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Ecology and Epidemiology

Predisposing Effect of Water Stress on the Severity of Phytophthora Root Rot in Safflower. J. M. Duniway, Department of Plant Pathology, University of California, Davis, CA 95616; Phytopathology 67:884-889. Accepted for publication 10 January 1977. Copyright 1977 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-67-884.

Safflower plants were grown in crocks of steamed soil and were inoculated with zoospores of Phytophthora cryptogea when they were 3-5 wk old. Water stress was induced in some of the plants by withholding water from the soil and the remaining plants were watered daily to maintain leaf water potentials between 4 and 6 bars. Symptoms of root rot always developed in the susceptible cultivar Nebraska 10 following inoculation. However, withholding water from Nebraska 10 plants to decrease leaf water potential to 13 or 17 bars before inoculation increased root rot caused by P. cryptogea, as measured by the severity of visible symptoms or as a decrease in the fresh weight of roots. Root rot did not develop in well-watered plants of the resistant cultivar Biggs following inoculation, but P. cryptogea did cause visible symptoms of root rot and significant reductions in fresh weight of roots in cultivar Biggs when leaf water potential was depressed to 9 or 12 bars before inoculation. With the exception of saturated soil, a variety of soil water regimes under which Nebraska 10 plants were maintained after inoculation were almost equally suitable for the development of severe root rot. The suitable water regimes included soil water potentials that gradually declined from 0.4 bar to 4, 8, or 12 bars and constant soil matric potentials between 0.05 and 0.37 bar. Depending on the methods used, saturated soil was either more or less suitable than the drier soils for disease development.

Additional keywords: water relations, Phytophthora drechsleri, Carthamus tinctorius.