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Enhanced Severity of Thielaviopis basicola Root Rot Induced in Soybean by the Herbicide Chloramben. May Lee, Department of Botany and Plant Pathology, Michigan State University, East Lansing, MI 48824, Present address of senior author: Department of Biology, Case-Western Reserve University, Cleveland, OH 44106; J. L. Lockwood, Department of Botany and Plant Pathology, Michigan State University, East Lansing, MI 48824. Phytopathology 67:1360-1367. Accepted for publication 8 April 1977. Copyright 1977 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-67-1360.

Of eight herbicides applied at recommended rates to soil infested with Thielaviopsis basicola in the greenhouse, chloramben, alachlor, and DNPB resulted in increased severity of soybean root rot, compared with that in plants grown in soil without chloramben. Disease enhancement by chloramben was studied further. In the greenhouse, in soil infested with chloramben at the rate of 2 mg/kg (=3.4 kg/ha =3 lb/acre) disease enhancement was shown with three soybean cultivars, four isolates of the pathogen, and with endoconidia and chlamydospores as inoculum. The effect was expressed over a soil temperature range of 14-26 C. In a field infested with T. basicola, disease severity was greater and plant stand and yield were less in soil treated with chloramben than in soil without herbicidal treatment. Germination of spores of T. basicola was 2-4 times higher in rhizospheres of chloramben-treated soybean seedlings than in rhizospheres of untreated seedlings. Exudates from roots of soybean seedlings grown axenically with chloramben supported more germination of T. basicola spores, and caused development of larger T. basicola lesions when applied to soybean seedlings than did exudates from untreated plants. Root exudates from chloramben-treated seedlings contained more electrolytes and amino acids than did control exudates. Carbohydrates, fatty acids, and nucleic acids were increased little or not at all by chloramben. Casamino acids applied to soybean seedlings growing in sterilized soil infested with T. basicola increased symptom severity as compared with that in soybeans not so treated. Possibilities of disease enhancement due to direct stimulation of the pathogen, to increased virulence of pathogen, or to changes in populations of other microorganisms were ruled out.