Urban distribution of Rhytisma acerinum (Pers.) Fries (tar spot) on sycamore

This study examined ecological aspects of the distribution of tar spot disease caused by the fungus Rhytisma acerinum (Pers.) Fries on Acer pseudoplatanus L. (sycamore). The host is abundant in Edinburgh and surrounding areas yet no tar spots were found in the city, although they were abundant on sycamore in the surrounding countryside, especially at sheltered sites. The vertical distribution from a localized source of leaf litter from infected sycamore showed an approximately linear relationship between tar spot index of attached leaves (TSI = no. of tar spots per 100 cm² of leaf) and the log of height of such infected foliage above the ground. The distribution of tar spots was controlled primarily by the presence or absence of overwintered sycamore leaves infected with R. acerinum. In Edinburgh these leaves are actively removed by man and passively by wind, so reducing the potential source of inoculum by the following spring. The presence of SO₂ was unimportant in the distribution of R. acerinum. An experiment to study the relative importance of current air concentrations of SO₂ on the incidence of infection by R. acerinum showed that the average SO₂ concentration of < 50 μg m^-3 hail no effect. The observed distribution of tar spot throughout the city and surrounding countryside could be explained entirely by distance from the nearest source of inoculum without involving a role for air pollutants.