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Relationship Between Localized Glyceollin Accumulation and Metalaxyl Treatment in the Control of Phytophthora Rot in Soybean Hypocotyls. G. Lazarovits, Assistant, Research Centre, Agriculture Canada, University Sub Post Office, London, Ontario N6A 5B7; E. W. B. Ward, principal plant pathologist, Research Centre, Agriculture Canada, University Sub Post Office, London, Ontario N6A 5B7. Phytopathology 72:1217-1221. Accepted for publication 9 February 1982. Copyright 1982 The American Phytopathological Society. DOI: 10.1094/Phyto-72-1217.

Soybean seedlings (cultivar Altona) were treated with the systemic fungicide metalaxyl by immersing the roots in solutions of 0.5, 1.0, 1.5, or 2.0 μg/ml for 18 hr. Controls were immersed in water only. After treatment, hypocotyls were inoculated with drops of zoospore suspensions of Phytophthora megasperma f. sp. glycinea (Pmg), race 4 (incompatible) or race 6 (compatible). Water drops were used on uninoculated controls. After incubation for 2472 hr, the reaction type (resistant or susceptible) was recorded and the tissues at the inoculation site were excised and analyzed for metalaxyl and for glyceollin (a phytoalexin) by bioautography, gas-liquid chromatography, and spectrophotometry. Plants inoculated with the incompatible race developed limited necrotic lesions regardless of metalaxyl treatment. Untreated plants inoculated with the compatible race 6 developed spreading water-soaked lesions. However, in metalaxyl-treated plants the compatible race produced limited necrotic lesions by 24 hr, similar to those produced with the incompatible race. These lesions remained restricted throughout 72 hr incubation, except in plants treated with 0.5 μg metalaxyl per milliliter. In the latter, lesions had spread by 48 hr and hypocotyls were extensively colonized by 72 hr. Generally, fungicide concentrations were greater in infected lesions than at similar sites that received water droplets instead of inoculum. Lesions in plants protected from the compatible race contained metalaxyl concentrations in excess of the in vitro EC90 for Pmg (0.8 μg/ml). This concentration was not reached in lesions in plants treated with only 0.5 μg metalaxyl per milliliter. Except in untreated plants inoculated with the compatible race, glyceollin concentrations exceeded the in vitro EC90 in all lesions examined. In spite of this, in plants only partially protected by 0.5 μg metalaxyl per milliliter, spread of the fungus was not restricted more than would be expected from the metalaxyl concentration alone. The results suggest that glyceollin does not contribute to inhibition of spread of the fungus under these conditions.