A Comparative Study of Two Components of the Poinsettia Root Rot Complex

Abstract

THE MOST IMPORTANT pathological problem encountered by commercial growers of poinsettias ( Euphorbia piilcheirima Willd.) is caused by a complex of fungi composed of Thielaviopsis basicola (Berk. & Br.) Ferr., Rhizoctonia solani Kuhn, and Pythium ultimum Trow. Collectively these three organisms cause a condition referred to as the poinsettia root rot complex. Since more research has been conducted on P. ultimum and R. solani than on T. basicola, the present research was originally designed to investigate some of the environmental factors affecting the growth of Thielaviopsis and the development of the root rot caused by it. However, another fungus, Chalaropsis thielavioides Peyronel, frequently was obtained in isolations from diseased greenhouse poinsettias. Since C. thielavioides had not been reported as being a part of the poinsettia root rot complex, the author decided to investigate its importance as a pathogen on poinsettias. Numerous similarities between C. thielavioides and T. basicola were evident. The object of this work was to compare the two fungi. Prior to undertaking such a study, the pathogenicity of Chalaropsis on poinsettias had to be established. A comparison of Chalaropsis and two isolates of Thielaviopsis was made to determine the effects of environment on the growth of the fungi and the ability of the two fungi to produce disease symptoms on poinsettias.