EFFECTS OF SOWING DATE AND VERNALIZATION ON THE GROWTH OF WINTER BARLEY AND ITS RESISTANCE TO POWDERY MILDEW (ERYSIPHE-GRAMINIS F-SP HORDEI)

Detailed studies on the production of individual leaves, and the devel opment of powdery mildew on them, were made in field plots of winter b arley sown on different dates. The greater severity of the disease on early-sown than on later-sown seedlings during the autumn and winter c an probably be explained mainly by changes in the abundance of inoculu m and the suitability of the weather for infection. Results from glass house experiments suggest that the differences may be reinforced by di rect effects of vernalisation on the susceptibility of seedlings to th e disease. Contrary effects of sowing date on mildew severity during s ummer are probably due to the progressively greater resistance to mild ew of the later-formed than of seedling leaves, and the earlier appear ance of corresponding leaves on early-sown than on later-sown plants. Early sowing can also increase the total number of leaves produced per stem. Therefore, because resistance of the leaves increases progressi vely, the maximum degree of resistance expressed by the later-formed ( e.g. flag) leaves will often be greater on early-sown than on later-so wn plants.