Introduction to Fungi, Third Edition

104 STRAMINIPILA: OOMYCOTA
Fig 5.20 Phytophthorainfestans.
(a) Sporangiophores penetrating a stoma of a
potato leaf. (b) Zoospores and zoospore cysts, one
formed inside a zoosporangium. (c) Intercellular
mycelium from a potato tuber showing the
finger-like haustoria penetrating the cell walls.
Note the thickening of the cell walls around the
haustorium.
those of Pythium. On the host plant, the sporangiophores
may emerge through the stomata,
as in P. infestans (Fig. 5.20a). The first sporangium
is terminal, but the hypha bearing it may push
it to one side and form further sporangia by
sympodial growth. Mature sporangia of most
species have a terminal papilla which appears as
a plug because it consists of material different
from the sporangial wall (Coffey & Gees, 1991).
In species of Phytophthora infecting aerial
plant organs, the sporangia are detached, possibly
aided by hygroscopic twisting of the sporangiophore
on drying, and are dispersed by wind
before germinating. In aquatic or soil-borne
forms, zoospore release commonly occurs
whilst the sporangia are still attached; internal
proliferation of attached sporangia may occur.
Whether deciduous or not, sporangia may
germinate either directly by means of a germ
tube, or by releasing zoospores. The latter seems
to be the original route because undifferentiated
sporangia contain pre-formed flagella within
their cytoplasm, and these are degraded under
unfavourable conditions leading to direct germination
(Hemmes, 1983; Erwin & Ribeiro, 1996).
The mode of germination is dependent on
environmental parameters. For example, in
P. infestans, uninucleate zoospores are produced
below 15°C whilst above 20°C multinucleate
germ tubes arise. Further, with increasing age
sporangia lose their capacity to produce zoospores
and tend to germinate directly. In
P. cactorum, sporangia have been preserved for
several months under moderately dry conditions.