The greater the area of a uniform host population with a single vertical resistance, the more dangerous that resistance becomes. This is because of an increased selection pressure for the matching parasite, and an increased loss when the matching does occur. The greater the area of uniformity of vertical resistance, therefore, the greater the danger. Conversely, the greater the area of a uniform host population with high horizontal resistances, the more effective the horizontal resistance becomes. This is because parasite interference declines as the area of a horizontally resistant host population increases, and it is least when the entire crop of a region has a high level of horizontal resistance in all of its cultivars. The greater the area of uniformity of horizontal resistance, therefore, the greater the security.