Pythium Root Rot of Seedling Geraniums Associated with Various Concentrations of Nitrogen, Phosphorus, and Sodium Chloride. L. A. Gladstone, Department of Plant Pathology, The Pennsylvania State University, University Park 16802. G. W. Moorman, Department of Plant Pathology, The Pennsylvania State University, University Park 16802. Plant Dis. 73:733-736. Accepted for publication 4 April 1989. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/PD-73-0733.

Seven-week-old seedling geraniums (Pelargonium × hortorum ‘Showgirl’) planted in soilless media received soluble fertilizer with each irrigation. A complete base fertilizer (20% nitrogen, 10% phosphorus, 20% potassium) was amended to supply nitrate nitrogen, ammonium nitrogen, or phosphorus in suboptimum, optimum, or excessive concentrations. In a separate treatment, sodium chloride was added to the base fertilizer to simulate the electrical conductivity of the excessive fertilizer solution. Half the plants in each treatment were inoculated with Pythium ultimum. Few of the noninoculated plants given excessive fertilization died and none of the plants given the suboptimum level of fertilization died, even when inoculated with P. ultimum. Mortality of the inoculated plants increased as the concentration of nitrogen or phosphorus increased. Mortality could not be associated with increased electrical conductivity of the saturated paste extract of the potting medium.