Greek epidemics of grapevine downy mildew are driven by local oosporic inoculum: a population biology approach ARTEMIS RUMBOU1 and CESARE GESSLER2 1. Plant Protection Institute, National Agricultural Research Foundation, P.O. Box 1303, 38001, Volos, Greece 2. Phytomedicine/Pathology, Institute of Plant Sciences, Swiss Federal Institute of Technology, 8092, Zürich, Switzerland Abstract Epidemics of grapevine downy mildew are usually assumed to be initiated by oospores, but the spread of the disease is commonly attributed to the secondary, asexual inoculum. Evidence from the investigation of population structures observed in 15 natural downy mildew epidemics which occurred in seven Greek vineyards and for three consecutive years, however, showed that primary infections from oospores play a major role in both the initiation and subsequent development of the epidemic. Furthermore, oosporic infections were found responsible also for the disease recrudescence in September. The role of secondary infections in the epidemic, although it differs depending on the region and on the sampling date, is mostly not important for the epidemic growth and pathogen dispersal. However, the clonal multiplication of some genotypes can, in rare instances, be of significance for the spread of the epidemic. The epidemic patterns that appeared can be grouped into a) the "continuous" epidemic pattern in humid areas, b) the “two-peak” epidemic in areas with dry summer, where bottleneck events during summer halt the disease and c) the “predominance-of-one-clone” pattern in islands/coasts, where the secondary infections have greater impact on disease growth than the oosporic ones. The present results, using both epidemiological and population genetics data, in combination with similar research accomplished under Central European conditions, constitute a first approach to Plasmopara viticola population biology   < back .