Effects of herbicides on Fusarium solani f. sp glycines and development ofsudden death syndrome in glyphosate-tolerant soybean

Sudden death syndrome of soybean, caused by Fusarium solani f. sp. glycines , is a disease of increasing economic importance in the United States. Alth ough the ecology of sudden death syndrome has been extensively studied in r elation to crop management practices such as tillage, irrigation, and culti var selection, there is no information on the effects of herbicides on this disease. Three herbicides (lactofen, glyphosate, and imazethapyr) commonly used in soybean were evaluated for their effects on the phenology of F. so lani f. sp. glycines and the development of sudden death syndrome in four s oybean cultivars varying in resistance to the disease and in tolerance to g lyphosate. Conidial germination, mycelial growth, and sporulation in vitro were reduced by glyphosate and lactofen. In growth-chamber and greenhouse e xperiments, there was a significant increase in disease severity and freque ncy of isolation of F: solani f. sp. glycines from roots of all cultivars a fter application of imazethapyr or glyphosate compared with the control tre atment (no herbicide applied). Conversely, disease severity and isolation f requency of F. solani f. sp. glycines decreased after application of lactof en. Across all herbicide treatments, severity of sudden death syndrome and isolation frequency were lower in disease-resistant than in susceptible cul tivars. Results suggest that glyphosate-tolerant and -nontolerant cultivars respond similarly to infection by F. solani f. sp. glycines after herbicid e application.