FREISLEBEN and LEIN (2) obtained a mutant line resistant against the mildew races 1, 2 and 4 after the exposure of dry seeds of a pure line of the barley variety "Haisa" to X-rays (12,000 r) . This is the first case of X-ray-induced resistance against plant diseases reported in the literature . This mildew-resistant mutant yielded less than the mother line, most probably as a consequence of the necrotic spots developed in advanced stages of growth . The mildew-resistance and the necrotic spots of this line were inherited together (NoTZEL, 13) .
In 1951, X-ray breeding in the barley variety "Vollkorn" was started at the Plant Nursery and Plant Breeding Station Probstdorf, N A (Austria), with the intention to produce and to select mildew-resistant mutants of practical value . "Vollkorn" is the widest distributed variety of brewing barley in Lower Austria and susceptible to the mildew races A, B, C, D, F as classified by ULONSKA (16) . The "Vollkorn"-line used in this experiment was originally selected by E . VON TsCHERMAK (Vienna) who in 1928 found a few individuals with shorter kernels and a somewhat shorter culm in a line of the variety "Hanna" . During the following 23 years of pedigree breeding no mildew-resistant type among 2,500 single plant progenies of this "Vollkorn" line was found .