Screening for spontaneous virulent mutants of erysiphe-graminis-f-sp-hordei on barley hordeum-vulgare lines with resistance genes ml-a-1 ml-a-6 ml-a-12 and ml-g

Seedlings of 4 barley lines with powdery mildew resistance genes Ml-al, Ml-a6, Mla12 or Ml-g were inoculated with powdery mildew culture CR3 which is avirulent to the 4 host lines. Inoculation density was 1.2 infectious conidia/mm2, and in total 50 million conidia were screened for the occurrence of virulent mutants. During 30 cycles of screening, 43 putative virulent mutants were selected, multiplied and tested. They were grouped in 5 genotypes according to the virulence spectrum. Based on virulence, mating type, biochemical tests and analyses of test crosses, 3 of the types were rejected as being of mutational origin, and the verification of the remaining 2 were not consistent with the expectations deduced from a gene-for-gene interaction. Provided that none of the genotypes found were of mutational origin, the spontaneous mutation frequency from avirulence to virulence in barley powdery mildew is below 2 .times. 10-8. A reconstruction experiment showed that the density of avirulent inoculum did not reduce the survival rate of rare virulent genotypes.