Introduction

Plant leaf surfaces (here termed the phyllosphere) are colonized by a wide range of bacteria, yeasts and filamentous fungi. They include epiphytic fungi, endophytes and pathogens. The total community and its structure are influenced by a range of abiotic and biotic factors. These include not just atmospheric pollutants but also wind, precipitation, water availability and pH. Microbial community structure may also be influenced by the presence of nutrients from insect honey dew, pollen, leaf exudates and organic debris. Interactions between abiotic and biotic factors can occur. For example, atmospheric pollutants have been demonstrated to markedly increase the weathering of conifer needle surfaces and to decrease the protective wax fibrillar structure (Rinallo et al., 1986), which could increase nutrient exudates in the phyllosphere and thus influence patterns of colonization and perhaps senescence of needles.

The impact of atmospheric pollutants on both epiphytic and endophytic fungi colonizing plant leaves has received increasing attention by being implicated directly or indirectly in ‘forest decline’ in parts of Europe where significant premature senescence and defoliation of forests occurred in the 1970s and 1980s (Boddy et al., this volume; Schutt & Cowling, 1985; Kandler, 1990). For example, Rehfuess and Rodenkirchen (1985) implicated the endophytes Lophodermium piceae and Rhizosphaera kalkhoffii in premature senescence and needle reddening disease. However, Butin and Wagner (1985) suggested that they may be only early colonizers of dying or dead needles and therefore not involved in such forest decline syndromes.