Controlled environment experiments on epidemics of barley mildew in different density host stands.

Controlled environment experiments on small epidemics of powdery mildew of barley, an air-borne disease caused by Erysiphe graminis f.sp. hordei, indicated that there was a direct linear relationship between host density and the rate of increase of disease within populations. Under the particular experimental conditions used, the overall infection rate was almost doubled (from 0.39 to 0.75 per unit per day) following an increase in density from 31 to 115 host units per m2. In separate experiments these overall epidemic rates were partitioned into two separate components related to inoculum transmission between plants and inoculum transmission within plants.