Ultrastructure of the Infection and Early Colonization ofPinus TaedabyCronartium QuercuumFormae SpecialesFusiforme

Abstract

Basidiospore germlings of Cronartium quercuum f. sp. fusiforme grew randomly on the surface of Pinus taeda hypocotyls. Polymorphic appressoria developed from germ tube apices and were the first-formed components of the intracellular infection structure apparatus. A narrowed infection peg developed from each appressorium and penetrated directly into an epidermal cell giving rise to a narrowed intracellular neck region and an expanded body which produced an infection hypha. Therefore, the intracellular infection structure apparatus consisted of an extracellular component, the appressorium, as well as the intracellular neck region, body, and infection hypha. The intracellular portions of the infection structure were bounded by a sheath and the invaginated host plasmalemma, possessed a simple perforate septum in the neck region, and a non-septate to variously septate body. The infection hypha exited the infected epidermal cell to colonize adjacent cells and tissues by either of two mechanisms: 1) intracellular hyphae from the infection hypha grew directly into adjacent cells without the formation of an intercellular phase, or 2) the infection hypha produced an extensive intercellular hyphal network. These intercellular hyphae ramified into the cortex and