Introduction to Fungi, Third Edition

656 USTILAGINOMYCETES: SMUT FUNGI AND THEIR ALLIES
Fig 23.16 Exobasidium japonicum. (a) Basidium on the lower surface of an evergreen azalea. (b) Freshly discharged aseptate
basidiospores. (c) Basidiospore after a few hours’ incubation on potato dextrose agar.The spore has formed a transverse septum
and has germinated. (d) Basidiospore after12 h.The germ tubes have branched and are budding off elongated yeast cells.
Fig 23.17 Lobed haustorium of Exobasidium sp.Three
interaction sites with the host cytoplasm are numbered. Site1
is shown in median section, and the interaction ring is visible
(arrowheads).Sites2and3areintangentialsection,showing
electron-dense deposits (d).Tubulovesicular cytoplasmic
structures involved in secretion of the electron-dense matter
are also visible (arrows).The penetration site of the host wall
is at the bottom of the picture (double arrowheads).
Reprinted from Begerow et al.(2002),Mycological Progress,
with permission of IHW-Verlag; original print kindly provided
by D. Begerow.
(Plates 12i,j). Infection commonly gives rise to
hypertrophied tissue. Keys and descriptions have
been given by Nannfeldt (1981) and Ing (1998).
Frequently infected host plants include members
of the Ericaceae, e.g. Vaccinium (bilberry, blueberry)
in heaths, moors and forests, and ornamental
azaleas and Rhododendron spp. in gardens.
Cosmopolitan examples are E. japonicum on
evergreen azaleas (Plate 12i; Fig. 23.16) and
E. vaccinii on Vaccinium spp. (Plate 12j). A second
group of host plants is the Theaceae family, with
the tea plant (Camellia sinensis) a prominent
casualty of E. vexans which adversely affects the
quantity and quality of the tea harvest (Gulati
et al., 1999).
Infections by Exobasidium spp. are easily
confused with symptoms caused by Taphrina
(Plate 4a,c), and these two genera, although
phylogenetically unrelated, have much in
common. Plant tissues infected by Exobasidium
are often swollen and show a pale or reddish
discoloration. There is an obvious hormonal
imbalance in the infected host tissue, although
no detailed examinations have been carried out.
Infected shoots are often affected in their reproductive
development (Wolfe & Rissler, 1999).
A whitish superficial layer of basidia is eventually
produced on the surface of infected host
tissue, and these carry about 2 8 sterigmata