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Tolerant and Root-Resistant Soybean Cultivars: Reactions to Phytophthora Rot in Inoculum-Layer Tests

Dep. of Agronomy, Ohio State Univ., Columbus, OH 43210

C. A. Thomas

USDA-ARS Beltsville Agric. Res. Ctr., Beltsville, MD 20705

W. J. Kenworthy

Dep. of Agronomy, Univ. of Maryland, College Park, MD 20742

*Corresponding author.

Race specificity may be involved in the reactions of certain soybean [Glycine max (L.) Merr.]cultivars previously characterized as tolerant to Phytophthora megasperma f. sp. glycinea Kuan & Erwin (Pmg). The objectives of this study were to determine if an inoculumlayer procedure could be used to differentiate race-specific reactions among soybean cultivars characterized as tolerant to Pmg and to distinguish root resistance reactions associated with tolerance from those conditioned by the Rps₂ gene. An inoculum-layer procedure was used in the greenhouse to determine the reactions of 21 soybean cultivars to Races 1 and 10 of Pmg. Some cultivars characterized as tolerant to Pmg and the cultivar CNS with the Rps₂ gene, which conditions race-specific root resistance to these Pmg races, were included to provide a comparison between various levels of seedling tolerance and root resistance. Seeds were planted 2.5 or 5.0 cm above an inoculum layer of lima bean agar cultures. Cultivar reaction to Pmg was recorded as percent survival of emerged seedlings 14 d after planting. Cultivars exhibited different levels of survival with Races 1 and 10, with variation also at the 2.5- and 5.0-cm inoculum layer depths. Seedling survival with Race 10 was significantly (P < 0.05) higher at the 5.0-cm compared with the 2.5-cm inoculum layer, whereas there was no consistent difference in survival with Race 1 among cultivars at the two depths. Nine cultivars exhibited high levels of seedling survival with Race 1 that were comparable with CNS. Only CNS and 'Emerald' survived both Races 1 and 10. Since the reactions of Emerald closely resemble those of CNS, they may be conditioned by the Rps2 gene. Results of the experiments indicate that it is difficult to distinguish between race-specific root resistance and high levels of tolerance when using the inoculum-layer test with certain cultivar/race/inoculum depth combinations. Limited information is available concerning differences in the expression of root infection between cultivars with race-specific root resistance and those with high levels of tolerance. The results emphasize the need for improved techniques to differentiate tolerance and root resistance, and to characterize precisely soybean lines or cultivars lacking major resistance genes.

Received for publication December 29, 1989.

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