Development of infection structures of Uromyces transversalis in leaves of the host and a nonhost

Substomatal vesicle development of Uromyces transversalis on its host, gladiolus, was studied with a scanning electron microscope. Development after penetration occurred in distinct stages; the single infection peg developed into a globose immature substomatal vesicle, which initiated, singly or in pairs, primary hyphae that frequently developed asynchronously. One of the observed elongated primary hyphae delimited a haustorial mother cell. Observations on the orientation of the long axis of the substomatal vesicle in relation to the stomatal slit showed development of the primary hyphae at right angles rather than acute angles or parallel. A similar trend was observed in the substomatal vesicle formed in a nonhost, corn.