Relationship of Mycelial Growth and Production of Oxalic Acid and Cell Wall Degrading Enzymes to Virulence inSclerotium Rolfsii

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Abstract
Forty-three single-basidiospore strains and 5 parental field isolates of S. rolfsii differed in growth rate, mycelial dry wt, and oxalic acid production, and in the amounts of endo-polygalacturonase (PG), endopectinmethylgalacturonase, and cellulase (Cx and C1) produced adaptively in culture. High activities of these enzymes also were detected in diseased tissue. Oxalic acid was produced early during growth in culture and was followed by a rapid increase in activity of endo-PG, concomitant with an increase in dry wt. Cx activity increased later during growth. The strains were separated into high (39.6%), moderate (37.2%), and weak (23.2%) virulence classes based on their relative abilities to induce water-soaking in host tissue, colonize carrot roots and infect sugar beet seedlings. The highly virulent strains characteristically had rapid growth rates and produced large quantities of oxalic acid and the cell wall degrading enzymes. Weakly virulent strains grew less rapidly and produced very low amounts of endo-PG but differences in levels of oxalic acid were not detected. The determinants of pathogenicity in S. rolfsii include the ability to produce sufficient quantities of oxalic acid and endo-PG and to grow rapidly, whereas virulence was highly correlated with endo-PG production and growth rate, provided a base level of oxalic acid also was produced. The C source on which 2 parental isolates were grown did not alter their virulence rating. However, tissue maceration by culture filtrates and disease development in vitro were reduced by 0.05 M CaCl2, which sequestered oxalic acid to form crystals of calcium oxalate and also reduced endo-PG activity.