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Effect of Nontreated Field Soil on Sporulation of Vesicular-Arbuscular Mycorrhizal Fungi Associated with Soybean. J. P. Ross, Plant pathologist, U. S. Department of Agriculture, Science and Education Administration, Agriculture Research, and Department of Plant Pathology, North Carolina State University, Raleigh 27650; Phytopathology 70:1200-1205. Accepted for publication 9 June 1980. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1980. DOI: 10.1094/Phyto-70-1200.

Experiments were conducted in which cultures of Glomus macrocarpus var. geosporus or Gigaspora gigantea were established on soybean plants growing in steamed or fumigated soil or sand amended with various amounts of nontreated soil or organic matter extracted from nontreated soil. Spore counts were made after 34 mo and expressed as number per gram of root. Amendments of 1 and 5% nontreated soil to steamed soil reduced Glomus sporulation almost three and six fold, respectively, compared with sporulation in steamed soil. Average chlamydospore size of Glomus was significantly less in soil amended with nontreated soil compared with that in steamed soil and was positively correlated with spore numbers. Additions of 5% nontreated soil decreased azygospore production of G. gigantea by 93%. Separation of cultures of Glomus in steamed soil from nontreated soil by means of a polycarbonate membrane (pore size of 0.2 μm) prevented sporulation suppression. Soybean seedling roots grown in nontreated soil for 1 wk did not transmit the sporulation suppressor when transplanted without soil into steamed soil. Organic matter extracted from nontreated soil manifested sporulation suppression similar to that of nontreated soil; the suppressive activity was eliminated when organic matter was heated for 15 min at 6575 C. Additions of small amounts of nontreated soil or organic matter to Glomus cultures in quartz sand induced 23 and 78-fold increases in sporulation, respectively, over that obtained with the same additions that had been steamed. Additions of cultures of actinomycetes isolated from nonsterile soil to cultures of Glomus or Gigaspora in steamed soil had little or no effect on sporulation. Additions of 4.6% by volume of nonfumigated soils, of four different classifications, to soil in plots which had been fumigated with methyl bromide also caused significant reductions in sporulation of mycorrhizal fungi. A clay soil induced a greater reduction of total Glomus sporulation than the three loam soils. The results of these studies explain in part why infection and sporulation on soybean roots in the field are only a fraction of those found in experiments with sterile soils.

Additional keywords: Glomus, Gigaspora.