2011 ; Thines et al. 2008 ), which contain impor-
tant pathogens of cereal crops in the semi-arid
tropics.
The vast majority ofdowny mildew species
are pathogens of so-called eudicot families,
although whether pathogen species are restricted
to a given host species or wider host families has
often been debated (Constaninescu 1991; Dick
2001; Thines et al.2009c). However, in recent
years molecular studies have helped clarify
host–pathogen relationships and confirmed that
most downy mildew species have very narrow
host ranges (Choi et al. 2007 , Riethmu ̈ller et al.
2002 ; Thines 2011 ; Thines et al.2009b;Voglmayr
2003 ; Voglmayr and Thines 2007 ;Voglmayretal.
2004 ), often encompassing only a few or even a
single species within a given host genus. Only a
comparatively small number of species, such as
Pseudoperonospora cubensiss. lat., are able to
infect more than one host genus (Choi et al.
2005 ;Go ̈ker et al. 2004 , 2007 ; Runge and
Thines 2011 , 2012 ; Runge et al.2011b, 2012 ).
The next recognized subgroup of downy mildews
are parasitic to members of the Brassicaceae;
they are known as theBrassicolous downy mil-
dewsand include the intensively studied genus
Hyaloperonospora(Fig.3.15o, p) (Tyler et al.
2007) and the genusPerofascia(Table3.5). The
third downy mildew subgroup (Table3.5)are
those with coloured (pigmented) conidia
(DMCC), which includes the economically
important species-rich genera Peronospora
(Garcı ́a-Bla ́zquez et al. 2008 ; Voglmayr and Con-
stantinescu 2008 )andPseudoperonospora
(Thines et al.2009c) and has over 500 listed
species. Genera such as Hyaloperonospora
(Fig.3.15o)andPeronosporaoften have elongate
digit-like haustoria(as inHyaloperonospora)
(Fig.3.15p), which are similar in morphology
and ultrastructure (Beakes et al. 1982 ; Hickey
and Coffey 1977 ) to those formed by hemibio-
trophicPhytophthoraspecies such asPh. infes-
tans(Coffey and Wilson 1983 ). The final major
monophyletic lineage is a diverse group known
as the downy mildews with smallpyriform haus-
toria(DMPH)(Fig.3.15m); it contains eight
genera (Table3.5), includingBasidiophora,Bre-
mia(Fig.3.15n),Benua,Paraperonospora,Plas-
mopara(Fig.3.15l), andPlasmoverna[for a
review see Thines et al. (2009c)], in addition to
NovotelnovaandProtobremia, which are closely
allied withBremia(Fig.3.15n).III. Selected Developmental and
Morphological TrendsA. Zoospore Characteristics- Zoospore Morphology and Flagellar Rootlet
Organization
Zoospore organization and flagellum micro-
morphology have been extensively reviewed
(Barr 1981 ; Barr and De ́saulniers 1989 ; Beakes
1987 , 1989 ; Dick 1990 ,2001a,b; Fuller 1990 ,
2001 ; Perkins 1976 ). The stramenopile zoo-
spore (Fig.3.16) shows a remarkably uniform
and conserved structure overall, supporting the
origin of this clade from a common flagellate
ancestor (Tsui et al. 2009 ). Stramenopile zoos-
pores range in size between 3 and 15mm (Dick
2001), and many are reniform with laterally
inserted flagella, as in the Labyrinthulomycota
and most oomycetes (Fig.3.16a, b, l–r). In most
stramenopiles the anterior flagellum is deco-
rated with two parallel rows of proteinaceous
tripartite tubular hairs (TTHs) (Vlk 1939 )
(Fig.3.16a, b, l, o–r), which reverse the flagel-
lum thrust, in effect pulling the zoospores
through the water (Dick2001a). Hyphochytrio-
mycota zoospores are small (3–5mm) and char-
acterized by their single anterior flagellum
decorated with TTHs (Fig.3.16d) (Dick2001a;
Fuller 1990 ; Karling 1977 ; Sparrow 1960 ).
There is some variation in the presence and
distribution of TTHs within the oomycete
clade. Zoospores of mostHaptoglossaspecies
lack TTHs (Fig.3.16m, n) (Beakes and Glock-
ling 1998 ), whilst genera such as Crypticola
(Frances et al. 1989 ) andLagenidium gigan-
teum(Domas et al. 1986 ) have only a single
row of TTHs along the anterior flagellum, and
Myzocytiopsis vermicolahas only a partial row
of TTHs proximal to the zoospore body (Glock-
ling and Beakes2006a). Thraustochytrid zoos-
pores are unusual amongst stramenopiles in
that the cell body is coated in small scales76 G.W. Beakes et al.