Occurrence of antibiotic resistance in the biological control of potato scab disease

Mutants of the pathogenic Streptomyces scabies strain RB4 that are resistant to an antibiotic produced by the potato-scab-suppressive isolate S. diastatochromogenes strain PonSSII arose spontaneously at a frequency of 10^-4. These mutants were also resistant to the antibiotic produced by the scab-suppressive S. scabies strain PonR. Eleven of these mutants were analyzed for differences in their fatty acid and DNA fingerprint profiles and in their abilities to produce the phytotoxin thaxtomin and to cause scab disease. Only minor differences between the mutants and the parent isolate were observed in fatty acid composition. By DNA fingerprint analysis, the mutants were closely related to the parent strain but most of the mutants were missing one or more of the PCR-amplified DNA bands. The mutants tended to cause less disease than the parent strain as measured by percentage lesion coverage. When the total number of lesions per tuber was quantified, the average level of disease caused by the mutants was comparable to that of the parent strain. Two of the 11 mutants lost pathogenicity and thaxtomin production altogether. These results indicate the occurrence of a high frequency of phenotypic and potential genotypic instability in the pathogen strain. The resulting variants are closely related but are resistant to the antibiotics produced by scab-suppressive streptomycetes and are generally less pathogenic.