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Low Concentrations of the Isoflavone Genistein Influence in vitro Asexual Reproduction and Growth of Phytophthora sojae. Elena G. Vedenyapina, Botanical Institute, Russian Academy of Sciences, St. Petersburg; Gene R. Safir(2), Brendan A. Niemira(3), and Thomas E. Chase(4). (2)(3)Department of Botany and Plant Pathology, Michigan State University, East Lansing 48824; (4)South Dakota State University, Plant Sciences Department, Brookings 57009. Phytopathology 86:144-148. Accepted for publication 10 October 1995. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/Phyto-86-144.

Isolates of Phytophthora sojae were evaluated for their sensitivity to a series of low concentrations of genistein (0.01, 0.1, 1, 3, and 10 ?g/ml) during various stages of growth and asexual reproduction. Genistein was applied during mycelial growth, formation of zoosporangia, release of zoospores, and zoospore germination. At 10 μ g/ml, genistein inhibited radial growth of all isolates in solid culture. Changes in fungal morphology, such as increased branching and swelling of hyphae, were observed at all concentrations of genistein tested. Growth of mycelium in broth containing 10 μ g of genistein per ml decreased colonies resulting from subsequently formed and germinated zoospores, even when genistein was removed during zoosporangia formation. Genistein also decreased zoosporangia production and release of zoospores. The strongest inhibition of P. sojae reproduction was obtained when genistein was applied during zoosporangia formation; this occurred at concentrations as low as 0.01 to 1 μ g/ml. Genistein in the medium containing zoospores caused a general reduction in asexual reproduction across isolates, with 10 μ g/ml causing the most dramatic reduction. There was strong evidence of intraspecific variation in the responses of the isolates of P. sojae in all processes tested.