Most plant diseases consist of delicate interactions between higher plants and microorganisms. Acidic precipitation represents an environmental stress that has been shown to affect expected development of some diseases and similar phenomena under experimental conditions. From the perspective of the 'disease triangle' framework, this impact may be expressed through increased plant susceptibility, decreased plant susceptibility through altered metabolic pathways, decreased pathogen activity or increased pathogenicity. Aerial plant parts are directly exposed to rainfall and the potential influence of acidic precipitation on pathogen and diseases of above ground tissues seems obvious. However, soilborne pathogens and root diseases may also be significantly altered by short-term acidification of the soil solution resulting from acid deposition, or by gradual changes in bulk soil chemical characteristics over a long period of time.