Mycology 12, 275-283. The effect of six different substrates on basidiospore germination and differentiation of infection structures was investigated in three species of Uromyces:U. appendiculatus var. appendiculatus, U. viciae-fabae, and U. vignae. Light and fluorescent microscopic observations were made after 3 days of incubation on the following substrates: 2 or 5% water agar, glass slide, collodion membrane, nitrocellulose sheet, and isolated host cuticle. Infection structure development up to the formation of a primary hypha was observed on hard substrates, such as 5% water agar, nitrocellulose sheet, and on isolated host cuticle. Addition of oil to the collodion membrane did not significantly increase the differentiation rate. The morphology and nuclear condition of these basidiospore infection structures were similar to those observed in host tissue. For the three directly penetrating species we studied, it appears that basidiospores respond to an unspecific stimulus, such as the thickness of host cuticle or the hardness of an artificial substrate. o 1988 Academic Press, Inc.

SUMMARY -The haustorium of the uredial stage of Cronartium flaccidum in Vincetoxicum hirundinaria was studied with TEM. The observations concern the haustoria! mother cell and its relation to the host cell wall, the penetration peg, the shape and the cytoplasm of the haustorium and some host cell organelles in relation to the haustorium. The presence of collars was noted. These features are similar to those described for host-dikaryotic haustorium relationship in other rusts. The extrahaustorial matrix however shows a unusual development. In fact, it wraps the whole length of haustoria! neck although it is separated from the haustoria! neck wall by the extrahaustorial membrane doubled in this region. The probable significance of this host-parasite interface, perhaps limited to the genus Cronartium, is discussed.