Biological Control of Pythium Damping-Off of Cucumbers withPythium nunn: Influence of Soil Environment and Organic Amendments

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Abstract
The effects of temperature, pH, matric potential, and organic amendments on the biological control of Pythium damping-off of cucumbers with Pythium nunn was investigated. Disease suppression was significant at 26 but not at 22 or 17.degree.C, even though reduction in inoculum density of P. ultimum was observed at all temperatures. Reduction in disease incidence and inoculum density of P. ultimum occurred at pH 6.7 but not at pH 5.0 or 6.0. Differences in disease and inoculum density of P. ultimum between treatments with and without P. nunn were significant at matric potentials of -20, -40, -100, -200, -500, and -1.000 mb. The addition of bean leaves, cotton leaves, alfalfa, or wheat straw together with P. nunn to raw soil infested with P. ultimum did not significantly influence disease incidence. In the treatment with oatmeal plus P. nunn, the slope of the regression line of disease incidence over time was significantly less than with other treatments. Also, saprophytic increase of P. nunn was greatest in the oatmeal treatment. When organic amendments were added alone to raw soil infested with P. ultimum, disease incidence and inoculum density of P. ultimum were initially greater than in treatments where P. nunn was added together with amendments. This evidence suggests that P. nunn can compete with P. ultimum for organic substrates and that P. nunn and P. ultimum occupy overlapping environmental niches.

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