Factors affecting incidence of damping-off of spinach seedlings caused by Pythium species

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Influences of moisture, electric conductivity (EC), and pH of soils on the incidence of damping-off of spinach seedlings caused by Pythium aphanidermatum and P. paroecandrum were studied. The incidence of damping-off increased considerably at high soil moisture, especially with more than 40% of soil water content. The incidence was not significantly different between upland fields and drained rice paddy fields where spinach had been cultivated for more than 3 consecutive years. A negative correlation was found between the incidence of damping-off and EC values of soils (r=-0.5094** in upland fields and r=-0.3601* in drained paddy fields), and also between plant height and EC values (r=-0.6491**). Hyphal growth of the pathogen on corn-meal agar was maximal at pH 6.6-7.0, decreased at pH 4.0, but not considerable at pH 9.0. Soil pH of spinach fields ranged from 5.5 to 7.5, and most were very close to the above optimum pH. However, no significant correlation was found between the incidence of damping-off and soil pH. The young seedlings were most sensitive to the disease and became resistant with the seedling age. Damping-off was severe within two weeks after sowing.
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