THE BLACK ROOTROT DISEASE OF TOBACCO.: II. PHYSIOLOGIC SPECIALIZATION OF THIELAVIOPSIS BASICOLA ON NICOTIANA TABACUM

Thielaviopsis basicola (Berk. & Br.) Ferraris exists in nature in two distinct forms, termed the brown and the gray wild type, which are differentiated on potato dextrose agar. Pathogenicity on tobacco was found to be a function of the wild type culture used. All gray wild type cultures were less pathogenic than the brown. The brown wild type cultures consisted of at least two physiologic races. Race I is found in the "old belt" of Ontario, and in Quebec, Ohio, and Connecticut. Race II is present in the "new belt" of Ontario and in Kentucky. Race II and all gray wild type cultures are less pathogenic than Race I. All cultural mutants were less pathogenic than wild type cultures.