GENETIC INSTABILITY OF RISHITIN METABOLISM AND TOLERANCE AND VIRULENCE ON POTATO-TUBERS OF A STRAIN OF GIBBERELLA-PULICARIS

Two strains of the potato pathogen Gibberella pulicaris were analysed for the ability to metabolize the phytoalexin rishitin and for virulen ce on potato tubers. While strain R-6380 was always virulent and able to metabolize the phytoalexin, strain R-7843 showed significant variat ion in both traits. Analysis of eight sectors of a radially grown cult ure of this strain revealed that six had a virulent, rishitin-metaboli zing phenotype, one had an avirulent, rishitin-nonmetabolizing phenoty pe, and one had an intermediate phenotype for both traits. Similar res ults were found for single spores isolated from one of two spore suspe nsions from different agar cultures. In all cases high virulence was a ssociated with fast metabolism of rishitin; weakly virulent strains on ly slowly degraded the phytoalexin. Restriction-fragment-length-polymo rphism (RFLP) analysis of sectors with contrasting phenotypes and meas urement of the nuclear DNA content from these sectors indicate that th e sectors were identical to the original strain (R-7843) with respect to these traits. It is therefore concluded that the variations observe d in virulence and in the ability to metabolize rishitin are the resul t of spontaneous genetic changes in strain R-7843. The concurrent chan ges in tolerance, metabolism and virulence are further indicators of a relationship among these traits and lend support to the hypothesis th at phytoalexin metabolism is important in pathogenicity.