8 juice agar cultures of Pmg 5 cm below the seed in composted, hardwood-bark media. Sterile V-8 juice agar was used in uninoculated controls. Ten seeds were planted in a 1-L polystyrene cup. Genotypes grown with and without Pmg were compared for number of live plants and for average plant height at 28 days after planting. Both the hill-plot and greenhouse disease evaluations were effective and reliable in identifying tolerant soybean genotypes. Field and greenhouse disease ratings were highly correlated with seed yield. Maturity of the genotypes significantly influenced seed yield but did not significantly influence field, hill-plot ratings or greenhouse ratings.

Key Words: Glycine max (L.) Merr. • Phytophthora megasperma Drechs. f. sp. glycinea Kuan and Erwin • Race-non-specific • Disease screening • Soybean disease

² Formerly associate professor, Dep. of Agronomy, Ohio Agric. Res. and Dev. Ctr., The Ohio State Univ. Wooster, OH 44691. Now research scientist III Asgrow Seed Co., Redwood Falls, MN 56283; and professor, Dep. of Plant Pathology, Ohio Agric. Res. adn Dev. Ctr., The Ohio State Univ., Wooster, OH 44691.

Received for publication April 1, 1983.

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