Genetic Instability of Rishitin Metabolism and Tolerance and Virulence on Potato Tubers of a Strain of Gibberella pulicaris

Two strains of the potato pathogen Gihherella pulicaris were analysed for the ability to metabolize the phytoalexiti rishitin and for virulence on potato tubers. While strain R-6380 was always virulent and able to metabolize the phytoalexin, strain R-7843 showed significant variation in both traits. Analysis of eight sectors of a radially grown culture of this strain revealed that six had a virulent, dshitin-metabolizing phenotype, one had an avirulent, rishitin-nonmetabolizing phetiotype, and one had an intermediate phenotype for both traits. Similar results were fovtnd for single spores isolated from one of two spore suspensions from different agar cultures. In all cases high virulence was associated with fast metabolism of rishitin; weakly virulent strains only slowly degraded the phytoalexin. Restriction-fragment-length-polymorphism {RFLP) analysis of sectors with contrasting phenotypes and measurement of the nuclear DN A content from these sectors indicate that the sectors were identical to the original strain (R-7843) with respect to these traits. It is therefore concluded that the variations observed in virulence and in the ability to metaboHze rishitin are the result of spontaneous genetic chatiges in strain R-7843, The concurrent changes in tolerance, metabolism and virulence are further indicators of a relationship among these traits and lend support to the hypothesis that phytoalexin metabolism is important in pathogenicity,