Influence of Soil Water Potential and Temperature on Severity of Pythium Root Rot of Snap Beans

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Abstract
Soil water and temperature had significant effects on the severity of root rot of snap beans [Phaseolus vulgaris] caused by P. ultimum. The disease was studied in pasteurized soil infested with the pathogen and maintained at fluctuating water potentials of 0 to -1.0, 0 to -5 and 0 to -12 bars, each at 15, 21 and 27.degree. C. At any one temperature, root rot severity increased as soil water potential increased. At any one water potential, root rot severity decreased as soil temperature increased. Pythium root rot was most severe and caused the greatest plant dry weight loss in soil at 15.degree. C and soil-water potential of 0 to -1 bar. When plants grown in infested soil for 15 days at 15.degree. C and at soil water potential of 0 to -1 bar were shifted to 27.degree. C and/or soil water potential 0 to -5 bars for another 15 days, they had greater dry weight and less root rot than did plants maintained in soil at 15.degree. C and 0 to -1 bar for 30 days. Damage from Pythium also occurred, although to a lesser extent, when plants were grown at 27.degree. C and soil water potential of 0 to -12 bars for 15 days and then shifted to a 15.degree. C and/or soil water potential of 0 to -1 bar. Soil populations of P. ultimum increased at all temperatures and soil water potentials tested.