Pathohistology of sunflower stems attacked by Diaporthe helianthi

Abstract

Nearly 7000 histological preparations of sunflower plant parts attacked by the holomorph Diaporthe helianthi – Phomopsis helianthi Munt.-Cvet., Mihaljčević et Petrov have been examined since 1980. After foliar infection the hyphae progress through the vascular system and then spread to invade other tissues. Hyphal masses that form in the host cortex represent pycnidial primordia. When fully differentiated the pycnidia expand towards the host periphery and rupture the epidermis. The stem cankers with the conidiomata of the fungus represent an advanced stage of pathogenesis. The nuclear phase change of the fungus occurs in the host pericycle, beneath the endodermis. Ascogonia can be observed beginning in the autumn; their development into protoperithecia and perithecia takes place slowly and unevenly during the subsequent months. Perithecial maturation in spring, when ascospores are abundantly released, must coincide with the onset of host vegetation to satisfy the nutritional needs of the anamorphic, parasitic phase of the fungus. Exceptions to this general scheme have been observed, with ascospore maturation occurring during the winter months.