The importance of dispersal mechanisms in the epidemiology of Phytophthora blights and downy mildews on crop plants

Epidemics of plant diseases share a basic structure with those of human and animal diseases; however, unlike most epidemics of people and animals, fungi or fungi-like organisms are of major importance as pathogens. Because of the relatively low value of an individual in most plant populations, botanical epidemiologists perform observational as well as designed studies repeatedly on epidemics in large populations. Plant disease epidemics develop from overlapping cycles of propagule dissemination, host encounter and infection, pathogenesis (including symptom development), and pathogen reproduction. initial infective units, or propagules, originate from survival structures formed between cropping seasons or during periods when weathers conditions are not suitable for disease initiation. Propagules may also arrive by long-distance transport from other geographic areas where conditions are suitable for the occurrence of continuous epidemic cycles. The importance of dispersal mechanisms in epidemic development are illustrated with three solanaceous pathosystems: Phytophthora blight of pepper (caused by Phytophthora capsici), blue mold of tobacco (caused by Peronospora tabacina), and late blight of potato (caused by Phytophthora infestans).