INCIDENCE OF A ROOT ROT FUNGUS IN A DISEASED ALFALFA SOIL OF CENTRAL ALBERTA

Abstract

Alfalfa seedlings were consistently diseased by inoculating 7-day-old seedlings, grown in sterilized sand in glass culture tube assemblies, with various soil dilutions from a diseased alfalfa soil from central Alberta. The root systems of stunted plants had reddish brown lesions, and in the more advanced stages lesions were dark and expanded more than 1 cm above and below the point of initiation. Eventually water-soaked areas developed, and the tissue collapsed. Isolation from lesions on the roots of stunted alfalfa revealed that virtually all lesions contained a fungus belonging to Pythiaceae which was classified as Phytophthora megasperma. Other fungi and bacteria were also isolated from some lesions but pathogenicity tests revealed they did not produce lesions or stunting, whereas the P. megasperma produced severe lesioning and severe stunting of the seedlings. Results indicate that P. megasperma is one of the causes of a complex disease in central Alberta that has become known as "alfalfa sickness."