Differential Changes in Host Specificity among MGR586 DNA Fingerprint Groups of Pyricularia Grisea

Abstract

The development of new races of the tice blast pathogen continues to circumvent the best efforts of plant breeders to develop blast resistant cultivars. Sampling field populations of the rice blast fungus fi aln 1990-1997 has indicated that four MGR586 DNA fingerprint groups (A, B, C, and D) occur in the contemporary rice blast pathogen population in Arkansas. Ail field isolates examined in fingerprint groups A and C had an 1B-49 virulence phenotype whereas most isolates in fingerprint groups B and D had an IC-17 virulence phenotype. However, several field isolates in fingerprint group B had an IG-I or an 1C-1k virulence phenotype; race IC-lk was virulent on the more recently released resistant cultivar Katy. Because race diversity was greater within the B fingerprint group based on evidence fr om the field isolates, it was hypothesized that spontaneous mutations affecting host specificity could occur more readily among isolates in fingerprint group B. Five representative isolates, one each from DNA fingerprint groups A, C, and D, and two isolates tan IG1 and an IC-17 isolate in fingerprint group B were inoculated on the cultivars M201, Newbonnet, and Katy. A marked strain (a sulfate nonutilizing mutant) of each isolate was used, along with DNA fingerprints, to rule out any possibility of cross-contamination during the inoculation tests. A single-spored isolate was recovered from 15-20 rare lesions observed on the resistant cultivar Katy after inoculation with all five isolates. None of the single-spore isolates recovered from the A or C isolate inoculations were virulent when re-inoculated onto Katy whereas 2 of 20 single-spored isolates from the D isolate inoculation were virulent when re-inoculated onto Katy. However, 13 of 16 single-spored isolates recovered from the B isolate (race IG I) inoculation and 17 of 19 single spored isolates recovered from the B isolate trace IC17) inoculation were virulent when re-inoculated onto Katy. The data indicate that spontaneous mutation to virulence on Katy occurred at a much higher frequency in the two isolates in the B fingerprint group than the isolates in fingerprint groups A, C, or D. Inoculations with additional isolates in all four fingerprint groups indicated that the frequency of the spontaneous host-specificity mutation was consistently higher among some, but not all isolates, in the B fingerprint group. Although circumstantial, the data indicate that virulent phenotypes capable of infecting Newbonnet and Katy originated from the resident population.