Teliospore germination, basidiospore formation and the infection process of Puccinia xanthii on Xanthium occidentale

Teliospore germination, basidiospore formation and the infection process of Puccinia xanthii on Xanthium occidentale were examined using light and scanning electron microscopy. Telisopores germinated within 30 min after hydration. The germ-tube differentiated into a four-celled metabasidium within 4 h of exposure to high relative humidity. A basidiospore was formed at the tip of a conical sterigma arising from each cell. Mature basidiospores left on the telial surface often produced secondary basidiospores. Basidiospores germinated by producing a single unbranched germ-tube of variable length on the leaf surface. The end of the germ-tube sometimes differentiated into an appressorium. Host penetration through the epidermis occurred at 3 to 6 h after inoculation and was associated with the formation of an intraepidermal vesicle. A septum was formed between the intraepidermal vesicle and the primary hypha at 15 h after inoculation. Primary hyphae ramified through the intercellular spaces of the palisade parenchyma cells or penetrated adjacent epidermal cells. The first terminal intracellular hypha (M haustorium) was observed at 24 h after inoculation. The terminal intracellular hypha arose from an unspecialized intercalary or terminal mother cell. Terminal intracellular hyphae showed considerable variation in shape and never grew out of the infected host's mesophyll cells.