The Effect of Mildew and Scald Infection on Yield and Quality of Barley 1

OWDERY mildew, Erysiphe graminis h.ordei MarP chal, and Rhynchosporium scald, Rhynchosporium secnlis (Oud.) Davis, rank with barley stripe as being the most destructive diseases of barley in California. While not as spectacular as barley stripe or such diseases as stem rust'and bunt of wheat, their widespread distribution and annual occurrence make them a potential threat to barley production year in and year out. The addition of mildew and scald resistance to the variety Atlas was a distinct advancement toward stabilizing barley production in California. The resultant variety, Atlas 46, has proved a useful tool in measuring the effect of mildew and scald on yield and quality, the importance of which has never been fully appreciated. Several investigators have established linear relationships between the incidence of certain diseases, which either kill the plants or replace their fruiting bodies, 'and reduction in yield (4, 7, 13, 14). Losses due to foliar diseases have been more difficult to measure. Dusting a portion of the crop to control or minimize disease development has been used frequently and fairly successfully (3, 8, 9, 10, 11, 12). Complete control of the diseases, however, is difficult to obtain; the plants, moreover, are subjected to unnatural conditions. Yarwood (16) found that dusting clover with sulfur increased the respiration of both healthy and mildewinfected leaflets. The development of essentially isogenic varieties which differ only in their disease reaction seems to be the most logical approach in measuring the effect of disease development on yield and quality. Two methods are available for obtaining such varieties, namely, backcrossing or the continuous selection in successive generations of plants heterozygous for the desired character. Of these, backcrossing appears to be the more meritorious. The resultant varieties, in addition to being available for commercial production, will have the same range of adaptation as the recurrent parents. Consequently, the fullest expression of the given character may be expected. Varieties developed in this manner, while not completely isogenic, attain a high degree of homozygosity for the genic complement of the recurrent parent. On a theoretical basis, i f progeny from a cross between two parents differing by 100 independent gene pairs are backcrossed seven times, 45.6% of the resulting plants would be homozygous for all genes of the recurrent parent and 99.8% would be homozygous for 96 or more of the recurrent genes. Linkage between genes would increase the percentage of homozygosity for all genes