Effects of Glyphosate on Calonectria crotalariae and Red Crown Rot of Soybean. D. K. Berner, Instructor, Department of Plant Pathology and Crop Physiology, Louisiana Agricultural Experiment Station, Louisiana State University Agricultural Center, Baton Rouge 70803. G. T. Berggren, and J. P. Snow. Professor, and Professor, Department of Plant Pathology and Crop Physiology, Louisiana Agricultural Experiment Station, Louisiana State University Agricultural Center, Baton Rouge 70803. Plant Dis. 75:809-813. Accepted for publication 16 February 1991. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/PD-75-0809.

Surfactant- and nonsurfactant-containing commercial herbicide formulations of glyphosate were evaluated for in vitro and in vivo effects on Calonectria crotalariae and red crown rot of soybean, respectively. Rates of 0.28, 0.56, 1.12, and 2.25 kg of glyphosate per hectare, corresponding to recommended rates for weed control in soybean (Glycine max), were used. Three pathogenic isolates of the fungus from soybean were grown on a selective medium amended with either water or the different rates of the herbicide. Both formulations of glyphosate inhibited mycelial growth by C. crotalariae. Additions of amino acids to medium amended with the nonsurfactant formulation produced a reversal of herbicide inhibition. Duplicated field trials showed a reduction in red crown rot incidence with preplant applications of low rates of glyphosate. These results, coupled with findings on the significant influence of previous season disease incidence on current red crown rot levels, indicate that glyphosate may be used simultaneously and efficaciously as a preplant herbicide for weed control and as a fungicide for the control of diseases caused by C. crotalariae.

Keyword(s): Cylindrocladium black rot, peanut.