Introduction to Fungi, Third Edition

616 UREDINIOMYCETES: UREDINALES (RUST FUNGI)
Fig 22.4 Appressorium formation by urediniospore germ tubes. (a) The runner bean rust Uromyces appendiculatus.Urediniospores
inoculated onto a polystyrene surface with artificial ridges (0.5 mm high) have germinated and differentiated appressoria upon
reaching the ridge. (b) The broad bean rust Uromyces viciae-fabae.The germ tube has followed topographical features of the host
surface until it has reached a stoma, over which an appressorium has formed. (a) Reprinted from Allen et al. (1991) with permission
by APS Press.Original print kindly provided by H.C. Hoch. (b) reprinted from Mendgen (1997), with kind permission of Springer
Science and Business Media.Original print kindly provided by K. Mendgen. Both images approximately to same scale.
Fig 22.5 Schematic comparison of infection processes in Uromyces. (a) Germinating basidiospore.The monokaryotic germ tube
infects directly through the epidermal wall without forming a pronounced appressorium.Growth is both inter- and intracellular.
Differentiated haustoria are not formed, and intracellular hyphae are regarded as M-haustoria. (b) Germinating urediniospore.
The dikaryotic germ tube forms an appressorium above a stoma and infects through the stomatal opening.Growth in planta is
mainly intercellular. Stalked globose haustoria (D-haustoria) arising from haustorial mother cells are the only intracellular organs
of note. Both M-haustoria and D-haustoria are surrounded by a matrix (not drawn). Based on Mendgen (1997) and Mendgen and
Hahn (2002).
(Gold & Mendgen, 1991). These are sometimes
called M-haustoria. In contrast, differentiated
haustoria are formed by dikaryotic hyphae
(D-haustoria), and valuable physiological work
has been performed on them. This is summarized
in the following section.
22.2.4 Thephysiologyofbiotrophyin
rust fungi
The dikaryotic haustorium of rust fungi
(Fig. 22.7) is functionally very similar to that of
the Erysiphales (see Fig. 13.5). The haustorial
mother cell emits a narrow penetration tube