The expression of partial resistance to Erysiphe graminis in spring barley

The development of powdery mildew on five barley varieties selected for partial resistance in the field and lacking identifiable, effective race-specific resistance factors was examined. Inoculation tests demonstrated that a reduction in the number of colonies was a major component of this resistance. Microscopic examination of segments from the fourth and fifth leaf revealed several stages at which colony development was arrested. The failure of individual penetrations to establish compatible colony growth was classified as being due either to hypersensitive or to non-hypersensitive resistance in host epidermal cells. Varying proportions of these two types of resistance mechanism were evident in all varieties examined, including the susceptible controls. In four resistant varieties, one or other mechanism predominated. The possibility of enhancing partial resistance by combining sources in which different mechanisms operate is discussed.