Pathogenic Variability

Considerable variation in morphology (Marukawa et al., 1975a; Morrall et al., 1972; Price and Calhoun, 1975a) and pathogenicity (Marciano et al., 1983; Morrall et al., 1972; Price and Calhoun, 1975a) has been observed among isolates of S. sclerotiorum. Variation in pathogenicity have been associated with the production of pectolytic enzymes (Hancock, 1966; Lumsden, 1976, 1979), cellulase (Lumsden, 1969), hemicellulase (Hancock, 1967), phosphatidase (Lumsden, 1970), and oxalic acid (Maxwell and Lumsden, 1970; Marciano et al., 1983; Noyes and Hancock, 1981). However, variation in these factors does not appear to be the primary determinant of pathogenicity as correlations have not always been apparent (Marciano et al., 1983; Morrall et al., 1972). In S. trifoliorum, a degenerate non-virulent isolate of the pathogen produces more protopectinase than a normal strain but only the normal isolate secretes a toxin and is considered virulent (Held, 1955). Little information is available on additional mechanisms that affect virulence among isolates of S. sclerotiorum.