The Variability of Pyrenochaeta terrestris Isolates Based on Isozyme Polymorphism, Cultural Characteristics and Virulence on Differential Onion Breeding Lines

Isolates of Pyrenochaeta terrestris from South Africa and the United States of America were evaluated for pathogenicity, isozyme polymorphism and cultural characteristics. Isozyme polymorphism indicated that the isolates of this pathogen, even from the same field, are highly variable. Based on variability in the enzymes: alcohol dehydrogenase, aldolase, malate dehydrogenase and sorbitol dehydrogenase, the isolates could be divided into five electrophoretic types. The virulence of isolates to onion breeding lines resistant, moderately resistant and susceptible to pink root was variable and differentiation depended on the isolate used in the evaluation. The isolates showed extensive variation in both the ability to form setose pycnidia in culture and in the cultural pigmentation.