Light and Electron Microscopic Studies on the Development of Soybean Rust (Phakopsora pachyrhizi Syd.) in Susceptible Soybean Leaves

The development of Phakopsora pachyrhizi from infection to uredospore development was studied by light‐ and electron microscopy. Uredospores germinated with a single germ tube, formed appressoria and infected always by direct, cuticular penetration. Penetration started with the formation of an appressorial cone which was continuous with the cell wall of the penetration hypha. The penetration hypha entered the epidermal cell, transversed it and reached the intercellular space of the mesophyll where the first septum was formed separating the penetration hypha from the primary hypha. Spread of the fungus in the tissue was followed in whole‐leaf mounts. The first haustorium was visible between 24 and 48 hours after inoculation. Haustoria were formed in mesophyll and epidermal cells. Occasionally, encapsulations of haustoria were observed. In sections of uredia two cells, obviously the pedicel and the sporogenous cell, were observed below developing uredospores. This indicates that uredosporogenesis in P. pachyrhizi is the same as known from other rust fungi. As in the case of other rusts partial septa and perforate septa were found in the sporogenous tissue. Perforate septa were also observed elsewhere in hyphae of the infected tissue.