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Roles of Matric and Osmotic Components of Water Potential and Their Interaction with Temperature in the Growth of Fusarium oxysporum in Synthetic Media and Soil. K. H. Brownell, Stauffer Chemical Co., P.O. Box 760, Mountain View, CA 94040; R. W. Schneider, Department of Plant Pathology and Crop Physiology, Louisiana State University, Baton Rouge 70803. Phytopathology 75:53-57. Accepted for publication 12 June 1984. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/Phyto-75-53.

Interactions of water potential, both matric and osmotic, and temperature as they affect radial growth of Fusarium oxysporum were studied. Water potential (φ) was adjusted osmotically with KCl and MgCl2 and matrically in sterile and nonsterile field soil and with polyethylene glycol (PEG) 6000, a suspended matricum. High temperatures were more inhibitory to growth in matrically adjusted than in osmotically adjusted media. Growth decreased steadily with decreases in soil matric φ; however, growth in osmotica was stimulated as φ, declined from - 8 to - 15 bars. At osmotic potentials <- 20 bars, MgCl2 as a solute was more inhibitory than KCl. PEG, for which a method of radial growth determinations is described, affected growth <- 10 bars similarly to the osmotica. The effects of temperature and φ on growth in sterile and nonsterile soil were similar. However, growth responses obtained on any of the synthetic media tested, adjusted either osmotically or matrically, did not adequately represent those obtained in soil. We conclude that inferences concerning the role of soil φ in fungal ecology cannot be drawn from studies in which φ is adjusted osmotically or by the use of a suspended matricum such as PEG. Techniques are described for measuring radial growth as a function of matric φ in sterile and nonsterile soil.