In Denmark powdery mildew susceptible spring barley varieties were replaced by major gene resistant varieties in between 1963-1974. Varieties with resistance from Hordeum laevigatum covered about 1/3 of the barley area in 1968-1974, and 51 and 71% in 1975 and 1976, respectively. The severity of powdery mildew of these varieties remained at 20-25% of that on susceptible varieties for 15 yr. Varieties with resistance from 'Arabische' and 'Lyallpur' cover about half the barley area shortly after their introduction in 1967 and 1971, respectively, but they disappeared again rapidly. The severity of powdery mildew on these varieties was low at first; however, it increased sharply when the varieties became widely distributed. The severity of powdery mildew on spring barley in Denmark is determined by the frequencies of the matching virulences in the initial inoculum, which is wind-borne to Denmark each spring from overwintering barley crops in Germany and the UK, the distribution of the resistances on the barley area in Denmark, and the selective advantage of the matching virulences. Because the virulences matching the above 3 resistances apparently are present in the initial inoculum with similar frequencies, the stability of the resistance from H. laevigatum may be ascribed to the intermediate level of resistance, and the instability of resistance from 'Arabische' and 'Lyallpur' to the high level of resistance, which, respectively, delays and favors rapid multiplication of the matching virulences during each growing season.
|Journal||The Royal Veterinary and Agricultural University|
|Publication status||Published - 1978|