Most plant diseases consists 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 incrased plant susceptibility (through physical wounding or impaired physiological activity), decreased plant susceptibility through altered metabolic pathways, decreased pathogen activity (from intolerance to low-pH conditions by vegetative or survival structures, or from increased activity of competitors) or increased pathogenicity (by stimulation from more favorable pH conditions for acidophilic organisms, or by elimination of competitors). Aerial plant parts are directly exposed to rainfall and the potential influence of acidic precipitation on pathogen and diseases of aboveground tissues seems obvious. However, soilborne pathogens and root diseases may also be significantly altered by short-term acidification of t he soil solution resulting from acid deposition, or by gradual changes in bulk soil chemical characteristics over a long period of time. although research in this area is only beginning, knowledge of the biological, ecological and economic impacts of diseases of agricultural crops and native vegetation suggests that acidic precipitation plays a significant role in the subtle interactions of plants and their parasites.
