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Ecology and Epidemiology

Active Sporangium Discharge by Peronospora destructor. Charles M. Leach, Professor, Department of Botany and Plant Pathology, Oregon State University, Corvallis 97331; Phytopathology 72:881-885. Accepted for publication 23 November 1981. Copyright 1982 The American Phytopathological Society. DOI: 10.1094/Phyto-72-881.

Sporangium discharge by Peronospora destructor on infected onion leaves was photographed and visually observed using special illumination. The sporangia, whether liberated spontaneously or triggered by vibration (a single light tap) consistently were propelled along essentially parallel trajectories perpendicular to leaf surfaces irrespective of the orientation of the leaf surface to gravity. These results could not be explained on the basis of the generally accepted hygroscopic-twisting mechanism and an alternative electrostatic mechanism is proposed. An electrostatic mechanism was indicated by the occurrence of a highly organized pattern of sporangium discharge perpendicular to leaf surfaces, positively charged sporangia, and the determination that modifications of leaf voltages significantly influenced discharge velocity. Velocities of sporangia liberated from leaves at 200V(+) DC were 129% greater than for those discharged from leaves without supplemental voltage applied and velocities for sporangia liberated from leaves at 800V(+) were 8082% greater than those for leaves at 200V. Active liberation of sporangia by P. destructor is similar to the active conidium release reported for Drechslera turcica.