et al. 2010 ), ~Halophytophthora(Hulvey et al.
2010 )and~Phytophthora(Runge et al.2011a),
all of which had previously been placed in the
Pythiaceae(Table3.4) (Dick2001a). The family
also includes the 19 genera ofdowny mildews
(Table 3.5). Most species form ovoid, often
deciduous, sporangiaon branchedaerial spor-
angiophores(Fig.3.15). InHalophytophthora,
Phytopythium,andPhytophthora,aswellasa
few zoosporic downy mildew species, the zoos-
pores are usually released from the sporangium
into a transient restrainingvesicle.However,in
the downy mildews (Fig.3.15k–s)thereisatrend
towards the suppression or complete loss of zoo-
spore formation. This has been confirmed by the
recent finding of a complete loss of zoospore-
associated genes inHyaloperonospora(Baxter et
al. 2010 ). Therefore, in many downy mildew
genera, including the two largest,Hyaloperonos-
poraandPeronospora, sporangia act as disper-
sive conidia that germinate solely by germ tubes
(Waterhouse 1973 ).
There is no doubt that this large and com-
plex family will require significant taxonomic
revision, but there is at present no consensus as
to how this should be achieved [see discussion
by Runge et al. (2011a)]. For convenience, we
divide this account of thePeronosporaceaeinto
three partswithout implying taxonomic signif-
icance but rather trophic similarities because,
apart from the downy mildews, all are paraphy-
letic. The first section includes the largely
saprotrophic generaPhytopythiumand ~Halo-
phytophthora, the second the largely hemibio-
trophic genus ~Phytophthora,and the third the
obligate biotrophic downy mildews. Recent
genomic studies have shown that both ~Phy-
tophthora(Bozkurt et al. 2012 ; Morgan and
Kamoun 2007 ; Raffaele et al. 2010 ; Tyler et al.
2006 ) and thedowny mildews (Baxter et al.
2010 ; Stassen et al. 2011 ; Tian et al. 2011 ) pro-
duce a huge range ofRxLR-type effectormole-
cules, which suppress host responses to
infection. The molecular basis for host–patho-
gen interactions is currently under intense
investigation (Jiang and Tyler 2012 ; Thines
and Kamoun 2010 ), and a detailed comparison
of obligate biotrophy in white blister rusts and
downy mildews was recently reviewed by
Kemen and Jones ( 2012 ).(iv) Part 1:Phytopythiumand
HalophytophthoraClades
The generaPhytopythium(e.g.Pp. ostracodes,
Fig. 3.15a–c) and ~Halophytophthora
(Fig.3.15d, e) form two relatively small clades
that diverge before clades containing most of
the phytopathogenic oomycetes.Phytopythium
(Bala et al. 2010 ) is synonymous with the almost
simultaneously namedOvatosporangium(Uzu-
hashi et al. 2010 ) and was formerly referred to
as the K-cladePythiums(Le ́vesque and de Cock
2004 ). Species in this clade have both morpho-
logical and physiological characteristics that
are intermediate betweenPythium andPhy-
tophthora (Bala et al. 2010 ). Species have
stronglypapillateovoid sporangia and large,
thick-walled oospores (such as inPp. ostra-
codes) (Fig.3.15c). Furthermore, some produce
elicitin-like holoproteins (Panabie`res et al.
1996 ) similar to those found inPhytophthora.
This clade includes both saprotrophs (Nechwa-
tal and Mendgen 2005 ) androot/stem-infecting
pathogens such asPp. vexansand the desig-
nated type speciesPp. sindham (Bala et al.
2010 ), which infects bananas. Consistent with
ancestral, Pythium-like characteristics and
ecology,Phytopythiumspp. form the earliest
monophyletic clade within Peronosporaceae
s. lat. (Fig.3.5b) (Bala et al. 2010 ; Uzuhashi
et al. 2010 ).
Also within Peronosporaceae s. lat. are
many ~Halophytophthoraspp., including the
species typeH. vesicula(Fig.3.15d, e). ~Halo-
phytophthora as originally defined (Ho and
Jong 1990 ; Nakagiri2002b) is a polyphyletic
genus that contains around 15 species (Hulvey
et al. 2010 ). Species in this genus are significant
saprotrophic decomposers of fallen mangrove
leaves in tropical or subtropical coastal ecosys-
tems (Ho et al. 1992 ; Nakagiri et al. 1994 ), and
some species were originally described as
~Phytophthoraspecies (Fell and Master 1975 ).
Recent investigations have revealed their pres-
ence also in salt marshes in northern Europe, in
the German Bight (Nigrelli and Thines 2013 ),Systematics of the Straminipila: Labyrinthulomycota, Hyphochytriomycota, and Oomycota 73