CYTOLOGICAL AND PHYSIOLOGICAL REASONS FOR THE LATENT TYPE OF INFECTION IN SUNFLOWER CAUSED BY PLASMOPARA-HALSTEDII

Lightmicroscopy was used to investigate cytological and physiological reasons for the latent type of infection of sunflowers with Plasmopara halstedii. Sections of the hypocotyl of inoculated, but healthy looki ng plants of compatible and incompatible host-pathotype combinations r evealed vital hyphae and oospores, thus indicating an uninterrupted li fe cycle of the pathogen. Plants with latent infections showed enhance d cell division activities which restricted the pathogen to the cortic al parenchyma. Additionally, hypersensitive reactions inhibited furthe r growth in the pith parenchyma. In contrast, for systemic infection a successful invasion by the pathogen of the pith parenchyma of the hyp ocotyl and a growth of the hyphae faster than the hypersensitive react ions of the host tissue are needed.