Cytological and physiological reasons for the latent type of infection in sunflower caused by Plasmopara halstedii.
Light microscopy was used to investigate the cytological and physiological reasons for the latent infection of sunflowers by P. halstedii. Sections of the hypocotyl of inoculated but healthy looking plants of compatible and incompatible host-pathotype combinations, revealed vital hyphae and oospores, suggesting an uninterrupted life cycle of the pathogen. Plants with latent infections showed enhanced cell division which restricted the pathogen to the cortical parenchyma. Additionally, hypersensitive reactions inhibited further growth in the pith parenchyma. In systemic infection, successful invasion of the pith parenchyma of the hypocotyl, and hyphae growth exceeding the hypersensitive reactions of the host tissue, are required.