Controlled environment experiments on epidemic rates of barley mildew in different mixtures of barley and wheat.

Controlled environment experiments on small epidemics of powdery mildew in mixtures of barley and wheat indicated that there was a relationship between the density of susceptible host units in the mixture and the rate of increase of disease within the stand best described by the equation: -r m =r s +clog e m; where r m and r s are epidemic rates in a mixture and a susceptible monoculture of the same overall stand density, m is the proportion of host plants in the mixture, and c is a constant. By the use of appropriate controls, it was shown that most of the reduction in epidemic rates in mixtures was attributable to the reduction in density of susceptible host units. Interception of air-borne inoculum by immune plants played a relatively minor role, although extrapolation of the results suggests that this factor might become significant if there was a sufficiently high proportion of immune to susceptible plants present.