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Effect of Preinoculation and Postinoculation Water Stress on the Severity of Phytophthora Root Rot in Processing Tomatoes. J. B. Ristaino, Graduate Research Assistant, Department of Plant Pathology, University of California, Davis 95616. J. M. Duniway, Professor, Department of Plant Pathology, University of California, Davis 95616. Plant Dis. 73:349-352. Accepted for publication 15 November 1988. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/PD-73-0349.

The effect of water stress imposed either before or after inoculation of processing tomatoes on the severity of Phytophthora root rot was evaluated under controlled conditions in greenhouse experiments. Three-week-old tomato seedlings or plants in early flowering stages were either inoculated with zoospores of Phytophthora parasitica or not inoculated. Plants either were stressed by withholding water until leaf water potentials reached 9 bars before or after inoculation or were watered regularly to maintain leaf water potentials above 4 bars. Severity of symptoms on roots, length of brown roots, and numbers of colonies of P. parasitica recovered from roots were greater in inoculated seedlings that were water-stressed either before or after inoculation. In the absence of water stress, older flowering plants were more resistant to infection than seedlings. Preinoculation water stress increased disease severity in flowering plants, whereas postinoculation water stress did not affect disease.

Keyword(s): Lycopersicon esculentum, water potential.