Introduction to Fungi, Third Edition

80 STRAMINIPILA: OOMYCOTA
Table 5.1. Summary of the most important groups of Oomycota and their characteristic features. Only the
last four groups are considered further in this book. Based on information provided by Dick (2001a,b) and
Kirk et al. (2001).
Order
Number
of species
Thallus and
reproduction
Ecology
Myzocytiopsidales
(incertae sedes)
Olpidiopsidales
(incertae sedes)
74 Holocarpic, later
coralloid or breaking
up into segments.
Zoospores, oospores.
21 Holocarpic, becoming
converted into a
sporangium. Zoospores,
oospores.
Rhipidiales 12 Eucarpic with rhizoids.
Zoospores, oospores.
Leptomitales 25 Constricted hyphae
producing sporangia.
Zoospores, oospores.
Saprolegniales
(see Section 5.2)
Pythiales
(see Section 5.3)
Peronosporales
(see Section 5.4)
Sclerosporaceae
(see Section 5.5)
132 Mycelium of wide stout
hyphae. Zoospores,
oospores.
4200 Mycelium of relatively
narrow hyphae.
Zoospores, oospores.
252 Intercellular mycelium with
haustoria.Differentiated
sporangiophores.
Zoospores or‘conidia’,
oospores.
22 Mycelium of very narrow
hyphae.Differentiated
sporangiophores.
Zoospores or‘conidia’,
oospores.
Parasites of invertebrates or algae.
Biotrophic parasites of Oomycota,
Chytridiomycota and algae.
Freshwater saprotrophs, facultatively
or obligately anaerobic.
Freshwater saprotrophs or parasites
of animals.
Saprotrophs or necrotrophic pathogens
of animals, plants and other organisms.
Saprotrophs or pathogens
(often necrotrophic) of plants, fungi and
animals.
Biotrophic plant pathogens, causing
downy mildews and other diseases.
Biotrophic pathogens of grasses, causing
downy mildews.
For thallus terminology, see Fig.6.1.
mechanisms of hyphal polarity and growth
regulation in Achlya and Saprolegnia (see Heath,
1995b; Hyde & Heath, 1997; Heath & Steinberg,
1999). Like other Oomycota but in contrast to the
Eumycota (Pfyffer et al., 1986; Rast & Pfyffer,
1989), these fungi are unable to synthesize
compatible osmotically active solutes such as
glycerol, mannitol and other polyols to maintain
their intrahyphal turgor pressure against fluctuating
external conditions. Under conditions of
water stress, the turgor pressure in hyphae of
Achlya and Saprolegnia approaches zero, yet hyphal
growth can still occur at least under laboratory
conditions because of the enhanced secretion of