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Growth of Two Root-Rot Fungi as Affected by Osmotic and Matric Water Potentials¹

ABSTRACT

Growth of Ophiobolus graminis in response to a range of water potentials was maximal with the wettest treatments (-1.2 to -1.5 bars), reduced by half at about -20 bars, and prevented at -45 to -50 bars. This pattern held for growth measured as colony diameters on agar media with osmotically-controlled water potentials, or as linear hyphal growth along the surface of straws in soil with matrically-controlled water potentials.

In contrast, growth of Fusarium roseum f. sp. cerealis ‘Culmorum’ was progressively stimulated as the osmotic potential was lowered over the range, -1.5 to -8.2 bars, then declined with further decrease in potential. The effect was independent of the solute type, either salts or sucrose in agar media, or with salts in soil. Stimulation did not occur when the fungus was subjected to a range of matric potentials in soil; rather the response was similar to O. graminis with maximal growth in the wettest soil. However, unlike O. graminis, growth of Culmorum was reduced by half at about -40 bars matric potential, or -45 to -50 bars osmotic potential, and was prevented at -70 to -75 bars matric potential, or about -80 bars osmotic potential. The differential effects of osmotic and matric potentials on Culmorum growth in autoclaved soil, and similarly the absence of an osmotic stimulatory effect on O. graminis probably relate to physiological factors inherent with the individual fungus.

The root and foot rot of wheat caused by Culmorum is generally most severe in the low rainfall (30 cm or less annual precipitation), non-irrigated areas of the Northwest USA whereas O. graminis is most destructive in areas of higher rainfall or on irrigated wheat or barley. The natural association of these two pathogens with dry or wet field conditions may relate to their respective responses to the soil- and plant-water potentials associated with the different environments.

¹ Cooperative investigations, Plant Science and the Soil and Water Conservation Research Divisions, ARS, USDA, and the Agricultural Exp. Sta. of Idaho, Montana, Oregon, Utah, and Washington. Scientific Paper no. 3616, College of Agriculture, Washington State Univ., Pullman. Research started in 1969 while Dr. Griffin was Visiting Professor in Plant Pathology, Washington State Univ., Pullman.

² Pathologist, Plant Science Research Division, and Soil Scientist, respectively, Soil & Water Conservation Research Div., ARS, USDA, Washington State Univ., Pullman, WA 99163.

³ Reader in Plant Pathology, Dep. of Agr. Botany, Univ. of Sydney, Australia. Dr. Griffin is presently with the Department of Forestry, The Australian National Univ., Canberra.

Received for publication April 6, 1971. Accepted for publication October 8, 1971.