Cytological and Physiological Reasons for the Latent Type of Infection in Sunflower Caused by Plasmopara halstedii

Lightmicroscopy was used to investigate cytologica! and physiological reasons for the latent type of infection of sunflowers with Plashiopara halstedii. Sections of the hypocotyl of inoculated, but healthy looking plants of compatible and incompatible host-pathotype combinations revealed vital hyphae and oospores, thus indicating an uninterrupted life cycle ofthe pathogen. Plants with latent infections showed enhanced cell division activities which restricted the pathogen to the cortical parenchyma. Additionally, hypersensitive reactions inhibited further growth in the pith parenchyma. In contrast, for systemic infection a successful invasion by the pathogen of the pith parenchyma of the hypocotyl and a growth of the hyphae faster than the hypersensitive reactions of the host tissue are needed.