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In Vitro Interactions of Fusarium and Verticillium Wilt Fungi with Water, pH, and Temperature. Juju B. Manandhar, U.S. AID Fellow, Plant Pathology Section, Khumal, Lalitpur, KTM, Nepal; G. W. Bruehl, Plant Pathologist, Department of Plant Pathology, Washington State University, Pullman 99163. Phytopathology 63:413-419. Accepted for publication 26 September 1972. DOI: 10.1094/Phyto-63-413.

Fusarium oxysporum f. sp. vasinfectum and the microsclerotial form of Verticillium albo-atrum pathogenic to cotton grew progressively slower in soil as the predominantly matric water potentials in soil were lowered below 0. In contrast, both fungi grew optimally at about 10 to 30 bars at 25 C in either agar media or in soil when water potential was regulated osmotically. The osmotic stimulation of both fungi was independent of the solute used and occurred in agar media when water potentials were adjusted with NaCl, KCl, CaCl2, MgCl2, a salt mixture, or sucrose, and on straw in sterile soil when water potentials were osmotically adjusted. At water potentials below the stimulatory range, growth declined linearly and was zero at near 100 bars for V. albo-atrum and at near 115 bars for F. oxysporum. A water stress of 40 to 60 bars reduced the growth of both to about half the maximum rate. Both fungi grew well at pH values ranging from 5.2 to 8.6, and no significant interactions of pH and water potential were observed in either fungus. A highly significant temperature × water interaction occurred in both wilt fungi: the magnitude of growth stimulation with slightly reduced osmotic water potential increased with rising temperature. At 35 C and pH 6, for example, F. oxysporum grew 2-4 times as fast at about 25 bars as at about 3 bars; at 35 C, Verticillium spp. did not grow on cornmeal dextrose agar, but when KCl was added to reduce the water potential to 30 to 40 bars, considerable growth occurred. No new in vitro responses to water, pH, temperature, or to interactions of these were observed that explain the different distributions of these wilt fungi in nature.