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The Influence of a Fungicide on the Epidemiology of Black Shank of Tobacco. M. E. Kannwischer, Graduate Research Assistant, Department of Plant Pathology, University of Florida, Gainesville, FL 32611; D. J. Mitchell, Associate Professor, Department of Plant Pathology, University of Florida, Gainesville, FL 32611. Phytopathology 68:1760-1765. Accepted for publication 16 June 1978. Copyright 1978 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-68-1760.

Inoculum densities near 0.5 propagule of Phytophthora parasitica var. nicotianae per gram of soil resulted in 93 to 100% mortality of susceptible tobacco plants and apparent infection rates(r) of 0.08 to 0.11/unit/day in the plant growth room, greenhouse, or field. Disease was initiated sooner in the field but the time required for 50 and 90% mortality to occur was shorter under artificial conditions. No black shank occurred under plant growth room conditions with 1 or more mg/kg (ppm, w/w) of an experimental fungicide, CGA 48988 (CIBA-GEIGY Corp.), incorporated into soil containing 0.5 propagule per gram of soil; under greenhouse conditions 93, 60, 7, and 0% of the susceptible tobacco plants died at 0.5 propagule per gram of soil plus 0, 1, 10, or 50 ppm of the fungicide. After approximately 130 ml of a 150-μg/ml solution of the fungicide was added to individual susceptible transplants in the field at the time of planting, disease development initially was delayed, but after 50 days it progressed more rapidly (r = 0.22/unit/day) than in nontreated plants (r = 0.10/unit/day). All of the nontreated and treated susceptible plants and 61 and 10% of the nontreated and fungicide-treated resistant plants, respectively, were dead at harvest. The fungicide treatment resulted in significant increases (P = 0.05) in yields (based on six successive leaf harvests per treatment) of susceptible and resistant plants over nontreated plants. Increase in rhizosphere populations of P. parasitica var. nicotianae from an initial field population of 0.75 propagule per gram of soil, coincided with rapid disease development in each treatment.

Additional keywords: Nicotiana tabacum, Phytophthora nicotianae var. nicotianae, Ridomil.