In analogy with the theory of island biogeography, the immigration rate of a pathogen is expected to decline with increasing isolation of a host patch and the local extinction rate of the disease to decline with increasing patch size. This was tested on U. violacea infecting V. vulgaris. Larger host patches had a higher incidence of disease than did smaller ones. No patches smaller than 35 individuals were infected. There was a positive correlation between patch size and infection rate in patches with Uetilago. Patch size was not correlated with mean size of Viscaria individuals. Median infection rate of host plant individuals of larger patches was 8% in an area with low density of Viscaria but 25% in another area with higher Viscaria density. Since infected plants become sterile, Ustilago acts as a population regulatory factor slowing down the increase and dispersal of local populations of Viscaria after they have reached a certain size.