The systemic nature of the disease of sunflower plants caused by Diaporthe helianthi, the leaf–petiole–stem route of the host invasion by the fungus, and the plant tissues that were successively affected were demonstrated through histological studies. After penetration into the host, the infection hyphae invade the intercellular spaces and terminal veinlets of the lamina and spread toward larger branches of the conducting system, the midrib, and the petiole. Xylem elements are invaded but are affected less by the fungus attack than the phloem and the parenchyma tissues, which disintegrate completely. Hyphae spread through the leaf axil to the tissues of the stem cortex, where pycnidia of the Phomopsis anamorph are initiated from internal masses of mycelium. Key words: systemic plant diseases, sunflower diseases, Diaporthe helianthi, Phomopsis helianthi.