Single spore infections with sunflower downy mildew

A technique was developed to select vital zoospores of Plasmopara halstedii after zoosporogenesis for the infection of sunflower leaf disks. Successfu l infections were performed with samples containing several zoospores origi nated from a single sporangium as well as with samples containing only one single zoospore. Infections were indicated by spontaneous sporulation at th e surface of inoculated sunflower leaf discs and were secured by transfer t o whole seedlings. Using this method it was possible to generate unithallic infections and genetically homogenous strains of sunflower downy mildew.