The development of infection structures of Plasmopara halstedii in suspensions of sunflower cells.
Zoospores germinate in water by a germ tube which elongates but does not differentiate. Zoospores in a suspension containing sunflower cells produced a short germ tube terminating in an appressorium flattened against a host cell. Zoospores which adhered to the Petri dish without contacting a host cell also produced appressoria-like structures, which in some cases differentiated further into structures resembling infection vesicles in P.-infected sunflower cells. Initiation of infection structures may require diffusible products from host cells but apparently does not require contact with the cells.